Binaural Beats

Interaural time difference From Wikipedia, the free encyclopedia The interaural time difference (or ITD) when concerning humans or animals, is the difference in arrival time of a sound between two ears. It is important in the localization of sounds, as it provides a cue to the direction or angle of the sound source from the head. If a signal arrives at the head from one side, the signal has further to travel to reach the far ear than the near ear. This pathlength difference results in a time difference between the sound's arrivals at the ears, which is detected and aids the process of identifying the direction of sound source. When a signal is produced in the horizontal plane, its angle in relation to the head is referred to as its azimuth , with 0 degrees (0°) azimuth being directly in front of the listener, 90° to the right, and 180° being directly behind. Different methods for measuring ITDs For an abrupt stimulus such as a click, onset ITDs are measured. An onset ITD is the time difference between the onset of the signal reaching two ears. A transient ITD can be measured when using a random noise stimulus and is calculated as the time difference between a set peak of the noise stimulus reaching the ears. If the stimulus used is not abrupt but periodic then ongoing ITDs are measured. This is where the waveforms reaching both ears can be shifted in time until they perfectly match up and the size of this shift is recorded as the ITD. This shift is known as the interaural phase difference (IPD) and can be used for measuring the ITDs of periodic inputs such as pure tones and amplitude modulated stimuli. An amplitude modulated stimulus IPD can be assessed by looking at either the waveform envelope or the waveform fine structure. Duplex theory The Duplex theory proposed by Lord Rayleigh (1907) provides an explanation for the ability of humans to localise sounds by time differences between the sounds reaching each ear (ITDs) and differences in sound level entering the ears (interaural level differences, ILDs). But there still lies a question whether ITD or ILD is prominent. The duplex theory states that ITDs are used to localise low frequency sounds, in particular, while ILDs are used in the localisation of high frequency sound inputs. However, the frequency ranges for which the auditory system can use ITDs and ILDs significantly overlap, and most natural sounds will have both high and low frequency components, so that the auditory system will in most cases have to combine information from both ITDs and ILDs to judge the location of a sound source.[1] A consequence of this duplex system is that it is also possible to generate so-called "cue trading" or "time–intensity trading" stimuli on headphones, where ITDs pointing to the left are offset by ILDs pointing to the right, so the sound is perceived as coming from the midline. A limitation of the duplex theory is that the theory does not completely explain directional hearing, as no explanation is given for the ability to distinguish between a sound source directly in front and behind. Also the theory only relates to localising sounds in the horizontal plane around the head. The theory also does not take into account the use of the pinna in localisation.(Gelfand, 2004)


Neurological effects on organisms

Transcript of Binaural Beats

  • Interaural time difference

    From Wikipedia, the free encyclopedia

    The interaural time difference (or ITD) when concerning humans or animals, is the difference inarrival time of a sound between two ears. It is important in the localization of sounds, as it providesa cue to the direction or angle of the sound source from the head. If a signal arrives at the head fromone side, the signal has further to travel to reach the far ear than the near ear. This pathlengthdifference results in a time difference between the sound's arrivals at the ears, which is detected andaids the process of identifying the direction of sound source.When a signal is produced in the horizontal plane, its angle in relation to the head is referred to asits azimuth, with 0 degrees (0) azimuth being directly in front of the listener, 90 to the right, and180 being directly behind.

    Different methods for measuring ITDs For an abrupt stimulus such as a click, onset ITDs are measured. An onset ITD is the time

    difference between the onset of the signal reaching two ears. A transient ITD can be measured when using a random noise stimulus and is calculated as

    the time difference between a set peak of the noise stimulus reaching the ears. If the stimulus used is not abrupt but periodic then ongoing ITDs are measured. This is

    where the waveforms reaching both ears can be shifted in time until they perfectly match upand the size of this shift is recorded as the ITD. This shift is known as the interaural phasedifference (IPD) and can be used for measuring the ITDs of periodic inputs such as puretones and amplitude modulated stimuli. An amplitude modulated stimulus IPD can beassessed by looking at either the waveform envelope or the waveform fine structure.

    Duplex theoryThe Duplex theory proposed by Lord Rayleigh (1907) provides an explanation for the ability ofhumans to localise sounds by time differences between the sounds reaching each ear (ITDs) anddifferences in sound level entering the ears (interaural level differences, ILDs). But there still lies aquestion whether ITD or ILD is prominent.The duplex theory states that ITDs are used to localise low frequency sounds, in particular, whileILDs are used in the localisation of high frequency sound inputs. However, the frequency ranges forwhich the auditory system can use ITDs and ILDs significantly overlap, and most natural soundswill have both high and low frequency components, so that the auditory system will in most caseshave to combine information from both ITDs and ILDs to judge the location of a sound source.[1] Aconsequence of this duplex system is that it is also possible to generate so-called "cue trading" or"timeintensity trading" stimuli on headphones, where ITDs pointing to the left are offset by ILDspointing to the right, so the sound is perceived as coming from the midline. A limitation of theduplex theory is that the theory does not completely explain directional hearing, as no explanation isgiven for the ability to distinguish between a sound source directly in front and behind. Also thetheory only relates to localising sounds in the horizontal plane around the head. The theory alsodoes not take into account the use of the pinna in localisation.(Gelfand, 2004)

  • Experiments conducted by Woodworth (1938) tested the duplex theory by using a solid sphere tomodel the shape of the head and measuring the ITDs as a function of azimuth for differentfrequencies. The model used had a distance between the 2 ears of approximately 2223 cm. Initialmeasurements found that there was a maximum time delay of approximately 660 s when the soundsource was placed at directly 90 azimuth to one ear. This time delay correlates to the wavelength ofa sound input with a frequency of 1500 Hz. The results concluded that when a sound played had afrequency less than 1500 Hz the wavelength is greater than this maximum time delay between theears. Therefore there is a phase difference between the sound waves entering the ears providingacoustic localisation cues. With a sound input with a frequency closer to 1500 Hz the wavelength ofthe sound wave is similar to the natural time delay. Therefore due to the size of the head and thedistance between the ears there is a reduced phase difference so localisations errors start to be made.When a high frequency sound input is used with a frequency greater than 1500 Hz, the wavelengthis shorter than the distance between the 2 ears, a head shadow is produced and ILD provide cues forthe localisation of this sound.Feddersen et al. (1957) also conducted experiments taking measurements on how ITDs alter withchanging the azimuth of the loudspeaker around the head at different frequencies. But unlike theWoodworth experiments human subjects were used rather than a model of the head. The experimentresults agreed with the conclusion made by Woodworth about ITDs. The experiments alsoconcluded that is there is no difference in ITDs when sounds are provided from directly in front orbehind at 0 and 180 azimuth. The explanation for this is that the sound is equidistant from bothears. Interaural time differences alter as the loudspeaker is moved around the head. The maximumITD of 660 s occurs when a sound source is positioned at 90 azimuth to one ear.

    The anatomy of the ITD pathwayThe auditory nerve fibres, known as the afferent nerve fibres, carry information from the organ ofCorti to the brainstem and brain. Auditory afferent fibres consist of two types of fibres called type Iand type II fibres. Type I fibres innervate the base of one or two inner hair cells and Type II fibresinnervate the outer hair cells. Both leave the organ of Corti through an opening called the habenulaperforata. The type I fibres are thicker than the type II fibres and may also differ in how theyinnervate the inner hair cells. Neurons with large calycal endings ensure preservation of timinginformation throughout the ITD pathway.Next in the pathway is the cochlear nucleus, which receives mainly ipsilateral (that is, from thesame side) afferent input. The cochlear nucleus has three distinct anatomical divisions, known asthe antero-ventral cochlear nucleus (AVCN), postero-ventral cochlear nucleus (PVCN) and dorsalcochlear nucleus (DCN) and each have different neural innervations.The AVCN contains predominant bushy cells, with one or two profusely branching dendrites; it isthought that bushy cells may process the change in the spectral profile of complex stimuli. TheAVCN also contain cells with more complex firing patterns than bushy cells called multipolar cells,these cells have several profusely branching dendrites and irregular shaped cell bodies. Multipolarcells are sensitive to changes in acoustic stimuli and in particular, onset and offset of sounds, as wellas changes in intensity and frequency. The axons of both cell types leave the AVCN as large tractcalled the ventral acoustic stria, which forms part of the trapezoid body and travels to the superiorolivary complex.A group of nuclei in pons make up the superior olivary complex (SOC). This is the first stage inauditory pathway to receive input from both cochleas, which is crucial for our ability to localise thesounds source in the horizontal plane. The SOC receives input from cochlear nuclei, primarily theipsilateral and contralateral AVCN. Four nuclei make up the SOC but only the medial superior olive(MSO) and the lateral superior olive (LSO) receive input from both cochlear nuclei.The MSO is made up of neurons which receive input from the low-frequency fibers of the left and

  • right AVCN. The result of having input from both cochleas is an increase in the firing rate of theMSO units. The neurons in the MSO are sensitive to the difference in the arrival time of sound ateach ear, also known as the interaural time difference (ITD). Research shows that if stimulationarrives at one ear before the other, many of the MSO units will have increased discharge rates. Theaxons from the MSO continue to higher parts of the pathway via the ipsilateral lateral lemniscustract.(Yost, 2000)The lateral lemniscus (LL) is the main auditory tract in the brainstem connecting SOC to theinferior colliculus. The dorsal nucleus of the lateral lemniscus (DNLL) is a group of neuronsseparated by lemniscus fibres, these fibres are predominantly destined for the inferior colliculus(IC). In studies using an unanesthetized rabbit the DNLL was shown to alter the sensitivity of the ICneurons and may alter the coding of interaural timing differences (ITDs) in the IC.(Kuwada et al.,2005) The ventral nucleus of the lateral lemniscus (VNLL) is a chief source of input to the inferiorcolliculus. Research using rabbits shows the discharge patterns, frequency tuning and dynamicranges of VNLL neurons supply the inferior colliculus with a variety of inputs, each enabling adifferent function in the analysis of sound.(Batra & Fitzpatrick, 2001) In the inferior colliculus (IC)all the major ascending pathways from the olivary complex and the central nucleus converge. TheIC is situated in the midbrain and consists of a group of nuclei the largest of these is the centralnucleus of inferior colliculus (CNIC). The greater part of the ascending axons forming the laterallemniscus will terminate in the ipsilateral CNIC however a few follow the commissure of Probstand terminate on the contralateral CNIC. The axons of most of the CNIC cells form the brachium ofIC and leave the brainstem to travel to the ipsilateral thalamus. Cells in different parts of the IC tendto be monaural, responding to input from one ear, or binaural and therefore respond to bilateralstimulation.The spectral processing that occurs in the AVCN and the ability to process binaural stimuli, as seenin the SOC, are replicated in the IC. Lower centres of the IC extract different features of theacoustic signal such as frequencies, frequency bands, onsets, offsets, changes in intensity andlocalisation. The integration or synthesis of acoustic information is thought to start in the CNIC.(Yost, 2000)

    Effect of a hearing lossA number of studies have looked into the effect of hearing loss on interaural time differences. Intheir review of localisation and lateralisation studies, Durlach, Thompson, and Colburn (1981),citedin Moore (1996) found a clear trend for poor localization and lateralization in people withunilateral or asymmetrical cochlear damage. This is due to the difference in performance betweenthe two ears. In support of this, they did not find significant localisation problems in individualswith symmetrical cochlear losses. In addition to this, studies have been conducted into the effect ofhearing loss on the threshold for interaural time differences. The normal human threshold fordetection of an ITD is up to a time difference of 10s (microseconds). Studies by Gabriel, Koehnke,& Colburn (1992), Husler, Colburn, & Marr (1983) and Kinkel, Kollmeier, & Holube (1991)(citedby Moore, 1996) have shown that there can be great differences between individuals regardingbinaural performance. It was found that unilateral or asymmetric hearing losses can increase thethreshold of ITD detection in patients. This was also found to apply to individuals with symmetricalhearing losses when detecting ITDs in narrowband signals. However, ITD thresholds seem to benormal for those with symmetrical losses when listening to broadband sounds.

  • Sound localizationSound localization refers to a listener's ability to identify the location or origin of a detected soundin direction and distance. It may also refer to the methods in acoustical engineering to simulate theplacement of an auditory cue in a virtual 3D space (see binaural recording, wave field synthesis).The sound localization mechanisms of the mammalian auditory system have been extensivelystudied. The auditory system uses several cues for sound source localization, including time- andlevel-differences between both ears, spectral information, timing analysis, correlation analysis, andpattern matching.These cues are also used by animals, but there may be differences in usage, and there are alsolocalization cues which are absent in the human auditory system, such as the effects of earmovements.

    Sound localization by the human auditory systemSound localization is the process of determining the location of a sound source. The brain utilizessubtle differences in intensity, spectral, and timing cues to allow us to localize sound sources.[1] [2] Localization can be described in terms of three-dimensional position: the azimuth or horizontalangle, the elevation or vertical angle, and the distance (for static sounds) or velocity (for movingsounds).[3] The azimuth of a sound is signalled by the difference in arrival times between the ears,by the relative amplitude of high-frequency sounds (the shadow effect), and by the asymmetricalspectral reflections from various parts of our bodies, including torso, shoulders, and pinnae.[3] Thedistance cues are the loss of amplitude, the loss of high frequencies, and the ratio of the direct signalto the reverberated signal.[3] Depending on where the source is located, our head acts as a barrier tochange the timbre, intensity, and spectral qualities of the sound, helping the brain orient where thesound emanated from.[2] These minute differences between the two ears are known as interauralcues.[2] Lower frequencies, with longer wavelengths, diffract the sound around the head forcing thebrain to focus only on the phasing cues from the source.[2] Helmut Haas discovered that we candiscern the sound source despite additional reflections at 10 decibels louder than the original wavefront, using the earliest arriving wave front.[2] This principle is known as the Haas effect, a specificversion of the precedence effect.[2] Haas measured down to even a 1 millisecond difference intiming between the original sound and reflected sound increased the spaciousness, allowing thebrain to discern the true location of the original sound. The nervous system combines all earlyreflections into a single perceptual whole allowing the brain to process multiple different sounds atonce.[4] The nervous system will combine reflections that are within about 35 milliseconds of eachother and that have a similar intensity.[4]

    Lateral information (left, ahead, right)For determining the lateral input direction (left, front, right) the auditory system analyzes thefollowing ear signal information:

    Interaural time differences Sound from the right side reaches the right ear earlier than the left ear. The auditory systemevaluates interaural time differences from

    Phase delays at low frequencies group delays at high frequencies

    Interaural level differences

  • Sound from the right side has a higher level at the right ear than at the left ear, because thehead shadows the left ear. These level differences are highly frequency dependent and theyincrease with increasing frequency.

    For frequencies below 800 Hz, mainly interaural time differences are evaluated (phase delays), forfrequencies above 1600 Hz mainly interaural level differences are evaluated. Between 800 Hz and1600 Hz there is a transition zone, where both mechanisms play a role.Localization accuracy is 1 degree for sources in front of the listener and 15 degrees for sources tothe sides. Humans can discern interaural time differences of 10 microseconds or less.[5] [6]

    Evaluation for low frequenciesFor frequencies below 800 Hz, the dimensions of the head (ear distance 21.5 cm, corresponding toan interaural time delay of 625 s), are smaller than the half wavelength of the sound waves. So theauditory system can determine phase delays between both ears without confusion. Interaural leveldifferences are very low in this frequency range, especially below about 200 Hz, so a preciseevaluation of the input direction is nearly impossible on the basis of level differences alone. As thefrequency drops below 80 Hz it becomes difficult or impossible to use either time difference orlevel difference to determine a sound's lateral source, because the phase difference between the earsbecomes too small for a directional evaluation.

    Evaluation for high frequenciesFor frequencies above 1600 Hz the dimensions of the head are greater than the length of the soundwaves. An unambiguous determination of the input direction based on interaural phase alone is notpossible at these frequencies. However, the interaural level differences become larger, and theselevel differences are evaluated by the auditory system. Also, group delays between the ears can beevaluated, and is more pronounced at higher frequencies; that is, if there is a sound onset, the delayof this onset between the ears can be used to determine the input direction of the correspondingsound source. This mechanism becomes especially important in reverberant environment. After asound onset there is a short time frame where the direct sound reaches the ears, but not yet thereflected sound. The auditory system uses this short time frame for evaluating the sound sourcedirection, and keeps this detected direction as long as reflections and reverberation prevent anunambiguous direction estimation.The mechanisms described above cannot be used to differentiate between a sound source ahead ofthe hearer or behind the hearer; therefore additional cues have to be evaluated.

    Sound localization in the median plane (front, above, back, below)The human outer ear, i.e. the structures of the pinna and the external ear canal, form direction-selective filters. Depending on the sound input direction in the median plane, different filterresonances become active. These resonances implant direction-specific patterns into the frequencyresponses of the ears, which can be evaluated by the auditory system (directional bands) for verticalsound localization. Together with other direction-selective reflections at the head, shoulders andtorso, they form the outer ear transfer functions.These patterns in the ear's frequency responses are highly individual, depending on the shape andsize of the outer ear. If sound is presented through headphones, and has been recorded via anotherhead with different-shaped outer ear surfaces, the directional patterns differ from the listener's own,and problems will appear when trying to evaluate directions in the median plane with these foreignears. As a consequence, frontback permutations or inside-the-head-localization can appear whenlistening to dummy head recordings,or otherwise referred to as binaural recordings.

  • Distance of the sound sourceThe human auditory system has only limited possibilities to determine the distance of a soundsource. In the close-up-range there are some indications for distance determination, such as extremelevel differences (e.g. when whispering into one ear) or specific pinna resonances in the close-uprange.The auditory system uses these clues to estimate the distance to a sound source:

    Sound spectrum : High frequencies are more quickly damped by the air than lowfrequencies. Therefore a distant sound source sounds more muffled than a close one, becausethe high frequencies are attenuated. For sound with a known spectrum (e.g. speech) thedistance can be estimated roughly with the help of the perceived sound.

    Loudness: Distant sound sources have a lower loudness than close ones. This aspect can beevaluated especially for well-known sound sources (e.g. known speakers).

    Movement: Similar to the visual system there is also the phenomenon of motion parallax inacoustical perception. For a moving listener nearby sound sources are passing faster thandistant sound sources.

    Reflections: In enclosed rooms two types of sound are arriving at a listener: The directsound arrives at the listener's ears without being reflected at a wall. Reflected sound hasbeen reflected at least one time at a wall before arriving at the listener. The ratio betweendirect sound and reflected sound can give an indication about the distance of the soundsource.

    Signal processingSound processing of the human auditory system is performed in so-called critical bands. Thehearing range is segmented into 24 critical bands, each with a width of 1 Bark or 100 Mel. For adirectional analysis the signals inside the critical band are analyzed together.The auditory system can extract the sound of a desired sound source out of interfering noise. So theauditory system can concentrate on only one speaker if other speakers are also talking (the cocktailparty effect). With the help of the cocktail party effect sound from interfering directions is perceivedattenuated compared to the sound from the desired direction. The auditory system can increase thesignal-to-noise ratio by up to 15 dB, which means that interfering sound is perceived to beattenuated to half (or less) of its actual loudness.

    Localization in enclosed roomsIn enclosed rooms not only the direct sound from a sound source is arriving at the listener's ears, butalso sound which has been reflected at the walls. The auditory system analyses only the directsound[citation needed], which is arriving first, for sound localization, but not the reflected sound,which is arriving later (law of the first wave front). So sound localization remains possible even inan echoic environment. This echo cancellation occurs in the Dorsal Nucleus of the LateralLemniscus (DNLL).In order to determine the time periods, where the direct sound prevails and which can be used fordirectional evaluation, the auditory system analyzes loudness changes in different critical bands andalso the stability of the perceived direction. If there is a strong attack of the loudness in severalcritical bands and if the perceived direction is stable, this attack is in all probability caused by thedirect sound of a sound source, which is entering newly or which is changing its signalcharacteristics. This short time period is used by the auditory system for directional and loudnessanalysis of this sound. When reflections arrive a little bit later, they do not enhance the loudness

  • inside the critical bands in such a strong way, but the directional cues become unstable, becausethere is a mix of sound of several reflection directions. As a result no new directional analysis istriggered by the auditory system.This first detected direction from the direct sound is taken as the found sound source direction, untilother strong loudness attacks, combined with stable directional information, indicate that a newdirectional analysis is possible. (see Franssen effect)

    AnimalsSince most animals have two ears, many of the effects of the human auditory system can also befound in animals. Therefore interaural time differences (interaural phase differences) and interaurallevel differences play a role for the hearing of many animals. But the influences on localization ofthese effects are dependent on head sizes, ear distances, the ear positions and the orientation of theears.

    Lateral information (left, ahead, right)If the ears are located at the side of the head, similar lateral localization cues as for the humanauditory system can be used. This means: evaluation of interaural time differences (interaural phasedifferences) for lower frequencies and evaluation of interaural level differences for higherfrequencies. The evaluation of interaural phase differences is useful, as long as it givesunambiguous results. This is the case, as long as ear distance is smaller than half the length(maximal one wavelength) of the sound waves. For animals with a larger head than humans theevaluation range for interaural phase differences is shifted towards lower frequencies, for animalswith a smaller head, this range is shifted towards higher frequencies.The lowest frequency which can be localized depends on the ear distance. Animals with a greaterear distance can localize lower frequencies than humans can. For animals with a smaller eardistance the lowest localizable frequency is higher than for humans.If the ears are located at the side of the head, interaural level differences appear for higherfrequencies and can be evaluated for localization tasks. For animals with ears at the top of the head,no shadowing by the head will appear and therefore there will be much less interaural leveldifferences, which could be evaluated. Many of these animals can move their ears, and these earmovements can be used as a lateral localization cue.

    Sound localization by odontocetesDolphins (and other odontocetes) rely on echolocation to aid in detecting, identifying, localizing,and capturing prey. Dolphin sonar signals are well suited for localizing multiple, small targets in a

    three dimensional aquatic environment by utilizing highly directional (3 dB beamwidth of about 10deg), broadband (3 dB bandwidth typically of about 40 kHz; peak frequencies between 40 kHz and120 kHz), short duration clicks (about 40 s). Dolphins can localize sounds both passively and

    actively (echolocation) with a resolution of about 1 deg. Cross modal matching (between vision andecholocation) suggests dolphins perceive the spatial structure of complex objects interrogatedthrough echolocation, a feat that likely requires spatially resolving individual object features andintegration into a holistic representation of object shape. Although dolphins are sensitive to small,

    binaural intensity and time differences, mounting evidence suggests dolphins employ positiondependent spectral cues derived from well developed head related transfer functions, for sound

    localization in both the horizontal and vertical planes. A very small temporal integration time (264s) allows localization of multiple targets at varying distances. Localization adaptations includepronounced asymmetry of the skull, nasal sacks, and specialized lipid structures in the forehead andjaws, as well as acoustically isolated middle and inner ears.

  • Sound localization in the median plane (front, above, back, below)For many mammals there are also pronounced structures in the pinna near the entry of the ear canal.As a consequence, direction-dependent resonances can appear, which could be used as an additionallocalization cue, similar to the localization in the median plane in the human auditory system. Thereare additional localization cues which are also used by animals.

    Head tiltingFor sound localization in the median plane (elevation of the sound) also two detectors can be used,which are positioned at different heights. In animals, however, rough elevation information isgained simply by tilting the head, provided that the sound lasts long enough to complete themovement. This explains the innate behavior of cocking the head to one side when trying to localizea sound precisely. To get instantaneous localization in more than two dimensions from time-difference or amplitude-difference cues requires more than two detectors.

    Localization with one ear (flies)The tiny parasitic fly Ormia ochracea has become a model organism in sound localizationexperiments because of its unique ear. The animal is too small for the time difference of soundarriving at the two ears to be calculated in the usual way, yet it can determine the direction of soundsources with exquisite precision. The tympanic membranes of opposite ears are directly connectedmechanically, allowing resolution of sub-microsecond time differences[7] [8] and requiring a newneural coding strategy.[9] Ho[10] showed that the coupled-eardrum system in frogs can produceincreased interaural vibration disparities when only small arrival time and sound level differenceswere available to the animals head. Efforts to build directional microphones based on the coupled-eardrum structure are underway.

    Bi-coordinate sound localization in owlsMost owls are nocturnal or crepuscular birds of prey. Because they hunt at night, they must rely onnon-visual senses. Experiments by Roger Payne[11] have shown that owls are sensitive to thesounds made by their prey, not the heat or the smell. In fact, the sound cues are both necessary andsufficient for localization of mice from a distant location where they are perched. For this to work,the owls must be able to accurately localize both the azimuth and the elevation of the sound source.

    ITD and ILDOwls must be able to determine the necessary angle of descent, i.e. the elevation, in addition toazimuth (horizontal angle to the sound). This bi-coordinate sound localization is accomplishedthrough two binaural cues: the interaural time difference (ITD) and the interaural level difference(ILD), also known as the interaural intensity difference (IID). The ability in owls is unusual; inground-bound mammals such as mice, ITD and ILD are not utilized in the same manner. In thesemammals, ITDs tend to be utilized for localization of lower frequency sounds, while ILDs tend tobe used for higher frequency sounds.ITD occurs whenever the distance from the source of sound to the two ears is different, resulting indifferences in the arrival times of the sound at the two ears. When the sound source is directly infront of the owl, there is no ITD, i.e. the ITD is zero. In sound localization, ITDs are used as cuesfor location in the azimuth. ITD changes systematically with azimuth. Sounds to the right arrivefirst at the right ear; sounds to the left arrive first at the left ear.In mammals there is a level difference in sounds at the two ears caused by the sound-shadowingeffect of the head. But in many species of owls, level differences arise primarily for sounds that are

  • shifted above or below the elevation of the horizontal plane. This is due to the asymmetry inplacement of the ear openings in the owl's head, such that sounds from below the owl reach the leftear first and sounds from above reach the right ear first.[12] IID is a measure of the difference in thelevel of the sound as it reaches each ear. In many owls, IIDs for high-frequency sounds (higher than4 or 5 kHz) are the principal cues for locating sound elevation.

    Parallel processing pathways in the brainThe axons of the auditory nerve originate from the hair cells of the cochlea in the inner ear.Different sound frequencies are encoded by different fibers of the auditory nerve, arranged alongthe length of the auditory nerve, but codes for the timing and level of the sound are not segregatedwithin the auditory nerve. Instead, the ITD is encoded by phase locking, i.e. firing at or near aparticular phase angle of the sinusoidal stimulus sound wave, and the IID is encoded by spike rate.Both parameters are carried by each fiber of the auditory nerve.[13]The fibers of the auditory nerve innervate both cochlear nuclei in the brainstem, the cochlearnucleus magnocellularis (mammalian anteroventral cochlear nucleus) and the cochlear nucleusangularis (see figure; mammalian posteroventral and dorsal cochlear nuclei). The neurons of thenucleus magnocellularis phase-lock, but are fairly insensitive to variations in sound pressure, whilethe neurons of the nucleus angularis phase-lock poorly, if at all, but are sensitive to variations insound pressure. These two nuclei are the starting points of two separate but parallel pathways to theinferior colliculus: the pathway from nucleus magnocellularis processes ITDs, and the pathwayfrom nucleus angularis processes IID.

  • Parallel processing pathways in the brain for time and level for sound localization in the owlIn the time pathway, the nucleus laminaris (mammalian medial superior olive) is the first site ofbinaural convergence. It is here that ITD is detected and encoded using neuronal delay lines andcoincidence detection, as in the Jeffress model; when phase-locked impulses coming from the leftand right ears coincide at a laminaris neuron, the cell fires most strongly. Thus, the nucleuslaminaris acts as a delay-line coincidence detector, converting distance traveled to time delay andgenerating a map of interaural time difference. Neurons from the nucleus laminaris project to thecore of the central nucleus of the inferior colliculus and to the anterior lateral lemniscal nucleus.In the sound level pathway, the posterior lateral lemniscal nucleus (mammalian lateral superiorolive) is the site of binaural convergence and where IID is processed. Stimulation of thecontralateral ear inhibits and that of the ipsilateral ear excites the neurons of the nuclei in each brainhemisphere independently. The degree of excitation and inhibition depends on sound pressure, andthe difference between the strength of the inhibitory input and that of the excitatory inputdetermines the rate at which neurons of the lemniscal nucleus fire. Thus the response of theseneurons is a function of the difference in sound pressure between the two ears.The time and sound-pressure pathways converge at the lateral shell of the central nucleus of theinferior colliculus. The lateral shell projects to the external nucleus, where each space-specific

  • neuron responds to acoustic stimuli only if the sound originates from a restricted area in space, i.e.the receptive field of that neuron. These neurons respond exclusively to binaural signals containingthe same ITD and IID that would be created by a sound source located in the neurons receptivefield. Thus their receptive fields arise from the neurons tuning to particular combinations of ITDand IID, simultaneously in a narrow range. These space-specific neurons can thus form a map ofauditory space in which the positions of receptive fields in space are isomorphically projected ontothe anatomical sites of the neurons.[14]

    Significance of asymmetrical ears for localization of elevationThe ears of many species of owls are asymmetrical. For example, in barn owls (Tyto alba), theplacement of the two ear flaps (operculi) lying directly in front of the ear canal opening is differentfor each ear. This asymmetry is such that the center of the left ear flap is slightly above a horizontalline passing through the eyes and directed downward, while the center of the right ear flap isslightly below the line and directed upward. In two other species of owls with asymmetrical ears,the saw-whet Owl and the long-eared owl, the asymmetry is achieved by different means: in sawwhets, the skull is asymmetrical; in the long-eared owl, the skin structures lying near the ear formasymmetrical entrances to the ear canals, which is achieved by a horizontal membrane. Thus, earasymmetry seems to have evolved on at least three different occasions among owls. Because owlsdepend on their sense of hearing for hunting, this convergent evolution in owl ears suggests thatasymmetry is important for sound localization in the owl.Ear asymmetry allows for sound originating from below the eye level to sound louder in the left ear,while sound originating from above the eye level to sound louder in the right ear. Asymmetrical earplacement also causes IID for high frequencies (between 4 kHz and 8 kHz) to vary systematicallywith elevation, converting IID into a map of elevation. Thus, it is essential for an owl to have theability to hear high frequencies. Many birds have the neurophysiological machinery to process bothITD and IID, but because they have small heads and low frequency sensitivity, they use bothparameters only for localization in the azimuth. Through evolution, the ability to hear frequencieshigher than 3 kHz, the highest frequency of owl flight noise, enabled owls to exploit elevationalIIDs, produced by small ear asymmetries that arose by chance, and began the evolution of moreelaborate forms of ear asymmetry.[15]Another demonstration of the importance of ear asymmetry in owls is that, in experiments, owlswith symmetrical ears, such as the screech owl (Otus asio) and the great horned owl (Bubovirginianus), could not be trained to locate prey in total darkness, whereas owls with asymmetricalears could be trained.[16]

    Neural interactionsIn vertebrates, inter-aural time differences are known to be calculated in the superior olivarynucleus of the brainstem. According to Jeffress,[17] this calculation relies on delay lines: neurons inthe superior olive which accept innervation from each ear with different connecting axon lengths.Some cells are more directly connected to one ear than the other, thus they are specific for aparticular inter-aural time difference. This theory is equivalent to the mathematical procedure ofcross-correlation. However, because Jeffress' theory is unable to account for the precedence effect,in which only the first of multiple identical sounds is used to determine the sounds' location (thusavoiding confusion caused by echoes), it cannot be entirely used to explain the response.Furthermore, a number of recent physiological observations made in the midbrain and brainstem ofsmall mammals have shed considerable doubt on the validity of Jeffress' original ideas [18]Neurons sensitive to ILDs are excited by stimulation of one ear and inhibited by stimulation of theother ear, such that the response magnitude of the cell depends on the relative strengths of the twoinputs, which in turn, depends on the sound intensities at the ears.

  • In the auditory midbrain nucleus, the inferior colliculus (IC), many ILD sensitive neurons haveresponse functions that decline steeply from maximum to zero spikes as a function of ILD.However, there are also many neurons with much more shallow response functions that do notdecline to zero spikes.

    Binaural fusion

    Binaural fusion (or binaural integration) is a cognitive process that involves the "fusion" ofdifferent auditory information presented binaurally, or to each ear. In humans, this process isessential in understanding speech as one ear may pick up more information about the speech stimulithan the other.The process of binaural fusion is important for computing the location of sound sources in thehorizontal plane (sound localization), and it is important for sound segregation.[1] Soundsegregation refers the ability to identify acoustic components from one or more sound sources.[2]The binaural auditory system is highly dynamic and capable of rapidly adjusting tuning propertiesdepending on the context in which sounds are heard. Each eardrum moves one-dimensionally; theauditory brain analyzes and compares movements of both eardrums to extract physical cues andsynthesize auditory objects.[3]When stimulation from a sound reaches the ear, the eardrum deflects in a mechanical fashion, andthe three middle ear bones (ossicles) transmit the mechanical signal to the cochlea, where hair cellstransform the mechanical signal into an electrical signal. The auditory nerve, also called thecochlear nerve, then transmits action potentials to the central auditory nervous system.[3]In binaural fusion, inputs from both ears integrate and fuse to create a complete auditory picture atthe brainstem. Therefore, the signals sent to the central auditory nervous system are representativeof this complete picture, integrated information from both ears instead of a single ear.Binaural fusion is responsible for what is known as the cocktail party effect, the ability of a listenerto hear a particular speaker against other interfering voices.[3]The binaural squelch effect is a result of nuclei of the brainstem processing timing, amplitude, andspectral differences between the two ears. Sounds are integrated and then separated into auditoryobjects. For this effect to take place, neural integration from both sides is required.[4]

  • AnatomyAs sound travels into the inner eardrum of vertebrate mammals, it encounters the hair cells that linethe basilar membrane of the cochlea in the inner ear.[5] The cochlea receives auditory informationto be binaurally integrated. At the cochlea, this information is converted into electrical impulses thattravel by means of the cochlear nerve, which spans from the cochlea to the ventral cochlear nucleus,which is located in the pons of the brainstem.[6] The lateral lemniscus projects from the cochlearnucleus to the superior olivary complex (SOC), a set of brainstem nuclei that consists primarily oftwo nuclei, the medial superior olive (MSO) and the lateral superior olive (LSO), and is the majorsite of binaural fusion. The subdivision of the ventral cochlear nucleus that concerns binaural fusionis the anterior ventral cochlear nucleus (AVCN).[3] The AVCN consists of spherical bushy cells andglobular bushy cells and can also transmit signals to the medial nucleus of the trapezoid body(MNTB), whose neuron projects to the MSO. Transmissions from the SOC travel to the inferiorcolliculus (IC) via the lateral lemniscus. At the level of the IC, binaural fusion is complete. Thesignal ascends to the thalamocortical system, and sensory inputs to the thalamus are then relayed tothe primary auditory cortex.[3] [7] [8] [9]

    FunctionThe ear functions to analyze and encode a sounds dimensions.[10] Binaural fusion is responsible

  • for avoiding the creation of multiple sound images from a sound source and its reflections. Theadvantages of this phenomenon are more noticeable in small rooms, decreasing as the reflectivesurfaces are placed farther from the listener.[11]

    Central auditory systemThe central auditory system converges inputs from both ears (inputs contain no explicit spatialinformation) onto single neurons within the brainstem. This system contains many subcortical sitesthat have integrative functions. The auditory nuclei collect, integrate, and analyze afferent supply,[10] the outcome is a representation of auditory space.[3] The subcortical auditory nuclei areresponsible for extraction and analysis of dimensions of sounds.[10]The integration of a sound stimulus is a result of analyzing frequency (pitch), intensity, and spatiallocalization of the sound source.[12] Once a sound source has been identified, the cells of lowerauditory pathways are specialized to analyze physical sound parameters.[3] Summation is observedwhen the loudness of a sound from one stimulus is perceived as having been doubled when heardby both ears instead of only one. This process of summation is called binaural summation and is theresult of different acoustics at each ear, depending on where sound is coming from. [4]The cochlear nerve spans from the cochlea of the inner ear to the ventral cochlear nuclei located inthe pons of the brainstem, relaying auditory signals to the superior olivary complex where it is to bebinaurally integrated.

    Medial superior olive and lateral superior oliveThe MSO contains cells that function in comparing inputs from the left and right cochlear nuclei.[13] The tuning of neurons in the MSO favors low frequencies, whereas those in the LSO favorhigh frequencies.[14]GABAB receptors in the LSO and MSO are involved in balance of excitatory and inhibitory inputs.The GABAB receptors are coupled to G proteins and provide a way of regulating synaptic efficacy.Specifically, GABAB receptors modulate excitatory and inhibitory inputs to the LSO.[3] Whetherthe GABAB receptor functions as excitatory or inhibitory for the postsynaptic neuron, depends onthe exact location and action of the receptor.[1]

    Sound localizationSound localization is the ability to correctly identify the directional location of sounds. A soundstimulus localized in the horizontal plane is called azimuth; in the vertical plane it is referred to aselevation. The time, intensity, and spectral differences in the sound arriving at the two ears are usedin localization. Localization of low frequency sounds is accomplished by analyzing interaural timedifference (ITD). Localization of high frequency sounds is accomplished by analyzing interaurallevel difference (ILD).[4]


    Binaural hearingAction potentials originate in the hair cells of the cochlea and propagate to the brainstem; both thetiming of these action potentials and the signal they transmit provide information to the SOC aboutthe orientation of sound in space. The processing and propagation of action potentials is rapid, andtherefore, information about the timing of the sounds that were heard, which is crucial to binaural

  • processing, is conserved.[15] Each eardrum moves in one dimension, and the auditory brainanalyzes and compares the movements of both eardrums in order to synthesize auditory objects.[3]This integration of information from both ears is the essence of binaural fusion. The binaural systemof hearing involves sound localization in the horizontal plane, contrasting with the monaural systemof hearing, which involves sound localization in the vertical plane.[3]

    Superior olivary complexThe primary stage of binaural fusion, the processing of binaural signals, occurs at the SOC, whereafferent fibers of the left and right auditory pathways first converge. This processing occurs becauseof the interaction of excitatory and inhibitory inputs in the LSO and MSO.[3] [13] [1] The SOCprocesses and integrates binaural information, in the form of ITD and ILD, entering the brainstemfrom the cochleae. This initial processing of ILD and ITD is regulated by GABAB receptors.[1]

    ITD and ILDThe auditory space of binaural hearing is constructed based on the analysis of differences in twodifferent binaural cues in the horizontal plane: sound level, or ILD, and arrival time at the two ears,or ITD, which allow for the comparison of the sound heard at each eardrum.[1] [3] ITD is processedin the LSO and results from sounds arriving earlier at one ear than the other; this occurs when thesound does not arise from directly in front or directly behind the hearer. ILD is processed in theMSO and results from the shadowing effect that is produced at the ear that is farther from the soundsource. Outputs from the SOC are targeted to the dorsal nucleus of the lateral lemniscus as well asthe IC.[3]

    Lateral superior oliveLSO neurons are excited by inputs from one ear and inhibited by inputs from the other, and aretherefore referred to as IE neurons. Excitatory inputs are received at the LSO from spherical bushycells of the ipsilateral cochlear nucleus, which combine inputs coming from several auditory nervefibers. Inhibitory inputs are received at the LSO from globular bushy cells of the contralateralcochlear nucleus.[3]

    Medial superior oliveMSO neurons are excited bilaterally, meaning that they are excited by inputs from both ears, andthey are therefore referred to as EE neurons.[3] Fibers from the left cochlear nucleus terminate onthe left of MSO neurons, and fibers from the right cochlear nucleus terminate on the right of MSOneurons.[13] Excitatory inputs to the MSO from spherical bushy cells are mediated by glutamate,and inhibitory inputs to the MSO from globular bushy cells are mediated by glycine. MSO neuronsextract ITD information from binaural inputs and resolve small differences in the time of arrival ofsounds at each ear.[3] Outputs from the MSO and LSO are sent via the lateral lemniscus to the IC,which integrates the spatial localization of sound. In the IC, acoustic cues have been processed andfiltered into separate streams, forming the basis of auditory object recognition.[3]

    Binaural fusion abnormalities in autismCurrent research is being performed on the dysfunction of binaural fusion in individuals withautism. The neurological disorder autism is associated with many symptoms of impaired brainfunction, including the degradation of hearing, both unilateral and bilateral.[16] Individuals withautism who experience hearing loss maintain symptoms such as difficulty listening to backgroundnoise and impairments in sound localization. Both the ability to distinguish particular speakers frombackground noise and the process of sound localization are key products of binaural fusion. They

  • are particularly related to the proper function of the SOC, and there is increasing evidence thatmorphological abnormalities within the brainstem, namely in the SOC, of autistic individuals are acause of the hearing difficulties.[17] The neurons of the MSO of individuals with autism displayatypical anatomical features, including atypical cell shape and orientation of the cell body as well asstellate and fusiform formations.[18] Data also suggests that neurons of the LSO and MNTBcontain distinct dysmorphology in autistic individuals, such as irregular stellate and fusiform shapesand a smaller than normal size. Moreover, a significant depletion of SOC neurons is seen in thebrainstem of autistic individuals. All of these structures play a crucial role in the proper functioningof binaural fusion, so their dysmorphology may be at least partially responsible for the incidence ofthese auditory symptoms in autistic patients.[17]

    Acoustic location

    Swedish soldiers operating an acoustic locator in 1940Acoustic location is the science of using sound to determine the distance and direction ofsomething. Location can be done actively or passively, and can take place in gases (such as theatmosphere), liquids (such as water), and in solids (such as in the earth).

    Active acoustic location involves the creation of sound in order to produce an echo, which isthen analyzed to determine the location of the object in question.

    Passive acoustic location involves the detection of sound or vibration created by the objectbeing detected, which is then analyzed to determine the location of the object in question.

    Both of these techniques, when used in water, are known as sonar; passive sonar and active sonarare both widely used.Acoustic mirrors and dishes, when using microphones, are a means of passive acoustic localization,but when using speakers are a means of active localization. Typically, more than one device is used,and the location is then triangulated between the several devices.As a military air defense tool, passive acoustic location was used from mid-World War I[1] to theearly years of World War II to detect enemy aircraft by picking up the noise of their engines. It wasrendered obsolete before and during World War II by the introduction of radar, which was far moreeffective (but interceptable). Acoustic techniques had the advantage that they could 'see' aroundcorners and over hills, due to sound refraction.The civilian uses include locating wildlife[2] and locating the shooting position of a firearm.[3]

  • Antiaircraft defence Sweden 1940Swedish soldiers operating an acoustic locatorSoldiers operating an acoustic airplane locator during World War 2, Trelleborg, Sweden, 1940.Before radar was invented, acoustic locator equipment was used to detect approaching enemyaircraft by listening for the sound of their engines.

    Military uses have included locating submarines[4] and aircraft.[5]The air-defense instruments usually consisted of large horns or microphones connected to theoperators ears using tubing, much like a very large stethoscope.[6] [7] Most of the work on anti-aircraft sound ranging was done by the British. They developed anextensive network of sound mirrors that were used from World War I through World War II.[8] [9] Sound mirrors normally work by using moveable microphones to find the angle that maximizes theamplitude of sound received, which is also the bearing angle to the target. Two sound mirrors atdifferent positions will generate two different bearings, which allows the use of triangulation todetermine a sound source's position.As World War II neared, radar began to become a credible alternative to the sound location ofaircraft. For typical aircraft speeds of that time, sound location only gave a few minutes of warning.[5] The acoustic location stations were left in operation as a backup to radar, as exemplified duringthe Battle of Britain.[10] Today, the abandoned sites are still in existence and are readily accessible.[8]After World War II, sound ranging played no further role in anti-aircraft operations.For enemy artillery spotting, see sound ranging.

    Active / passive locatorsActive locators have some sort of signal generation device, in addition to a listening device. Thetwo devices do not have to be located together.

    SonarSONAR (Sound Navigation And Ranging) or sonar is a technique that uses soundpropagation under water (or occasionally in air) to navigate, communicate or to detect other vessels.There are two kinds of sonar active and passive. A single active sonar can localize in range andbearing as well as measuring radial speed. However, a single passive sonar can only localize inbearing directly, though target motion analysis can be used to localize in range, given time. Multiplepassive sonars can be used for range localization by triangulation or correlation, directly.For more information on this item, see the article on Sonar.

    Biological echo locationDolphins, whales and bats use echolocation to detect prey and avoid obstacles.

    Time-of-arrival localizationHaving speakers/ultrasonic transmitters emitting sound at known positions and time, the position ofa target equipped with a microphone/ultrasonic receiver can be estimated based on the time of

  • arrival of the sound. The accuracy is usually poor under non-line-of-sight conditions, where thereare blockages in between the transmitters and the receivers. [11]

    Seismic surveysSeismic surveys involve the generation of sound waves to measure underground structures. Sourcewaves are generally created by percussion mechanisms located near the ground or water surface,typically dropped weights, vibroseis trucks, or explosives. Data are collected with geophones, thenstored and processed by computer. Current technology allows the generation of 3D images ofunderground rock structures using such equipment.For more information, see Reflection seismology.

    EcotracerEcotracer is an acoustic locator that was used to determining the presence and position of ships infog. Some could detect targets at distances up to 12 kilometers. Static walls could detect aircraft upto 30 miles away.

    TypesThere were four main kinds of system:[12]

    Personal/wearable horns Transportable steerable horns Static dishes Static walls

    ImpactAmerican acoustic locators were used in 1941 to detect the Japanese attack on the fortress island ofCorregidor in the Philippines.

    OtherBecause the cost of the associated sensors and electronics is dropping, the use of sound rangingtechnology is becoming accessible for other uses, such as for locating wildlife.[13]

    Coincidence detection in neurobiologyFor the electronic device, see Coincidence circuit.Coincidence detection in the context of neurobiology is a process by which a neuron or a neuralcircuit can encode information by detecting the occurrence of timely simultaneous yet spatiallyseparate input signals. Coincidence detectors are important in information processing by reducingtemporal jitter, reducing spontaneous activity, and forming associations between separate neuralevents. This concept has led to a greater understanding of neural processes and the formation ofcomputational maps in the brain.

  • Principles of coincidence detectionCoincidence detection relies on separate inputs converging on a common target. Consider a basicneural circuit with two input neurons, A and B, that have excitatory synaptic terminals convergingon a single output neuron, C (Fig. 1). If each input neuron's EPSP is subthreshold for an actionpotential at C, then C will not fire unless the two inputs from A and B are temporally close together.Synchronous arrival of these two inputs may push the membrane potential of a target neuron overthe threshold required to create an action potential. If the two inputs arrive too far apart, thedepolarization of the first input may have time to drop significantly, preventing the membranepotential of the target neuron from reaching the action potential threshold. This exampleincorporates the principles of spatial and temporal summation. Furthermore, coincidence detectioncan reduce the jitter formed by spontaneous activity. While random sub-threshold stimulations byneuronal cells may not often fire coincidentally, coincident synaptic inputs derived from a unitaryexternal stimulus will ensure that a target neuron fires as a result of the stimulus.

    Fig. 1: Two EPSP's innervated in rapid successionsum to produce a larger EPSP or even an action potential in the postsynaptic cell.

    Fig. 2: If a sound arrives at the left ear before the right ear,the impulse in the left auditory tract will reach X soonerthan the impulse in the right auditory tract reaches Y.Neurons 4 or 5 may therefore receive coincident inputs.

    Synaptic plasticity and associativityn 1949, Donald Hebb postulated that synaptic efficiency will increase through repeated andpersistent stimulation of a postsynaptic cell by a presynaptic cell. This is often informallysummarized as "cells that fire together, wire together". The theory was validated in part by thediscovery of long-term potentiation. Studies of LTP on multiple presynaptic cells stimulating apostsynaptic cell uncovered the property of associativity. A weak neuronal stimulation onto a

  • pyramidal neuron may not induce long-term potentiation. However, this same stimulation pairedwith a simultaneous strong stimulation from another neuron will strengthen both synapses. Thisprocess suggests that two neuronal pathways converging on the same cell may both strengthen ifstimulated coincidentally.

    Molecular mechanism of long-term potentiationLTP in the hippocampus requires a prolonged depolarization that can expel the Mg2+ block ofpostsynaptic NMDA receptors. The removal of the Mg2+ block allows the flow of Ca2+ into thecell. A large elevation of calcium levels activate protein kinases that ultimately increase the numberof postsynaptic AMPA receptors. This increases the sensitivity of the postsynaptic cell to glutamate.As a result, both synapses strengthen. The prolonged depolarization needed for the expulsion ofMg2+ from NMDA receptors requires a high frequency stimulation (Purves 2004). Associativitybecomes a factor because this can be achieved through two simultaneous inputs that may not bestrong enough to activate LTP by themselves.Besides the NMDA-receptor based processes, further cellular mechanisms allow of the associationbetween two different input signals converging on the same neuron, in a defined timeframe. Upon asimultaneous increase in the intracellular concentrations of cAMP and Ca2+, a transcriptionalcoactivator called TORC1 (CRTC1) becomes activated, that converts the temporal coincidence ofthe two second messengers into long term changes such as LTP (Kovacs KA; Steullet, P; Steinmann,M; Do, KQ; Magistretti, PJ; Halfon, O; Cardinaux, JR (2007), "TORC1 is a calcium- and cAMP-sensitive coincidence detector involved in hippocampal long-term synaptic plasticity.", PNAS 104(11): 47005, doi:10.1073/pnas.0607524104, PMC 1838663, PMID 17360587). This cellularmechanism, through calcium-dependent adenylate cyclase activation, might also account for thedetection of the repetitive stimulation of a given synapse.

    Molecular mechanism of long-term depressionLong-term depression also works through associative properties although it is not always thereverse process of LTP. LTD in the cerebellum requires a coincident stimulation of parallel fibersand climbing fibers. Glutamate released from the parallel fibers activates AMPA receptors whichdepolarize the postsynaptic cell. The parallel fibers also activate metabotropic glutamate receptorsthat release the second messengers IP3 and DAG. The climbing fibers stimulate a large increase inpostsynaptic Ca2+ levels when activated. The Ca2+, IP3, and DAG work together in a signaltransduction pathway to internalize AMPA receptors and decrease the sensitivity of the postsynapticcell to glutamate (Purves 2004).

  • Animal echolocation

    A depiction of the ultrasound signals emitted by a bat, and the echo from a nearby object.Echolocation, also called biosonar, is the biological sonar used by several kinds of animals.Echolocating animals emit calls out to the environment and listen to the echoes of those calls thatreturn from various objects near them. They use these echoes to locate and identify the objects.Echolocation is used for navigation and for foraging (or hunting) in various environments. Someblind humans have learned to find their way using clicks produced by a device or by mouth.Echolocating animals include some mammals and a few birds; most notably microchiropteran batsand odontocetes (toothed whales and dolphins), but also in simpler form in other groups such asshrews, one genus of megachiropteran bats (Rousettus) and two cave dwelling bird groups, the so-called cave swiftlets in the genus Aerodramus (formerly Collocalia) and the unrelated OilbirdSteatornis caripensis.

    Early researchThe term echolocation was coined by Donald Griffin, whose work with Robert Galambos was thefirst to conclusively demonstrate its existence in bats in 1938.[1] [2] Long before that, however, the 18th century Italian scientist Lazzaro Spallanzani had, by means of aseries of elaborate experiments, concluded that bats navigate by hearing and not by vision.[3]Echolocation in odontocetes was not properly described until two decades later, by Schevill andMcBride.[4]

    PrincipleEcholocation is the same as active sonar, using sounds made by the animal itself. Ranging is doneby measuring the time delay between the animal's own sound emission and any echoes that returnfrom the environment. The relative intensity of sound received at each ear as well as the time delaybetween arrival at the two ears provide information about the horizontal angle (azimuth) fromwhich the reflected sound waves arrive.[5]Unlike some man-made sonars that rely on many extremely narrow beams and many receivers tolocalize a target (multibeam sonar), animal echolocation has only one transmitter and two receivers(the ears). Echolocating animals have two ears positioned slightly apart. The echoes returning to thetwo ears arrive at different times and at different loudness levels, depending on the position of the

  • object generating the echoes. The time and loudness differences are used by the animals to perceivedistance and direction. With echolocation, the bat or other animal can see not only where it is goingbut also how big another animal is, what kind of animal it is, and other features.[citation needed]

    BatsMicrobats use echolocation to navigate and forage, often in total darkness. They generally emergefrom their roosts in caves, attics, or trees at dusk and hunt for insects into the night. Their use ofecholocation allows them to occupy a niche where there are often many insects (that come out atnight since there are fewer predators then) and where there is less competition for food, and wherethere are fewer other species that may prey on the bats themselves.Microbats generate ultrasound via the larynx and emit the sound through the open mouth or, muchmore rarely, the nose. The latter is most pronounced in the horseshoe bats (Rhinolophus spp.).Microbat calls (helpinfo) range in frequency from 14,000 to well over 100,000 Hz, mostly beyondthe range of the human ear (typical human hearing range is considered to be from 20 Hz to20,000 Hz). Bats may estimate the elevation of targets by interpreting the interference patternscaused by the echoes reflecting from the tragus, a flap of skin in the external ear.[6] There are twohypotheses about the evolution of echolocation in bats. The first suggests that laryngealecholocation evolved twice in Chiroptera, once in the Yangochiroptera and once in the Horseshoebats (Rhinolophidae).[7] [8] The second proposes that laryngeal echolocation had a single origin inChiroptera, was subsequently lost in the family Pteropodidae (all megabats), and later evolved as asystem of tongue-clicking in the genus Rousettus.[9]Individual bat species echolocate within specific frequency ranges that suit their environment andprey types. This has sometimes been used by researchers to identify bats flying in an area simply byrecording their calls with ultrasonic recorders known as "bat detectors". However echolocation callsare not always species specific and some bats overlap in the type of calls they use so recordings ofecholocation calls cannot be used to identify all bats. In recent years researchers in several countrieshave developed "bat call libraries" that contain recordings of local bat species that have beenidentified known as "reference calls" to assist with identification.Since the 1970s there has been an ongoing controversy among researchers as to whether bats use aform of processing known from radar termed coherent cross-correlation. Coherence means that thephase of the echolocation signals is used by the bats, while cross-correlation just implies that theoutgoing signal is compared with the returning echoes in a running process. Today most - but not all- researchers believe that they use cross-correlation, but in an incoherent form, termed a filter bankreceiver.When searching for prey they produce sounds at a low rate (10-20 clicks/second). During the searchphase the sound emission is coupled to respiration, which is again coupled to the wingbeat. Thiscoupling appears to dramatically conserve energy as there is little to no additional energetic cost ofecholocation to flying bats.[10] After detecting a potential prey item, microbats increase the rate ofpulses, ending with the terminal buzz, at rates as high as 200 clicks/second. During approach to adetected target, the duration of the sounds is gradually decreased, as is the energy of the sound.

    Calls and ecologyBats belonging to the suborder Microchiroptera (microbats) occupy a diverse set of ecologicalconditions - they can be found living in environments as different as Europe and Madagascar, andhunting for food sources as different as insects, frogs, nectar, fruit, and blood. Additionally, thecharacteristics of an echolocation call are adapted to the particular environment, hunting behavior,and food source of the particular bat. However, this adaptation of echolocation calls to ecologicalfactors is constrained by the phylogenetic relationship of the bats, leading to a process known as

  • descent with modification, and resulting in the diversity of the Microchiroptera today.[11] [12] [13] [14] [15] [16]

    Acoustic featuresDescribing the diversity of bat echolocation calls requires examination of the frequency andtemporal features of the calls. It is the variations in these aspects that produce echolocation callssuited for different acoustic environments and hunting behaviors.[17] [18] [19] [20] [21]

    Frequency Modulation and Constant Frequency: Echolocation calls can be composed oftwo different types of frequency structures: frequency modulated (FM) sweeps, and constantfrequency (CF) tones. A particular call can consist of one, the other, or both structures. AnFM sweep is a broadband signal that is, it contains a downward sweep through a range offrequencies. A CF tone is a narrowband signal: the sound stays constant at one frequencythroughout its duration.

    Intensity: Echolocation calls have been measured at intensities anywhere between 60 and140 decibels.[22] Certain microbat species can modify their call intensity mid-call, loweringthe intensity as they approach objects that reflect sound strongly. This prevents the returningecho from deafening the bat.[23] Additionally, the so-called "whispering bats" have adaptedlow-amplitude echolocation so that their prey, moths, which are able to hear echolocationcalls, are less able to detect and avoid an oncoming bat[24]

    Harmonic composition: Calls can be composed of one frequency, or multiple frequenciescomprising a harmonic series. In the latter case, the call is usually dominated by a certainharmonic ("dominant" frequencies are those present at higher intensities than otherharmonics present in the call).[citation needed]

    Call duration: A single echolocation call (a call being a single continuous trace on a soundspectrogram, and a series of calls comprising a sequence or pass) can last anywhere from 0.2to 100 milliseconds in duration, depending on the stage of prey-catching behavior that thebat is engaged in. For example, the duration of a call usually decreases when the bat is in thefinal stages of prey capture this enables the bat to call more rapidly without overlap of calland echo. Reducing duration comes at the cost of having less total sound available forreflecting off objects and being heard by the bat.[citation needed]

    Pulse interval: The time interval between subsequent echolocation calls (or pulses)determines two aspects of a bat's perception. First, it establishes how quickly the bat'sauditory scene information is updated. For example, bats increase the repetition rate of theircalls (that is, decrease the pulse interval) as they home in on a target. This allows the bat toget new information regarding the target's location at a faster rate when it needs it most.Secondly, the pulse interval determines the maximum range that bats can detect objects. Thisis because bats can only keep track of the echoes from one call at a time; as soon as theymake another call they stop listening for echoes from the previously made call.[25] Forexample, a pulse interval of 100 ms (typical of a bat searching for insects) allows sound totravel in air roughly 34 meters so a bat can only detect objects as far away as 17 meters (thesound has to travel out and back). With a pulse interval of 5 ms (typical of a bat in the finalmoments of a capture attempt), the bat can only detect objects up to 85 cm away. Thereforethe bat constantly has to make a choice between getting new information updated quicklyand detecting objects far away.

  • FM Signal AdvantagesThe major advantage conferred by an FM signal is extremely precise range discrimination, orlocalization, of the target. J.A. Simmons demonstrated this effect with a series of elegantexperiments that showed how bats using FM signals could distinguish between two separate targetseven when the targets were less than half a millimeter apart. This amazing ability is due to thebroadband sweep of the signal, which allows for better resolution of the time delay between the calland the returning echo, thereby improving the cross correlation of the two. Additionally, if harmonicfrequencies are added to the FM signal, then this localization becomes even more precise.[26] [27] [28] [29] One possible disadvantage of the FM signal is a decreased operational range of the call. Because theenergy of the call is spread out among many frequencies, the distance at which the FM-bat candetect targets is limited.[30] This is in part because any echo returning at a particular frequency canonly be evaluated for a brief fraction of a millisecond, as the fast downward sweep of the call doesnot remain at any one frequency for long.[31]

    CF Signal AdvantagesThe structure of a CF signal is adaptive in that it allows the CF-bat to detect both the velocity of atarget, and the fluttering of a target's wings as Doppler shifted frequencies. A Doppler shift is analteration in sound wave frequency, and is produced in two relevant situations: when the bat and itstarget are moving relative to each other, and when the target's wings are oscillating back and forth.CF-bats must compensate for Doppler shifts, lowering the frequency of their call in response toechoes of elevated frequency - this ensures that the returning echo remains at the frequency towhich the ears of the bat are most finely tuned. The oscillation of a target's wings also producesamplitude shifts, which gives a CF-bat additional help in distinguishing a flying target from astationary one. (Schnitzler and Flieger 1983; Zupanc 2004; Simmons and Stein 1980; Grinnell1995; Neuweiler 2003; Jones and Teeling 2006)Additionally, because the signal energy of a CF call is concentrated into a narrow frequency band,the operational range of the call is much greater than that of an FM signal. This relies on the factthat echoes returning within the narrow frequency band can be summed over the entire length of thecall, which maintains a constant frequency for up to 100 milliseconds.[32] [33]

    Acoustic environments of FM and CF signals

    FM: An FM component is excellent for hunting prey while flying in close, clutteredenvironments. Two aspects of the FM signal account for this fact: the precise targetlocalization conferred by the broadband signal, and the short duration of the call. The first ofthese is essential because in a cluttered environment, the bats must be able to resolve theirprey from large amounts of background noise. The 3D localization abilities of the broadbandsignal enable the bat to do exactly that, providing it with what Simmons and Stein (1980)call a "clutter rejection strategy." This strategy is further improved by the use of harmonics,which, as previously stated, enhance the localization properties of the call. The shortduration of the FM call is also best in close, cluttered environments because it enables thebat to emit many calls extremely rapidly without overlap. This means that the bat can get analmost continuous stream of information essential when objects are close, because theywill pass by quickly without confusing which echo corresponds to which call. (Neuweiler2003; Simmons and Stein 1980; Jones and Teeling 2006; Fenton 1995)

    CF: A CF component is often used by bats hunting for prey while flying in open, clutter-free

  • environments, or by bats that wait on perches for their prey to appear. The success of theformer strategy is due to two aspects of the CF call, both of which confer excellent prey-detection abilities. First, the greater working range of the call allows bats to detect targetspresent at great distances a common situation in open environments. Second, the length ofthe call is also suited for targets at great distances: in this case, there is a decreased chancethat the long call will overlap with the returning echo. The latter strategy is made possible bythe fact that the long, narrowband call allows the bat to detect Doppler shifts, which wouldbe produced by an insect moving either towards or away from a perched bat. (Neuweiler2003; Simmons and Stein 1980; Jones and Teeling 2006; Fenton 1995)

    Neural mechanisms in the brainBecause bats use echolocation to orient themselves and to locate objects, their auditory systems areadapted for this purpose, highly specialized for sensing and interpreting the stereotypedecholocation calls characteristic of their own species. This specialization is evident from the innerear up to the highest levels of information processing in the auditory cortex.

    Inner ear and primary sensory neuronsBoth CF and FM bats have specialized inner ears which allow them to hear sounds in the ultrasonicrange, far outside the range of human hearing. Although in most other aspects, the bat's auditoryorgans are similar to those of most other mammals, certain bats (horseshoe bats, Rhinolophus spp.and the moustached bat, Pteronotus parnelii) with a constant frequency (CF) component to theircall (known as high duty cycle bats) do have a few additional adaptations for detecting thepredominant frequency (and harmonics) of the CF vocalization. These include a narrow frequency"tuning" of the inner ear organs, with an especially large area responding to the frequency of thebat's returning echoes (Neuweiler 2003).The basilar membrane within the cochlea contains the first of these specializations for echoinformation processing. In bats that use CF signals, the section of membrane that responds to thefrequency of returning echoes is much larger than the region of response for any other frequency.For example, in [34], the horseshoe bat, there is a disproportionately lengthened and thickenedsection of the membrane that responds to sounds around 83 kHz, the constant frequency of the echoproduced by the bat's call. This area of high sensitivity to a specific, narrow range of frequency isknown as an "acoustic fovea".[35]Odontocetes (toothed whales and dolphins) have similar cochlear specializations to those found inbats. Odontocetes also have the highest neural investment of any cochleae reported to date withratios of greater than 1500 ganglion cells/mm of basilar membrane.Further along the auditory pathway, the movement of the basilar membrane results in thestimulation of primary auditory neurons. Many of these neurons are specifically "tuned" (respondmost strongly) to the narrow frequency range of returning echoes of CF calls. Because of the largesize of the acoustic fovea, the number of neurons responding to this region, and thus to the echofrequency, is especially high.[36]

    Inferior colliculusIn the Inferior colliculus, a structure in the bat's midbrain, information from lower in the auditoryprocessing pathway is integrated and sent on to the auditory cortex. As George Pollak and othersshowed in a series of papers in 1977, the interneurons in this region have a very high level ofsensitivity to time differences, since the time delay between a call and the returning echo tells thebat its distance from the target object. Especially interesting is that while most neurons respondmore quickly to stronger stimuli, collicular neurons maintain their timing accuracy even as signal

  • intensity changes.These interneurons are specialized for time sensitivity in several ways. First, when activated, theygenerally respond with only one or two action potentials. This short duration of response allowstheir action potentials to give a very specific indication of the exact moment of the time when thestimulus arrived, and to respond accurately to stimuli that occur close in time to one another. Inaddition, the neurons have a very low threshold of activation they respond quickly even to weakstimuli. Finally, for FM signals, each interneuron is tuned to a specific frequency within the sweep,as well as to that same frequency in the following echo. There is specialization for the CFcomponent of the call at this level as well. The high proportion of neurons responding to thefrequency of the acoustic fovea actually increases at this level.[37] [38] [39]

    Auditory cortexThe auditory cortex in bats is quite large in comparison with other mammals.[40] Variouscharacteristics of sound are processed by different regions of the cortex, each providing differentinformation about the location or movement of a target object. Most of the existing studies oninformation processing in the auditory cortex of the bat have been done by Nobuo Suga on themustached bat, Pteronotus parnellii. This bat's call has both CF tone and FM sweep components.Suga and his colleagues have shown that the cortex contains a series of "maps" of auditoryinformation, each of which is organized systematically based on characteristics of sound such asfrequency and amplitude. The neurons in these areas respond only to a specific combination offrequency and timing (sound-echo delay), and are known as combination-sensitive neurons.The systematically organized maps in the auditory cortex respond to various aspects of the echosignal, such as its delay and its velocity. These regions are composed of "combination sensitive"neurons that require at least two specific stimuli to elicit a response. The neurons varysystematically across the maps, which are organized by acoustic features of the sound and can betwo dimensional. The different features of the call and its echo are used by the bat to determineimportant characteristics of their prey. The maps include:

    FM-FM area: This region of the cortex contains FM-FM combination-sensitive neurons. Thesecells respond only to the combination of two FM sweeps: a call and its echo. The neurons in theFM-FM region are often referred to as "delay-tuned," since each responds to a specific time delaybetween the original call and the echo, in order to find the distance from the target object (therange). Each neuron also shows specificity for one harmonic in the original call and a differentharmonic in the echo. The neurons within the FM-FM area of the cortex of Pteronotus areorganized into columns, in which the delay time is constant vertically but increases across thehorizontal plane. The result is that range is encoded by location on the cortex, and increasessystematically across the FM-FM area.[41] [42] [43] [44]

    CF-CF area: Another kind of combination-sensitive neuron is the CF-CF neuron. Theserespond best to the combination of a CF call containing two given frequencies a call at30 kHz (CF1) and one of its additional harmonics around 60 or 90 kHz (CF2 or CF3) andthe corresponding echoes. Thus, within the CF-CF region, the changes in echo frequencycaused by the Doppler shift can be compared to the frequency of the original call to calculatethe bat's velocity relative to its target object. As in the FM-FM area, information is encodedby its location within the map-like organization of the region. The CF-CF area is first splitinto the distinct CF1-CF2 and CF1-CF3 areas. Within each area, the CF1 frequency isorganized on an axis, perpendicular to the CF2 or CF3 frequency axis. In the resulting grid,each neuron codes for a certain combination of frequencies that is indicative of a specificvelocity[45] [46] [47]

  • DSCF area: This large section of the cortex is a map of the acoustic fovea, organized byfrequency and by amplitude. Neurons in this region respond to CF signals that have beenDoppler shifted (in other words, echoes only) and are within the same narrow frequencyrange to which the acoustic fovea responds. For Pteronotus, this is around 61 kHz. This areais organized into columns, which are arranged radially based on frequency. Within a column,each neuron responds to a specific combination of frequency and amplitude. Suga's studieshave indicated that this brain region is necessary for frequency discrimination.[48] [49] [50]

  • Toothed whalesBiosonar is valuable to Toothed whales (suborder odontoceti), including dolphins, porpoises, riverdolphins, killer whales and sperm whales, because they live in an underwater habitat that hasfavourable acoustic characteristics and where vision is extremely limited in range due to absorptionor turbidity.Cetacean evolution consisted of three main radiations. Throughout the middle and late Eoceneperiods (49-31.5 million years ago), archaeocetes, primitive toothed Cetacea that arose fromterrestrial mammals with the creation of aquatic adaptations, were the only known archaic Cetacea.[51] These primitive aquatic mammals did not possess the ability to echolocate, although they didhave slightly adapted underwater hearing.[52] The morphology of acoustically isolated ear bones inbasilosaurid archaeocetes indicates that this order had directional hearing underwater at low to midfrequencies by the late middle Eocene.[53] However, with the extinction of archaeocete at the onsetof the Oligocene, two new lineages in the early Oligocene period (31.5-28 million years ago)compromised a second radiation. These early mysticete (baleen whales) and odontocete can bedated back to the middle Oligocene in New Zealand.[51] Based on past phylogenies, it has beenfound that the evolution of odontocetes is monophyletic, suggesting that echolocation evolved onlyonce 36 to 34 million years ago.[53] Dispersal rates routes of early odontocetes includedtransoceanic travel to new adaptive zones. The third radiation occurred later in the Neogene, whenpresent dolphins and their relatives evolved to be the most common species in the modern sea.[52]The evolution of echolocation could be attributed several theories. There are two proposed drivesfor the hypotheses of cetacean radiation, one biotic and the other abiotic in nature. The first,adaptive radiation, is the result of a rapid divergence into new adaptive zones. This results indiverse, ecologically different clades that are incomparable.[54] Clade Neocete (crown cetacean)has been characterized by an evolution from archaeocetes and a dispersion across the world'soceans, and even estuarites and rivers. These ecological opportunities were the result of abundantdietary resources with low competition for hunting.[55] This hypothesis of lineage diversification,however, can be unconvincing due to a lack of support for rapid speciation early in cetacean history.A second, more abiotic drive is better supported. Physical restructuring of the oceans has played arole in echolocation radiation. This was a result of global climate change at the Eocene-Oligoceneboundary; from a greenhouse to an icehouse world. Tectonic openings created the emergence of theSouthern ocean with a free flowing Antarctic Circumpolar current.[56] [57] [58] [59] These eventsallowed for a selection regime characterized by the ability to locate and capture prey in turbid riverwaters, or allow odontocetes to invade and feed at depths below the photic zone. Further studieshave found that echolocation below the photic zone could have been a predation adaptation to dielmigrating cephalopods.[53] [60] Since its advent, there has been adaptive radiation especially in theDelphinidae family (dolphins) in which echolocation has become extremely derived.[61]One specific type of echolocation, narrow-band high frequency (NBHF) clicks, evolved at least fourtimes in groups of odontocetes, including pygmy sperm whale (Kogiidae) and porpoise(Phocoenidae) families, Pontoporia blainvillei, the genus Cephalorhynchus, and part of the genusLagenorhynchus.[62] [63] These high frequency clicks likely evolved as adaptation of predatoravoidance, as they inhabit areas that have many killer whales and the signals are inaudible to killerwhales due to the absence of energy below 100 kHz. Another reason for variation in echolocationfrequencies is habitat. Shallow waters, where many of these species live, tend to have more debris;a more directional transmission reduces clutter in reception.[63]Toothed whales emit a focused beam of high-frequency clicks in the direction that their head ispointing. Sounds are generated by passing air from the bony nares through the phonic lips.[64]These sounds are reflected by the dense concave bone of the cranium and an air sac at its base. Thefocused beam is modulated by a large fatty organ known as the 'melon'. This acts like an acousticlens because it is composed of lipids of differing densities. Most toothed whales use clicks in aseries, or click train, for echolocation, while the sperm whale may produce clicks individually.

  • Toothed whale whistles do not appear to be used in echolocation. Different rates of click productionin a click train give rise to the familiar barks, squeals and growls of the bottlenose dolphin. A clicktrain with a repetition rate over 600 per second is called a burst pulse. In bottlenose dolphins, theauditory brain response resolves individual clicks up to 600 per second, but yields a gradedresponse for higher repetition rates.It has been suggested that some smaller toothed whales may have their tooth arrangement suited toaid in echolocation. The placement of teeth in the jaw of a bottlenose dolphin, as an example, arenot symmetrical when seen from a vertical plane, and this asymmetry could possibly be an aid inthe dolphin sensing if echoes from its biosonar are coming from one side or the other.[65] [66] However, this idea lacks experimental support.Echoes are received using complex fatty structures around the lower jaw as the primary receptionpath, from where they are transmitted to the middle ear via a continuous fat body.[67] [68] Lateralsound may be received though fatty lobes surrounding the ears with a similar density to water.Some researchers believe that when they approach the object of interest, they protect themselvesagainst the louder echo by quietening the emitted sound. In bats this is known to happen, but herethe hearing sensitivity is also reduced close to a target.Before the echolocation abilities of "porpoises" were officially discovered, Jacques Yves Cousteausuggested that they might exist. In his first book, The Silent World (1953, pp. 206207), he reportedthat his research vessel, the lie Monier, was heading to the Straits of Gibraltar and noticed a groupof porpoises following them. Cousteau changed course a few degrees off the optimal course to thecenter of the strait, and the porpoises followed for a few minutes, then diverged toward mid-channelagain. It was obvious that they knew where the optimal course lay, even if the humans didn't.Cousteau concluded that the cetaceans had something like sonar, which was a relatively new featureon submarines.

    Oilbirds and swiftletsOilbirds and some species of swiftlet are known to use a relatively crude form of echolocationcompared to that of bats and dolphins. These nocturnal birds emit calls while flying and use thecalls to navigate through trees and caves where they live.[69] [70]

    Shrews and tenrecsMain article: Shrews#EcholocationTerrestrial mammals other than bats known to echolocate include two ge