Auditory System

Assist. Prof. A.A. Maharramov

Auditory System

Auditory system

Auditory system

Auditory system

Auditory system

Auditory system

Vestibular System

Human Auditory System

The Auditory System

Outer Ear• The outer ear is comprised of the pinna (given the Latin origins, the plural is pinnae) and external auditory canal (also called the external auditory meatus). The eardrum (also called the tympanic membrane) marks the transition between the outer ear and the middle ear.

Anatomy of the Auditory SystemThe hearing structures of the ear are divided into three parts:The outer ear consists of the external ear structure, called the pinna, and the external ear canal.The middle ear contains the tympanic membrane, or eardrum, and three small bones known as the ossicles.The inner ear contains the cochlea and connected bony structures, called the "semicircular canals."These structures work together to transmit sound from outside the ear to the auditory nerve, which send the information as an electrical impulse to the brain, where it can be processed and assigned meaning.

Outer earMain article: Outer earThe folds of cartilage surrounding the ear canal are called the pinna. Sound waves are reflected and attenuated when they hit the pinna, and these changes provide additional information that will help the brain determine the direction from which the sounds came. A single malfunction of an organ in the ear can cause a person to become deaf.The sound waves enter the auditory canal, a deceptively simple tube. The ear canal amplifies sounds that are between 3 and 12 kHz. At the far end of the ear canal is the eardrum (or tympanic membrane), which marks the beginning of the middle ear.

Middle earMain article: Middle earSound waves traveling through the ear canal will hit the tympanic membrane, or eardrum. This wave information travels across the air-filled middle ear cavity via a series of delicate bones: the malleus (hammer), incus (anvil) and stapes (stirrup). These ossicles act as a lever and a teletype, converting the lower-pressure eardrum sound vibrations into higher-pressure sound vibrations at another, smaller membrane called the oval (or elliptical) window. The malleus articulates with the tympanic membrane via the manubrium, where the stapes articulates with the oval window via its footplate. Higher pressure is necessary because the inner ear beyond the oval window contains liquid rather than air. The sound is not amplified uniformly across the ossicular chain. The stapedius reflex of the middle ear muscles helps protect the inner ear from damage. The middle ear still contains the sound information in wave form; it is converted to nerve impulses in the cochlea.

Main article: Inner earThe inner ear consists of the cochlea and several non-auditory structures. The cochlea has three fluid-filled sections, and supports a fluid wave driven by pressure across the basilar membrane separating two of the sections. Strikingly, one section, called the cochlear duct or scala media, contains endolymph, a fluid similar in composition to the intracellular fluid found inside cells. The organ of Corti is located in this duct on the basilar membrane, and transforms mechanical waves to electric signals in neurons. The other two sections are known as the scala tympani and the scala vestibuli; these are located within the bony labyrinth, which is filled with fluid called perilymph, similar in composition to cerebrospinal fluid. The chemical difference between the two fluids (endolymph & perilymph) is important for the function of the inner ear due to electrical potential differences between potassium and calcium ions.

Human factors involving the auditory system Virtual reality attempts to offer realistic stimuli for all human senses. Given the current state of technology, the details of realism are often trade-offs for real-time management of the system components. The auditory system is no exception. While the human ear is capable of hearing a multitude of distinct sounds, the ear can only concentrate on listening to one particular sound at a given time.

This physiological constraint has lead to studies in "auditory cognition". Auditory cognition analyzes such issues as attending to auditory events, remembering and recognizing sound sources and events, and perceptions of acoustic sequences. The theories behind auditory cognition attempt to explain how the brain processes and/or filters out certain sounds.

These studies are useful in Virtual Reality systems, because they allow the developers to mimic selective sounds in the background while the user attends to a specific important sound. For example, if the virtual environment imitates the city of New York, the sound generators need not orchestrate the sounds of taxi horns, subway noise, the rustling of people, and other typical background noise. These may be merged as one sound that is played in the background, since the user may not want to attend to any individual sound in this sound cluster. However, the user may be interested in hearing his name being called among the noise. The layering of this sound over the background noise is a sound to which the user would selectively attend. In this example, the sound of interest (i.e. the name being called) would require the use of 3D sound. However, the background noises need not take advantage of the 3D sound capabilities, since these sounds seem to surround the user, and the user does not consciously attempt to locate the sound source of each individual noise.

This section provides some sample theories on how sounds of interest can be made to be attention-holding in the virtual environment.

Broadbent's Theory Broadbent's Filter Theory of Attention has been used as the basis for most selective attention models. In his theory, sound information is passed through a number of sensory channels. These "channels" are vaguely defined as having some distinct neural representation in the brain. Their representations may be based on a number of sound attributes, such as pitch, loudness or spatial position characteristics. Broadbent postulates that the sound channels lead into the short-term memory portion of the brain, where a particular channel may then be filtered based on the desired sound attributes. This filter allows only one of the channels to lead to the long-term memory store and any output mechanisms necessary to respond to the input channel.

This theory arose from the cocktail party situation in which a guest must filter out all distracting sounds to concentrate on one conversation. Several studies have suggested different conditions under which the filter switches to listen to a different channel. For instance, when your name is heard in the middle of a current conversation shows some priority over which channel is filtered. Some experiments have suggested that the switching time of the filter between channels is to be of the order of 0.25 seconds [Moray, 1970].

Treisman's Theory Treisman's theory modifies Broadbent's theory, by proposing that the input selections bypass the short-term memory area of the brain to arrive immediately to a filter which is sensitive to a sound's physical characteristics. This filter eliminates most unattended sound channels, but allows a subset of channels to enter a series of nodes in the brain, known as "dictionary units". This network of dictionary units becomes a pattern matcher, where similar-sounding stimuli trigger signals to the listener's output activity mechanism in the brain.

Deutsch and Deutsch's Theory Deutsch and Deutsch's Response Selection Theory of Selective Attention alters Treisman's theory by omitting the initial filter for physical characteristics. Within the dictionary network, each signal is analyzed and recognized for its importance. The importance of the signal fires a proportional signal to the brain's output activity mechanism. Hence, the sound that captures the listener's attention is the sound that bears the heaviest signal. Applying this theory to hearing your name during a conversation shows that tuning into your name when it is called has importance.

INTENSITY OF SOUND

• I = 10 log(I/I0)

• I0 = 3x10-5W/m2