The Central Auditory System

Ozarks Technical Community CollegeHIS 120

The Central Nervous System•The central nervous system contains

•the brain•the spinal cord

•The average adult brain weighs about 3 pounds•The brain contains about 100 billion neurons that transmit information about our bodies up to our brains, where that information is processed

Image from: faculty.washington.edu

The Brain

Image from: faculty.vassar.edu

•Our brains are divided into four lobes:• Frontal• Temporal• Parietal• Occipital

•The temporal lobe is where auditory information is sent to be processed

The Skull

Our brains are protected by our skull

The structures of the ear are housed in the temporal bone

Image from: cnx.org

Important Anatomy Terms to KnowExternal vs. Internal Auditory CanalAfferent vs. EfferentIpsilateral vs. Contralateral

External vs. Internal Auditory CanalYou know that the EAC is part of the outer

earThe internal auditory canal (IAC) or internal

auditory meatus is an opening in the temporal bone where the facial nerve (CN VII) and auditory nerve (CN VIII) travel to the brainstem

Afferent vs. EfferentAfferent refers to the ascending pathway of sensory

information that travels from our peripheral system to our central system. Another way to think about this is the transmission of information about all of our senses up to our brain. (i.e. when you touch a hot pan, the afferent nervous system sends the pain signal to your brain)Afferent =“up, up, and away”

Efferent refers to the descending pathway of information from our brain to a part of our body. (i.e. after your brain receives the pain signal from touching the hot pan, an efferent message is sent to your hand to pull away from the pan)

Ipsilateral vs. ContralateralIpsilateral = same sideContralateral = opposite side

Examples:When using nasal spray in the right nostril,

you should point the tip of the nasal spray at the corner of the ipsilateral (right) eye

During audiometric testing of the right ear, you present masking noise to the contralateral ear (left ear in this case) in order to isolate the test ear.

Onto the central auditory system…

So what happens to sound after it reaches the cochlea?Most of our auditory system is AFFERENTSound travels from our cochlea UP:

to the auditory nerve, thenTo the brainstem, thenTo the brain (where it is perceived as sound)

Information from one ear, reaches both sides of the brainOur PRIMARY auditory cortex is usually

located in the left hemisphere, as are our primary speech centers

Afferent Auditory Pathway•Cochlea•CN VIII•Brainstem:

•Cochlear Nucleus•Superior Olivary Complex•Lateral Lemniscus•Inferior Colliculus (crossover)•Medial Geniculate Body

•Brain:•Auditory cortex/Temporal Lobe

Afferent Auditory Pathway

Image from: Martin & Clark online

Efferent Auditory PathwaysDescending nerve fibers from the auditory cortex (brain)

to the brainstem and cochleaThe auditory efferent system is not well understood and

remains somewhat of a mysteryOur auditory efferent system is responsible for:

Sending inhibitory messages to the outer hair cells in the cochlea to control stiffness and membrane vibration

Innervation of the stapedius and tensor tympani muscles in the middle ear

Our efferent system is believed to play a role in:detection of signals in noise, protection in noise-induced

cochlear damage, development of hearing and processing of complex auditory signals

Efferent Auditory Pathways

Image from: Martin & Clark online

Central Auditory ProcessesOur central auditory system is responsible for the following

auditory processes:Sound localization and lateralizationAuditory discrimination

Determining the difference between different speech soundsAuditory pattern recognition

i.e. identifying a melodyTemporal (timing) aspects of hearing

Temporal integration Temporal discrimination (e.g., temporal gap detection) Temporal ordering Temporal masking

Auditory performance in competing acoustic signalsAuditory performance with degraded acoustic signals

ASHA, 1995