Central Auditory Nervous System Anatomy And Physiology

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2 main parts of the nervous system

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brain and spinal cord

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3 major levels of the brain

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cerebrum
cerebellum
brainstem

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cerebrum

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-largest
-most complex
-controls thought, learning, sensory info
- diff lobes

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cerebellum

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-coordinates movement
-posture
-balance
-muscle coordination

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brainstem

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-connects cerebrum with spinal cord
- responsible for motor and automatic functions

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cortex- location, function, appearance

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-the outermost layer of the cerebrum
-final processing of acoustic stimuli
-folded ridges and depressions

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ridges of cortex

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-gyri (gyrus)

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primary auditory cortex location

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superior temporal gyrus

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heschl's gyrus location

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transverse temporal gyrus

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depression of cortex- function

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fissures (sulci, sulcus)
major sulci divide cortex into lobes

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sylvian fissure

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lateral fissure- divides temporal lobe almost completely

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nucleus

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a collection of cell bodies in the CNS

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tract

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a bundle of axons in the CNS

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major landmarks in the auditory pathway in the CNS

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-cochlear nucleus
-superior olivary complex
-inferior colliculus
-medial geniculate body in thalamus
-auditory cortex in superior temporal gyrus

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major tract of superior olivary complex

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lateral lemniscus

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tracts of the medial geniculate body

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auditory radiations- deliver to auditory cortex

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what are the 2 ways that tracts in the CNS ascend? what do they sometimes do?

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ipsilaterally
contralaterally
projections sometimes skip levels

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what is the minimum # of synapses to get from cochlea to primary auditory cortex? path?

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- 4 synapses minimum
- cochlea > cochlear nucleus > contralateral inferior colliculus > medial geniculate body > auditory cortex

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Cochlear nucleus (CN)

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-3 major subdivisions
-tonotopically arranged
-numerous types of cells with different functions

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CN plays a major role in the...

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generation of wave 3 of the auditory brainstem response

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CN can be compromised by

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acoustic neuromas
vestibular schwannomas growing into cerebellopontine angle
hyperbilirubinemia

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superior olivary complex (SOC)

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recieves most input from CN
first location in CANS where binaural representation (Left and right SOCs) from monaural input is noted
major involvement in generation of auditory brainstem response and acoustic reflex

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nuclei in SOC

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small
diffuse
difficult to locate
not studied as extensively as other sites in brainstem

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localization

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determining apparent direction/distance of a sound

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lateralization

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determining the apparent direction of a sound as being left or right of the frontal-medial plane of the head

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auditory fusion

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combine info received at both ears into a fused auditory perception (auditory scene analysis)

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what plays a major role in localization, lateralization, and auditory fusion? why?

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SOC plays major role bc it is the first structure in the CANS to receive input from the two ears it allows analysis of the different neural impulses
-time of arrival (IAT difference)
-intensity of the original signal (IAI/IAL difference)

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Auditory brainstem response
waves 1 and 2 generated by the ____.
wave 3 genereated by the ___.
wave 4 generated by the____.

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auditory nerve
cochlear nucleus
superior olivary complex

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acoustic reflex- reflex arc, type of input

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external auditory canal- middle ear -cochlea- auditory nerve- ventral cochlear nucleus- superior olivary complex
ipsilateral and bilateral input

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presence of the acoustic reflex requires major participation of the...

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SOC

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LSO and MSO

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lateral and medial superior olive are tonotopically organized

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what is the lateral lemniscus? locaed? formed by? contain? organized? involved in what process?

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major lateral auditory pathway
in pons and midbrain
formed by fibers of the cochlear nuclei and superior olivary complex
contains efferent and afferent fibers
tonotopically organized
involved in generation of auditory brainstem response

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what is the role of the lateral lemniscus?

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wave 5 in auditory brainstem response- predominant wave that arises from neural activity in lateral lemniscus

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what is inferior colliculus? location?

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key auditory region
largest structure in auditory brainstem
in midbrain in the brainstem

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what does the inferior colliculus do? how organized?

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connects afferent fibers from lower brainstem structures
highly tonotopic
codes intensity and temporal aspects of the signal
involved in sound localization

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what happens if there is damage to the inferior colliculus?

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major deficits to auditory function

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how are inferior colliculus and lateral lemniscus related?

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-lateral lemniscus fiber pathway coures to inferior colliculus in midbrain
-both represent the main auditory pathway and auditory structures in brainstem above SOC

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whats located in the medial geniculate body? how organized?

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everything ascending synapse here
different divisions

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ventral division of medial geniculate body

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holds greater concentration of auditory fibers
play larger role in hearing

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medial geniculate body has 2 connections from...

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ipsilateral inferior colliculus
contralateral inferior colliculus

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primary auditory cortex is made of? location?

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-made of gray matter bc cortical fibers are unmyelinated
-located in superior temporal gyrus (heschls gyrus)
-near sylvian fissure
-larger on left than right hemisphere

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where does the auditory cortex recieve information from?

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receives neural input from the medial geniculate body

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what do you call the pathway connecting the medial geniculate body to the auditory cortex?

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thalamo-cortical pathway

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how many layers are of the auditory cortex?

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6

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functions of the auditory cortex

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-responds well to speech stimuli
-vital for understanding speech in noise (degraded speech)
-necessary for localization and lateralization

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auditory cortex and speech stimuli

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important and final role in processing of speech
esp dichotically presented speech (2 different stimuli presented one to each ear)
stronger responses to speech stimuli in left hemisphere

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2 valuable tools that can be used to evaluate the integrity of the auditory cortex

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PET, fMRI

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where are the connections from in the primary auditory cortex?

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-ipsilateral medial geniculate body
-projections from primary auditory cortex on one side to primary auditory cortex on other side via corpus callosum

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what is the corpus callosum? what happens if compromised?

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-large bundle of fibers that connect the 2 cerebral hemispheres
-deficits in dichotic listening

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central auditory processing

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-work in this area expanded since 60s
-question whether CAP disorders exist

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if CAP disorders arent real, what are they?

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reflections of a language disorder that is manifested when stimuli are presented auditorily

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Jerger- 3 points of evidence for existence of auditory perceptual disorders

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-significant evidence from children and adults with lesions of CANS
- listening problems of elderly can be associated with age related changes in CANS
-examples of person whose only complaint is inability to hear well in difficult listening situations with no speech/language defecits

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central auditory processing and ASHA

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1995- consensus statement that agrees upon definition of and best practice related to the diagnosis and management of children with CAPD

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taskforce defined CAP as (6)

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auditory system mechanisms and functions responsible for the following behavioral phenomena
-sound localization and lateralization
-auditory discrimination
-auditory pattern recognition
-temporal aspects of audition
-auditory performance decrements w competing acoustic signals
- auditory performance decrements w degraded acoustic signals

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these behavioral phenomena apply to what types of stimuli

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verbal and nonverbal

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2 propositions by Jerger

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-name change from CAPD to APD
-broader definition of APD

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jergers definition of APD

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deficit in the processing of info specific to the auditory modality (difficulty processing auditory information)

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2 contrasting views of APD

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-1 questions existence of CAPD
-other limits CAPD to disorders strictly related to processing acoustic-phonetic features of speech

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deficit in auditory attention

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-difficulty with the underlying cognitive process that allows the listener to focus selectively on stimulus of interest while ignoring irrelevant competing stimuli
-must assess individuals auditory attention skills rather than observable behaviors

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preparatory attention

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listener errs in how attention is to be directed initially so all subsequent processing is wasted

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selective attention

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critical when listening in presence of background noise
rarely occurs in the absence of other processing deficits
deficits in the ability to maintain and segregate acoustic streams

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divided attention

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individual must process and recall 2 or more competing messages (ex. note taking)
requires listener process and retain at least twice amount of information as in selective attention task

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selective attention and divided attention

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any process that affects selective attention will have a greater effect on divided attention

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sustained attention

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requires an individual process information over a long period of time
mental expenditure over time may eventually exceed individuals capacity and result in diminished performance
many test for capd are short/have breaks so not catch this defecit

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tolerance fading memory

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difficulty both in listening in noise and in recalling earlier presented info

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integration defecits

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deficit in ability to integrate acoustic or linguistic information across different processing regions or modalities (integrating acoustic and visual information)
associated with corpus callosum deficiency

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sequencing defecits

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observed on responses to pattern recognition testing
inability to follow directions
disorganized
poor spelling

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focus of auditory processing disorder assessment

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describe functional auditory performance deficits
assessment must be individualized
team approach

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the test battery approach

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for CAP
incorporate tests that allow identification of a person with CAPD in a timely cost effective manner
derive a composite profile for that person
likely to involve both behavioral and electrophyiological measures
CAPDs are a heterogeneous group
no single test will look at all various functions of the CANS

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3 factors to consider during test selection

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age appropriate
stimuli used- effected by HL?
purpose of test- screening or diagnosis?

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management of APD

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behavioral intervention
augmentative listening devices
classroom amplification
specific listening strategies
strategy teaching
counseling
interdisciplinary approach