CHAPTER 15

Central Auditory Nervous System

Central Nervous System Structures

• Nucleus = a group of nerve cell bodies

• Fiber Tract = a group of axons

Major Components of the Central Auditory Nervous System (CANS)

• VIIIth cranial nerve

• Cochlear Nucleus

• Superior Olivary Complex

• Lateral Lemniscus

• Inferior Colliculus

• Medial Geniculate Body

• Primary Auditory Cortex

Brainstem

Thalamus

Mid-brain

Temporal Lobe

<Trapezoid Body>

Mid-Saggital View of Brain

Pons

Cerebellum

4th Ventricle

Thalamus

Corpus Callosum

• MedGen Body

• Inf Coll

• Lat Lemn

• SOC

• Coch Nuc

• VIIIth CN

Neural Web-Sites

http://rprcsgi.rprc.washington.edu/

neuronames/hierarchy.html

http://www.ets.uidaho.edu/med532/start.htm

Section Thru Brainstem Shows Cochlear Nucleus

The Superior Olivary Complex

Connections To the Superior Olivary Complex

Superior Olivary Processing Supports Localization

• Lateral SO-- Interaural Intensity Differences

• Medial SO-- Interaural Time Differences

(These are the two primary acoustic cues for localizing sounds)

Dorsal (back) Side of Brainstem

• Thalamus

(medial geniculate)• Inferior Colliculus• 4th Ventricle• Area of Pons

Inferior Colliculus

Thalamus in Purple

Auditory Radiations Connect

• Medial Geniculate Body (in purple)

to• Primary Auditory

Cortex (in blue)

Lateral-Superior view of brain

Primary Auditory Cortex (AI):superior surface of the temporal lobe

6 Cortical Layers

• Thalamic inputs >IV

• project to pyramidal cells in layer III

• Divergence from III– within AI– other cortical areas– contra AI

• V and VI >>thalamus &IC

Cortical Neurons

• Tonotopically and Spatiotopically organized

• Highly Adaptable

• Sensitive to CHANGES in Frequency and Intensity– Coding virtual pitch– demodulating complex signals (e.g. speech)

Cortical Processing

• Pattern Recognition

• Duration Discrimination

• Localization of Sounds

• Selective Attention

Cerebral Dominance/Laterality

• Language Processing in the left hemisphere.

(Remember the right ear has the strongest connections to the left hemisphere)

• Most people show a right-ear advantage in processing linguistic stimuli

Neurophysiological Measures

• Gross Evoked Potentials-- Voltage changes in response to auditory stimulation recorded from the scalp

• Single-Unit Measures-- Voltage (or other) changes recorded within a neuron

Auditory Evoked Potentials

• Recorded in different time intervals (“epochs”) following a sound

• Earlier epochs come from lower in the system

• Later epochs come from higher in the system

Examples of AEP Epochs

• Electrocochleography-- within 5 milliseconds

• Auditory Brainstem Response-- thru 10 ms

• Middle Latency Response-- thru 75 ms

• Auditory Late Response-- thru 200 ms

Auditory Brainstem Response

Time (ms)

Am

plit

ude

(V

)

Wave V Latency

I IIIII

IVV

Amp V

0 10

GENERATORS of ABR WAVES

I

II

III

IV

V

= Distal VIIIth nerve

= Medial VIIIth nerve

= Cochlear Nucleus

= Superior Olivary Complex

= Lateral Lemniscus & Inferior Colliculus

The Acoustic Reflex

Afferent:• VIIIth nerve

• Cochlear Nucleus

• Superior Olivary Complex

Efferent:• VIIth nerve nucleus

• VIIth nerve

• Stapedius muscle