Central Auditory Nervous System 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 <Trapezoid Body> Brainstem Mid-brain Thalamus Temporal Lobe

Mid-Saggital View of Brain 4th Ventricle Corpus Callosum Cerebellum Thalamus Pons

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 The auditory cortex, as most other neocortex, has been divided into six layers. Layer IV receives input from the thalamus. Layer I also receives input from MMGB. Cells in layer IV project to pyramidal cells in layer III. From layer III, auditory information diverges to several different locations including other layers of AI, contralateral AI (via corpus callosum), other primary auditory cortex layers I, II, IV and V. Layers V and VI project out of the auditory cortex.

Cortical Neurons Tonotopically and Spatiotopically organized Highly Adaptable Sensitive to CHANGES in Frequency and Intensity Coding virtual pitch demodulating complex signals (e.g. speech) Tuning: Wide variability, some broad, some sharp w/ multi freq Maps: Tonotopic: Iso-frequency Columns. Binaural/Monaural columns at right angles to tonotopic columns. (Also referred to as binaural summation and suppression columns). Callosal connex: in irregular bands. Response Patterns: Temporal: On//On-off//Off//Through Level specificity: very non linear rate-intensity functions. Responses to FM: either to direction or rate. Responses to AM: tuning to specific rate.

Cortical Processing Pattern Recognition Duration Discrimination Localization of Sounds Selective Attention Difficult to assign specific functional roles. a) lesion studies provide some info * lesions affect localization ability in opposite hemifield. * lesions affect performance in contralateral ear in attention studies i) but man may or may not equal animal performance ii) and lesions in man are typically diffuse, not isolated. b) single cell responses show correlation to functions i) responses to interaural time differences which show up at SOC earliest, are preserved in auditory cortex. ii) Primary Auditory cortex fiber responses show preferential response to certain transient intervals, but do not show response to fundamental. :: Q.E.D., pitch info not necessarily disrupted by lesions of primary auditory cortex

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 IV V III II I Amplitude (V) Amp V Wave V Latency Time (ms) 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: Efferent: VIIIth nerve Cochlear Nucleus Superior Olivary Complex Efferent: VIIth nerve nucleus VIIth nerve Stapedius muscle