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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 <Trapezoid Body> Superior Olivary Complex Lateral Lemniscus Inferior Colliculus Medial Geniculate Body Primary Auditory Cortex 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 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 IV Amplitude (V) III I V II Amp V Wave V Latency Time (ms) 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: Efferent: • VIIIth nerve • VIIth nerve nucleus • Cochlear Nucleus • VIIth nerve • Superior Olivary Complex • Stapedius muscle