The Auditory Nervous System

Transcription

1 Processing in The Superior Olivary Complex The Auditory Nervous System Cortex Cortex Alan R. Palmer MGB Excitatory GABAergic IC Glycinergic Interaural Level Differences Medial Geniculate Body Inferior Colliculus Medical Research Council Institute of Hearing Research University Park Nottingham NG7 2RD, UK DNLL DCN Nuclei of the Lateral Lemniscus Lateral Interaural Lemniscus Time Differences PVCN Lateral Superior Olive Cochlea AVCN MNTB Superior Olive Medial Superior Olive Medial of the Trapezoid Body Binaural cues for Localising Sounds in Space time Interaural Time Differences (ITDs) Interaural Level Differences (ILDs) Binaural Mechanisms of Sound Localization Interaural time (or phase) difference at low frequency are initially analysed in the by coincidence detectors connected by a delay line system. Interaural level differences at high frequency are initially analysed in the LSO by input that is inhibitory from one ear and excitatory from the other. Binaural Hearing The ability to extract specific forms of auditory information using two ears, that would not be possible using one ear only. 1

2 Advantages of Two Ears PARALLEL PROCESSING OF INFORMATION IN THE COCHLEAR NUCLEUS To medial superior olive: information about sound To inferior colliculus: information about pinna localisation using timing (and possibly time coding of speech) sound transformations Improved detection / increased loudness Removing interference from echoes Improved detection of sounds in interfering backgrounds Spatial localization Detection of auditory motion To lateral superior olive: information about sound localisation using interaural intensity To medial nucleus of the trapezoid body: information about sound localisation using interaural intensity Input from cochlear nerve Either commisural or to inferior colliculus information about sound level and voice pitch To inferior colliculus: information about complex sounds (possibly place coding of speech) Interaural Level Difference Pathway 2 db Excitatory Inhibitory 7 μs + + _ + + Nordlund Interaural level differences (high frequency) 2

3 Caspary and Finlayson (1991) Irvine (1986) Interaural time differences (low frequency) Sound lev vel (db SPL) 1 Ipsilateral 1 Contralateral The discharges of cochlear nerve fibres to low- frequency sounds are not random; they occur at particular times (phase locking) Frequency (khz) Caird and Klinke 1983 Evans (1975) 3

4 PARALLEL PROCESSING OF INFORMATION IN THE COCHLEAR NUCLEUS To medial superior olive: information about sound To inferior colliculus: information about pinna localisation using timing (and possibly time coding of speech) sound transformations Response Interaural Time Difference To lateral superior olive: information about sound localisation using interaural intensity To medial nucleus of the trapezoid body: information about sound localisation using interaural intensity Either commisural or to inferior colliculus information about sound level and voice pitch To inferior colliculus: information about complex sounds (possibly place coding of speech) Input from cochlear nerve Interaural Time Difference Pathway The coincidence detection model of Jeffress (1948) is the widely accepted model for low-frequency sound localisation ALT TAB Department of Neurophysiology,University of Wisconsin Response Ipsilateral Interaural Time Difference Contralateral Barn Owl: Konishi et al

5 Semicircular Canals Window Semicircular Can als Window Se mi cir cul ar Canals Window Se mi cir cul ar Canals Window μs Time Delay μs Left Ear Right Ear Large calyx synaptic ending Auditory Nerve Activity μs Time Delay Pena et al 21 Matches between the inputs from the two ears in the Barn Owl Laminaris 3 μs Arrives at left ear 3 μs later than at the right Left Ear Right Ear Large calyx synaptic ending Auditory Nerve Activity 3 μs Time Delay Coincident spikes Fischer and Pena 29 Pathways for analysing interaural time differences μs Time Delay Arrives at left ear 3 μs later than at the right To inferior colliculus Excitatory 3 μs μs Left Ear Right Ear Left Ear Right Ear Large calyx synaptic ending Auditory Nerve Activity Large calyx synaptic ending 3 μs Time Delay μs Time Delay Coincident spikes 5

6 ITD (μs) Bekius et al 1999 Interaural Phase Sensitivity in the Best Delay Noise BF tones 1 ms 1 ms Guinea Pig Palmer et al., 199 Cat Yin et al., 1986 Yin and Chan (1988) Palmer et al 199 Smith et al 1993 Distribution of peaks of ITD functions in response to interaurally-delayed noise Physiological range 8 rones Number of Neur Interaural Delays (μs) McAlpine Jiang and Palmer 21 6

7 McAlpine, Jiang and Palmer 1996 Grothe 23 1/8 1/4 1/2 cycle 1/16 McAlpine Jiang and Palmer 21 Brand et al., Hz 5 Hz ed Response Normalis ITD (μs) 7 Hz 1. khz 1.4 khz Brand et al ITD (μs) McAlpine Jiang and Palmer 21 ITD (μs) 7

8 Distribution of steepest slopes of ITD functions in response to interaurally-delayed noise Physiological range Number of Ne urones Descending pathways Interaural Delays (μs) McAlpine Jiang and Palmer 21 ormalised Response N Interaural Time Difference (μs) aural Time Difference (μs) Intera Frequency (khz) Interaural Phase Difference (cycles) McAlpine Jiang and Palmer 21 Spangler and Warr 1991 ITD processing is BF-dependent. ITD functions are steepest around midline. The consequence of this is that: As ITD increases across the physiological range the activity at any frequency increases Warr 1978, Warr and Guinan

9 Spoendlin 1971 Wiederhold and Kiang 1971 Function of the descending or centrifugal innervation Protection from acoustic trauma Control of the mechanical state of the cochlea Control of the mechanical state of the cochlea Involvement in selective attention Detection of complex signal in noise 9