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The Auditory Nervous System Processing in The Superior Olivary Complex Cortex Cortex MGB Alan R. Palmer Medical Research Council Institute of Hearing Research University Park Nottingham NG7 2RD, UK Excitatory IC GABAergic Glycinergic Interaural Level Differences DNLL PVCN Cochlea AVCN Inferior Colliculus Nuclei of the Lateral Lemniscus Lateral Lemniscus Interaural Time Differences Cochlear Nucleus DCN MSO MNTB Advantages of Two Ears Medial Geniculate Body • Improved detection / increased loudness • Removing interference from echoes • Improved detection of sounds in interfering backgrounds • Spatial localization • Detection of auditory motion Lateral Superior Olive Medial Superior Olive Medial Nucleus of the Trapezoid Body Superior Olive Binaural cues for Localising Sounds in Space 20 dB time 700 μs Interaural Time Differences (ITDs) Interaural Level Differences (ILDs) Nordlund Binaural Mechanisms of Sound Localization • Interaural time (or phase) difference at low frequency are initially Binaural Hearing analysed in the MSO 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. The ability to extract specific forms of auditory information using two ears, ears that would not be possible using one ear only. Interaural level differences ((high g frequency) q y) 1 PARALLEL PROCESSING OF INFORMATION IN THE COCHLEAR NUCLEUS To inferior colliculus: information about pinna sound transformations To medial superior olive: information about sound localisation using timing (and possibly time coding of speech) To lateral superior olive: information about sound localisation using interaural intensity Either commisural or to inferior colliculus information about sound level and voice pitch To medial nucleus of the trapezoid body: information about sound localisation using interaural intensity To inferior colliculus: information about complex sounds (possibly place coding of speech) Input from cochlear nerve Interaural Level Difference Pathway Caspary and Finlayson (1991) Excitatory Inhibitory + _ + Interaural time differences ((low frequency) q y) + Ipsilateral Contralateral 100 The discharges of cochlear nerve fibres to lowlowfrequency sounds are not random; they occur at particular times (phase (phase locking). locking). 100 Sound levvel (dB SPL) + Irvine (1986) 20 0.125 20 32 0.125 32 Frequency (kHz) Caird and Klinke 1983 Evans (1975) 2 Response PARALLEL PROCESSING OF INFORMATION IN THE COCHLEAR NUCLEUS To medial superior olive: information about sound localisation using timing (and possibly time coding of speech) To inferior colliculus: information about pinna sound transformations 0 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 Pena et al 2001 The coincidence detection model of Jeffress (1948) is the widely accepted model for lowlow-frequency sound localisation Matches between the inputs from the two ears in the Barn Owl Nucleus Laminaris ALT TAB Department of Neurophysiology,University of Wisconsin Fischer and Pena 2009 Response Pathways for analysing interaural time differences Ipsilateral Excitatory To inferior colliculus 0 Interaural Time Difference Left Ear + Semicircular Canals Cochlear Nucleus Cochlear Nucleus + + Right Ear + Window MSO Contralateral Large calyx synaptic ending Barn Owl: Konishi et al 1988 3 0 μs Time Delay 0 μs -600 Cochlear Nucleus Left Ear Cochlear Nucleus -300 0 ITD (μs) 300 600 Right Ear Se micir cul ar Can als W in d ow MSO Auditory Nerve Activity Large calyx synaptic ending 0 μs Time Delay Bekius et al 1999 Interaural Phase Sensitivity in the MSO Arrives at left ear 300 μs later than at the right Best Delay Noise BF tones 300 μs Cochlear Nucleus Left Ear Cochlear Nucleus Right Ear Se micircular Can als 1 ms 1 ms Guinea Pig Palmer et al., 1990 Win dow MSO Cat Yin et al., 1986 Auditory Nerve Activity Large calyx synaptic ending 300 μs Time Delay Coincident spikes Yin and Chan (1988) Arrives at left ear 300 μs later than at the right 0 μs Time Delay Palmer et al 1990 Smith et al 1993 Distribution of peaks of ITD functions in response to interaurally-delayed noise 300 μs 0 μs Physiological range Left Ear Cochlear Nucleus 80 Right Ear Se micircula r Can als Win dow MSO Auditory Nerve Activity Number of Neurrones Cochlear Nucleus 60 40 20 Large calyx synaptic ending 300 μs Time Delay 0 μs Time Delay 0 -500 0 500 1000 Interaural Delays (μs) Coincident spikes McAlpine Jiang and Palmer 2001 4 Distribution of steepest slopes of ITD functions in response to interaurally-delayed noise Physiological range Number of Neurones 80 60 40 20 0 -500 0 500 1000 Interaural Delays (μs) Grothe 2003 McAlpine, Jiang and Palmer 1996 1/8 1/4 McAlpine Jiang and Palmer 2001 1/2 cycle 0.5 Interaaural Time Difference (μs) Noormalised Response 1/16 0.8 0.6 0.4 0.2 0.0 -1000 -500 0 500 1000 Interaural Time Difference (μs) McAlpine Jiang and Palmer 2001 Interaural Phase Difference (cycles) 600 1.0 500 0.4 400 0.3 300 0.2 200 0.1 100 0 0.0 0.00 0.25 0.50 0.75 1.00 1.25 1.50 Frequency (kHz) McAlpine Jiang and Palmer 2001 Brand et al., 2002 325 Hz ITD processing is BFBF-dependent. 500 Hz Normalised Response ITD functions are steepest around midline. 700 Hz -1000 -500 0 500 The consequence of this is that: 1000 ITD (μs) 1.0 kHz As ITD increases across the physiological range the activity at any frequency increases 1.4 kHz Brand et al 2002 -1000 -500 0 ITD (μs) 500 1000 -1000 -500 0 500 1000 McAlpine Jiang and Palmer 2001 ITD (μs) 5 Descending pathways Spoendlin 1971 Spangler and Warr 1991 Wiederhold and Kiang 1971 Function of the descending or centrifugal innervation • Protection from acoustic trauma • Control of the mechanical state of the cochlea • Involvement in selective attention • Detection of complex signal in noise Warr 1978, Warr and Guinan 1979 6