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The Frequency Representation on the Cochlea is Preserved in Every Nucleus of the Central Auditory System, and thus the Auditory System is Tonotopically Organized auditory cortex Auditory cortex medial geniculate Inferior colliculus cochlear nucleus Auditory nerve Cochlear nucleus Medial geniculate Medial geniculate Inferior colliculus medial geniculate Inferior colliculus superior olive Superior olive Superior olive Inferior colliculus Cochlear nucleus cochlear nucleus Auditory nerve Each cell type in the cochlear nucleus uniquely transforms an incoming spike train into an output that is different from the input cells in cochlear nucleus Projections form the parallel pathways in ascending auditory system to nucleus 1 to nucleus 2 to nucleus 3 to nucleus 4 to nucleus 5 cells in cochlear nucleus to nucleus 1 to nucleus 2 to nucleus 3 to nucleus 4 to nucleus 5 Projections from each cell group in the cochlear nucleus are tontopically organized to nucleus 1 to nucleus 2 to nucleus 3 to nucleus 4 to nucleus 5 Lateral Superior Olive (LSO) and Medial Superior Olive (MSO) are both binaural nuclei that process the cues for sound localization excitatory GABAergic Inferior Colliculus glycinergic dorsal intermediate ventral Cochlear nucleus superior olivary complex LSO MSO Cochlea auditory nerve bushy cells MNTB Processing of interaural time disparities for localizing low frequencies Processing of interaural intensity disparities for localizing high frequencies With high frequencies, the ears and head block some of the sound, making the sound louder in one ear than the other, which creates interaural intensity differences (IIDs) Right Base Left Base left ear louder Right Base Left Base Equally intense at both ears Right Base Left Base Right ear louder Right Base Left Base With low frequencies, sound waves just bend around the head and ears so there is no difference in sound intensity at the two ears Right Base Left Base With low frequencies, however, the sound arrives at one ear earlier than it does at the other ear, which creates interaural time differences (ITDs) Right Base Left Base Sound arrives at left ear first- left leads Right Base Left Base Arrives at both ears at the same time Right Base Left Base Sound arrives at right ear first- right leads Right Base Left Base Medial Superior Olive MSO Processing of interaural time disparities for localizing low frequencies bushy cell Spikes 10-20 microsec ITD Lateral Superior Olive LSO Processing of interaural intensity disparities for localizing high frequencies Spikes bushy cell IID Formation of EI Property in LSO monaural spike count Normalized Spike Count 1.0 Inferior Colliculus 0.5 DNLL 0 Excit ear louder 0 Inhib ear louder Interaural intensity disparity IID Cochlear nucleus + LSO Cochlea MNTB Formation of EI Property in LSO monaural spike count Normalized Spike Count 1.0 X Inferior Colliculus 0.5 DNLL 0 Excit ear louder 0 Inhib ear louder IID Cochlear nucleus Cochlea + LSO + MNTB Formation of EI Property in LSO monaural spike count Normalized Spike Count 1.0 0.5 X Inferior Colliculus X DNLL 0 Excit ear louder 0 Inhib ear louder IID Cochlear nucleus Cochlea + LSO + MNTB Formation of EI Property in LSO monaural spike count Normalized Spike Count 1.0 0.5 X Inferior Colliculus X DNLL X 0 Excit ear louder 0 Inhib ear louder IID Cochlear nucleus Cochlea + LSO + MNTB Formation of EI Property in LSO monaural spike count Normalized Spike Count 1.0 0.5 X Inferior Colliculus X DNLL X 0 X Excit ear louder 0 Inhib ear louder IID Cochlear nucleus Cochlea + LSO + MNTB Formation of EI Property in LSO monaural spike count IID Function Normalized Spike Count 1.0 0.5 X Inferior Colliculus X DNLL X 0 X X Excit ear louder 0 Inhib ear louder IID Cochlear nucleus Cochlea + LSO + MNTB Spikes Spikes Spikes Normalized spike count Spikes louder in excitatory ear + 0 IID louder in inhibitory ear Cochlear Nucleus LSO + MNTB Spikes Spikes Spikes Spikes all cells fire louder in excitatory ear + 0 IID louder in inhibitory ear Cochlear Nucleus LSO + MNTB louder in excitatory ear + 0 IID louder in inhibitory ear Cochlear Nucleus LSO + MNTB Spikes Spikes Spikes Spikes louder in excitatory ear + 0 IID louder in inhibitory ear Cochlear Nucleus LSO + MNTB Spikes Spikes Spikes Spikes louder in excitatory ear + 0 IID louder in inhibitory ear Cochlear Nucleus LSO + MNTB louder in excitatory ear + 0 IID louder in inhibitory ear Cochlear Nucleus LSO + MNTB Low frequencies Frequencies below 1000 Hz Discharges are phase locked to every cycle of the sinusoidal signal Frequencies from 1000-3000 Hz Discharges are phase locked but not to every cycle High frequencies- above about 3000 Hz Discharges are not phase locked tone burst Raster display of phase-locked discharges evoked by 5 presentations of a tone burst time (ms) Post-stimulus time(PST) histogram time (ms) right ear phase-locked discharges left ear spikes at right ear ITD spikes at left ear Due to phase-locking, the timing of spikes in the auditory nerve fibers from the two ears accurately represents, and thereby preserves, the ITD in the auditory pathway Interaural time disparities have to be processed on a frequency-by-frequency basis right ear left ear . . constant phase difference between two ears right ear left ear ITD phase difference between two ears would continuously change right ear left ear ITD In 1948 Lloyd Jeffress, a professor in the Psychology Department at The University of Texas at Austin, proposed a model could explain how low frequency sounds are localized. The model includes: 1) structural features, i.e., delay lines resulting from differences in axonal lenghts; 2) The neuronal process of coincidence detection. Specifically, the requirement that action potentials arrive at a target neuron simultaneously to activate the binaural neuron. 3) Topographically organized selective features that allows sound location to be repesented as a place of maximal activity. 4) How all of those features are activated by interaural time disparities, the cues animals use to localize low frequency sounds. Medial Superior Olive MSO Spikes MSO neurons are sensitive to Interaural time disparities of 10-20 µs -40 0 +40 Interaual time disparity (µsec) Medial Superior Olive MSO Spikes MSO neurons are sensitive to Interaural time disparities of 10-20 µs -40 0 +40 Interaual time disparity (µsec) Medial Superior Olive MSO Spikes MSO neurons are sensitive to Interaural time disparities of 10-20 µs -40 0 +40 Interaual time disparity (µsec) Medial Superior Olive MSO Spikes Freq 1 0 Freq 2 Spikes ITD (µsec) 0 Freq 3 Spikes ITD (µsec) 0 ITD (µsec) Spikes Freq 4 0 ITD (µsec) Jeffress Model MSO