cells in cochlear nucleus The inferior colliculus is a nexus in the ascending auditory system and has a complexity comparable to VI in the visual system The principal nuclei in the brainstem auditory pathway. Each nucleus uniquely transforms an incoming spike train into an output that is different from the input hair cell auditory nerve fiber cells in cochlear nucleus descending cortico-collicular projections to thalamus and then to cortex excitatory GABAergic Inferior Colliculus glycinergic dorsal intermediate ventral nuclei of the lateral lemniscus Cochlear nucleus superior olivary complex LSO MSO Cochlea auditory nerve MNTB

cells in cochlear nucleus The inferior colliculus is a nexus in the ascending auditory system has a complexity comparable to VI in the visual system The principal nuclei in the brainstem auditory pathway. Each nucleus uniquely transforms incoming an spike train into an output that is different from the input hair cell auditory nerve fiber cells in cochlear nucleus descending cortico-collicular projections to thalamus and then to cortex excitatory GABAergic Inferior Colliculus glycinergic dorsal intermediate ventral nuclei of the lateral lemniscus Cochlear nucleus MSO superior olivary complex LSO Cochlea auditory nerve MNTB

The Frequency Representation on the Cochlea is Preserved in Every Nucleus of the Central Auditory System, and thus the Auditory System is Tonotopically Organized Inferior colliculus Medial geniculate Superior olive Cochlear nucleus Auditory cortex Cochlea Auditory nerve auditory cortex medial geniculate Inferior colliculus cochlear nucleus superior olive

Each cell type in the cochlear nucleus uniquely transforms an incoming spike train into an output that is different from the input to nucleus 1 to nucleus 2 to nucleus 3 to nucleus 4 to nucleus 5 Projections form the parallel pathways in ascending auditory system cells in cochlear nucleus

cells in cochlear nucleus to nucleus 1 to nucleus 2 to nucleus 3 to nucleus 4 to nucleus 5 cells in cochlear nucleus 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 LSO MSO MSO Cochlea auditory nerve MNTB bushy cells

for localizing low frequencies for localizing high frequencies Processing of interaural time disparities for localizing low frequencies Processing of interaural intensity disparities for localizing high frequencies

interaural intensity differences (IIDs) 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) Base Right Left

left ear louder Base Right Left

Equally intense at both ears Base Right Left

Right ear louder Base Right Left

With low frequencies, sound waves just bend around the head and ears so there is no difference in sound intensity at the two ears Base Right Left

With low frequencies, however, the sound arrives at one ear earlier than it does at the other ear, which creates interaural time differences (ITDs) Base Right Left

Sound arrives at left ear first- left leads Base Right Left

Arrives at both ears at the same time Base Right Left

Sound arrives at right ear first- right leads Base Right Left

Low frequencies are localized with interaural time disparities (ITDs) High frequencies are localized with interaural intensity disparities (IIDs)

for localizing low frequencies for localizing high frequencies Processing of interaural time disparities for localizing low frequencies Processing of interaural intensity disparities for localizing high frequencies ITD IID Spikes 10-20 microsec Medial Superior Olive MSO Lateral Superior Olive LSO bushy cell bushy cell

Formation of EI Property in LSO DNLL Inferior Colliculus Cochlear nucleus Cochlea MNTB + Interaural intensity disparity IID Excit ear louder Inhib ear louder Normalized Spike Count 1.0 0.5 monaural spike count Formation of EI Property in LSO

Formation of EI Property in LSO DNLL Inferior Colliculus Cochlear nucleus Cochlea MNTB + LSO IID Excit ear louder Inhib ear louder Normalized Spike Count 1.0 0.5 monaural spike count X Formation of EI Property in LSO

Formation of EI Property in LSO DNLL Inferior Colliculus Cochlear nucleus Cochlea MNTB + LSO IID Excit ear louder Inhib ear louder Normalized Spike Count 1.0 0.5 monaural spike count X Formation of EI Property in LSO

Formation of EI Property in LSO DNLL Inferior Colliculus Cochlear nucleus Cochlea MNTB + LSO IID Excit ear louder Inhib ear louder Normalized Spike Count 1.0 0.5 monaural spike count X Formation of EI Property in LSO

Formation of EI Property in LSO DNLL Inferior Colliculus Cochlear nucleus Cochlea MNTB + LSO IID Excit ear louder Inhib ear louder Normalized Spike Count 1.0 0.5 monaural spike count X Formation of EI Property in LSO

Formation of EI Property in LSO IID Excit ear louder Inhib ear louder Normalized Spike Count 1.0 0.5 monaural spike count X IID Function Inferior Colliculus X X DNLL X X Cochlear nucleus + LSO Cochlea + MNTB

+ Normalized spike count Spikes Cochlear Nucleus LSO MNTB IID louder in excitatory ear inhibitory ear Spikes LSO MNTB + Cochlear Nucleus

all cells fire + Spikes Cochlear Nucleus LSO MNTB IID louder in excitatory ear inhibitory ear all cells fire Spikes

+ Cochlear Nucleus LSO MNTB IID louder in excitatory ear inhibitory ear

+ Spikes Cochlear Nucleus LSO MNTB IID louder in excitatory ear inhibitory ear Spikes

+ Spikes Cochlear Nucleus LSO MNTB IID louder in excitatory ear inhibitory ear Spikes

+ Cochlear Nucleus LSO MNTB IID louder in excitatory ear inhibitory ear

High frequencies- above about 3000 Hz Low frequencies High frequencies- above about 3000 Hz Discharges are phase locked but not to every cycle Discharges are not phase locked Discharges are phase locked to every cycle of the sinusoidal signal Frequencies below 1000 Hz Frequencies from 1000-3000 Hz

Post-stimulus time(PST) histogram tone burst Raster display of phase-locked discharges evoked by 5 presentations of a tone burst tone presentation time (ms) spike count time (ms) Post-stimulus time(PST) histogram

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 left ear right ear . . constant phase difference between two ears ITD right ear left ITD right ear left phase difference between two ears would continuously change

Medial nucleus of the trapezoid body 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. MSO LSO Medial Superior Olive Lateral Superior Olive MNTB Medial nucleus of the trapezoid body

Interaual time disparity (µsec) MSO neurons are sensitive to Interaural time disparities of 10-20 µs Spikes -40 +40 Interaual time disparity (µsec) MSO LSO Medial Superior Olive Lateral Superior Olive

Interaual time disparity (µsec) MSO neurons are sensitive to Interaural time disparities of 10-20 µs Spikes -40 +40 Interaual time disparity (µsec) MSO LSO Medial Superior Olive Lateral Superior Olive MNTB Medial nucleus of the trapezoid body

Interaual time disparity (µsec) MSO neurons are sensitive to Interaural time disparities of 10-20 µs Spikes -40 +40 Interaual time disparity (µsec) MSO LSO Medial Superior Olive Lateral Superior Olive MNTB Medial nucleus of the trapezoid body

Freq 1 Freq 2 Freq 3 Freq 4 Spikes Spikes ITD (µsec) Spikes ITD (µsec) Freq 1 Spikes ITD (µsec) Spikes Freq 2 ITD (µsec) Spikes Freq 3 Freq 4 ITD (µsec) Spikes

Jeffress Model MSO