Southern Federal University A.B.Kogan Research Institute for Neurocybernetics Detailed Analysis and Modeling of Neurons and Neural Networks Lab. The Necessary and Sufficient Conditions for Population Coding of Interaural Time Differences by Trough-Type Neurons Viacheslav Vasilkov, Ruben Tikidji-Hamburyan 2010

Moscow Rostov-on-Don Sochi

Southern Federal University A.B.Kogan Research Institute for Neurocybernetics

Laboratory for Detailed Analysis and Modeling of Neurons and Neural Networks

Psychophysical tests

How can we explain these results? Coincidence detectors: the Jeffress type models the equalization-cancellation type model ? (population coding)

What do we have? from: Michael Pecka “Functional roles of synaptic inhibition in auditory temporal processing” EI IE from: W. Liu, N. Suga, J Comp Physiol A (1997) 181:

Can EI/IE neurons detect ITD? Kidd and Kelly, J. Neurosci., 1996, 16(22):7390 –7397

Simple analytical explanation τ=1ms, c=1solid line:τ i =7.5ms, ω i =0.2,τ e =2.5ms, ω e =0.17; dash line:τ i =7.5ms, ω i =0.21,τ e =2.5ms, ω e =0.15; dash-dot line:τ i =7.5ms, ω i =0.22,τ e =2.5ms, ω e =0.13,. ITD = 0 msITD = 0,3 msITD = 0,6 ms

u max (a.u.) ITD (ms) Population coding: ITD is coded by population spikes rate Trough wideness and EI-inputs balance

Which conditions are sufficient to code? The problems: ● slowness (sluggishness) of neurons ● noisiness of neurons ● variance in length of axons from previous levels ● jitter in input signals

The basic model Trough-type activity Two input elements (VCN cells) Excitatory projections Inhibitory projections Left and Right populations of EI neurons

Two-gradient model

Normal distribution of synaptic conductances

Intrinsic noise currents NG

Arbitrary static delay lines

The Jitter

Disturbance allows detecting microsecond ITD

StaticDynamic Trough wideness modulation Trough position modulation

StaticDynamic Trough wideness modulation Trough position modulation

Quality assessment

Quality assessment for mixed disturbance

SUMMARY relatively large populations of trough-type neurons with slow integration of excitatory inhibitory inputs can discriminate time disparity of spike arrivals discrimination acuity is sufficient for microsecond ITD processing for sufficient detection homogeneity of each population must be disturbed the different disturbance factors can result in similar population code different disturbance factors may be mixed in a real nerve system there is an optimal magnitude of such disturbances

Can this model help us to explain the results of psychophysical tests?

Why two populations? Degenerative code beyond the biologically relevant range

Echo Two different clicks“Frozen” localization Clear localization Model-based explanations of psychophysical results

The future study: ➢ Cellular properties: ➢ using proper ion channels; ➢ synaptic input specification; ➢ peaker/trougher/tweener neuron types; ➢ dendritic morphology. ➢ Auditory structure features: ➢ afferent projections to neurons/nuclei; ➢ elaboration of local neural circuits, including: globular and spherical bush cells at VCN, inhibitory neurons and specific synaptic transmissions at MNTB, etc.; ➢ tonotopic organization ➢ integration with well known models (Jeffress, EC) ➢ Further testing and formalization: ➢ periodical signals ➢ enveloped random signals ➢ mixed input types: EIE, EIEI,... ➢ jitter sources ➢ Fokker–Planck formalization with simplified neuron models

Thanks a lot for your attention

It is easy to appreciate the importance to an animal of identifying the location of a sound source. For example, imagine hearing a snapping twig in the middle of the woods on a dark night. The ability to ascertain whether the sound originated from the left or right side may determine whether the animal survives predation or starvation. For Homo sapiens living in modern society with few predators outside of our own species, sound localization is perhaps not as critical, except for occasions like a American stepping off the curb on a busy London thoroughfare and hearing the sudden blaring horn of a rapidly oncoming cab from an unexpected direction. Tom C.T. YIN, 2002 It is easy to appreciate the importance to an animal of identifying the location of a sound source. For example, imagine hearing a snapping twig in the middle of the woods on a dark night. The ability to ascertain whether the sound originated from the left or right side may determine whether the animal survives predation or starvation. For Homo sapiens living in modern society with few predators outside of our own species, sound localization is perhaps not as critical, except for occasions like a American stepping off the curb on a busy London thoroughfare and hearing the sudden blaring horn of a rapidly oncoming cab from an unexpected direction. Tom C.T. YIN, 2002