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The case of the missing pitch templates: How harmonic templates emerge in the early auditory system Shihab Shamma and David Klein, 2000

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Background I

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Background II: Pitch Percept Theories Spectral Pitch Theories - Central Pattern Recognition theory (Goldstein 1973) 1.spectral profile along cochlea => pitch 2.Input spectrum internally stored spectral templates Temporal Theory 1.temporal response of auditory channel => pitch 2.final estimate = combination across the channels Limitations : lack of biological evidence learning of templates

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Goal and Outline: Goal: provide biologically plausible model for how and possibly where temples for identification of pitch come about. Note: Periodic Pitch perception is the focus Outline : I.Mathematical Model II.Why temples emerge III.Potential biological structures that underline the model IV.Conclusions

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Mathematical Model Analysis Stage Coincidence Detection Stage

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Mathematical Model: Analysis Stage Cochlear Filter Bank U(t,x) = s(t)*h(t,x) h( ) – cochlear filter impulse response Hair Cell Filtering & Rectification R(t,x) =g(U(t,x))=g(s(t)*h(t,x)) g( ) – hair cell nonlinearity S(t) Spectral Sharpening – lateral inhibition effects Y(t,x) = r(t,x) – r(t, x-1) Temporal Sharpening – enhancement of synchrony of phase- locking in various cells of CN Z(t,x)

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Mathematical Model: Analysis StagE S(t) Z(t,x) Spectral Sharpening – lateral inhibition effects Y(t,x) = r(t,x) – r(t, x-1) Temporal Sharpening – enhancement of synchrony of phase- locking in CN

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Mathematical Model: Coincidence Detection Stage Z(t,x) T ij =∑ N,t |C ij (t)|

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Model Simulations 1. Outputs of channels that are harmonically related are strongly correlated 2. Number of harmonics in template decreases with increasing fundamental frequency (as phase- locking diminishes beyond 1kHz) Broadband NoiseRandom Click Train

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Why Do Templates Emerge Nonlinear-transformation of the filter outputs => distortions necessary for template formation 1. Phase – locking of AN 2. Half-wave rectification For coincidence to occur, the phase needs to be the same - phase of cochlear traveling wave 1. Accumulates phase rapidly near resonance (i.e. CF) 2. Near CF widely different phases are in closely spaced location => high probability that at least one pair of channels will coincide and correlate. Cochlear frequency selectivity is critical for the formation of the harmonic templates: 1. Sharper filters => clearer high order harmonic peaks in the templates.

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Physiological Correlates of the Model CD cells in column have same CF + input from higher CFs CD cells with input from their CF + dendritic tree which spans higher CFs Input to coincidence detector: spectrally well resolved and phase locked Onset & Primary like units in low CF regions of CN Coincidence Detector: limniscus nuclei or the IC Octopus Cell in CN

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Conclusions Model for forming harmonic templates in the early stages of the auditory system with broadband noise stimulation, and without need for neural delay lines and other temporal structures is presented. Phase-locking of the filter responses is a critical factor in the template formation. Nonlinear transformation of the filter responses is essential in generating the ‘‘distortion’’ harmonics that ultimately form the templates. High spectral resolution improves the representation of the harmonic peaks in the templates. Phase delays of the trav eling wave provide locally phase shifted copies of the responses at each CF Formation of the harmonic templates is solely determined by the intrinsic properties of the cochlear filters and coincidences. That is, given enough time, the same templates will emerge for any broadband stimulus whether it is noise, harmonic sequences, or impulses.

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