The case of the missing pitch templates: How harmonic templates emerge in the

early auditory system

Shihab Shamma and David Klein, 2000

Background I

Background II: Pitch Percept Theories

Spectral Pitch Theories - Central Pattern Recognition theory (Goldstein 1973)1. spectral profile along cochlea => pitch2. Input spectrum internally stored spectral templates

Temporal Theory1. temporal response of auditory channel => pitch2. final estimate = combination across the channels

Limitations :• lack of biological evidence• learning of templates

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 ModelII. Why temples emergeIII. Potential biological structures that underline the modelIV. Conclusions

Mathematical Model

Analysis Stage

Coincidence Detection Stage

Mathematical Model: Analysis Stage

Cochlear Filter BankU(t,x) = s(t)*h(t,x)h( ) – cochlear filterimpulse 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)

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

Mathematical Model: Coincidence Detection Stage

Z(t,x)

Z(t,x)

Tij=∑N,t|Cij(t)|

Model Simulations1. 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 Noise Random Click Train

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.

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 lockedOnset & Primary like units in low CF regions of CN

Coincidence Detector: limniscus nuclei or the IC

Octopus Cell in CN

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 traveling 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.