Harmonicity Winner-Take-All
The cochlea is a sensitive membrane structure in the inner ear that performs a
particular type of frequency analysis.
While most applications treat the cochlea like an FFT, this is a poor description.
The point is that for a given input frequency, many points on the cochlea respond, but
With different amplitudes and different phases.
1) While it is true that the cochlea extracts frequency, the tuning is broad.
2) Unlike the FFT and spectrogram representations, frequency tuning is found
logarithmically represented along the length of the cochlea and
in the hair cell population.
3) At the frequencies for much of speech, the hair cells in the cochlea can
report the relative phases of the waveform for a particular frequency
Linear vs. Log Frequency Coding
Harmonic sounds
Spectrogram
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Cochleagram
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Because many phases of a tone are found in the cochlear representation, a
harmonic sound will excite a subset of cochlear hair cells to fire in synchrony
at a period equal to the fundamental frequency.
cos(x), cos(2x), cos(3x)
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Hair cells from the cochlea - different best frequencies
Inhibitory
neuron
Hair cells from the cochlea - different best frequencies
Inhibitory
neuron
By making the inhibitory neuron leaky, temporally-jittered inputs leak away before their
contributions add up.
When synchronized inputs come in, those neurons fire together, driving the inhibitory
neuron at the same time. This barrage of inputs make the inhibitory cell fire immediately.
The inhibitory cell then shuts everyone down. Only the synchronized pattern gets
through.
Two harmonic sounds summed
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Can we pull these apart?
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Chip Idea:
1. Given a cochlea circuit in PSPICE, design the hair cells and inhibitory network
neurons and demonstrate the synchrony concept.
2. Show that harmonic sounds are preferentially passed through and that the
network can briefly lock onto one harmonic sound over another.