Outline: Frequency Tuning and Pitch Perception

Overview

Ear as a Fourier Analyzer

Theories of Pitch Perception

Place Theory = Fourier Analysis
Telephone or Volley Theory = neurons fire in synchrony with the input signal
There is some evidence for both theories

Place Theory

Cochlear mechanics: traveling waves on the basilar membrane (von Békésy)

Thus, each position on the basilar membrane responds to a bandpass version of the input

Oval window is the high frequency end

Helicotrema is the low frequency end

The bandwidth is broader at higher sound intensities

This mechanical filtering accounts for much of the tuning of the hair cells

Outer hair cell feedback results in tighter tuning

Auditory nerve tuning is tighter through lateral inhibition

Tonotopic organization preserved in all known areas (including at least 4 or 5 in cortex)

Psychophysical evidence: critical band masking

Volley Theory

Phase-locked neural firing

Phase-locking is seen as high as 5 KHz

Evidence for place versus volley theories

At moderate to loud levels, cell tuning becomes broad, so use pool of activity or use volley information

Volley fails completely above 5 KHz, but works at high intensities for low-frequency signals

So, it may involve a combination of the two
White's cochlear implants: perceived pitch increases with change of place or change of stimulus frequency

Complex tones and pitch perception

Periodicity pitch and the missing fundamental

Shepard's auditory illusion

Perceived pitch

Logarithmic scale (one octave increase = double fundamental frequency)