Cortical Processing of Pitch: Excitation or Inhibition?

Abstract

Problem

Although tonotopicity provides frequency representation, it does not provide an explicit representation of pitch for complex sounds. For conventional stimuli, it is not possible to vary pitch without varying the spectrum of the sound. It is possible using a regular interval noise(RIN). RIN isolates the neural response associated with the perception of pitch. Several recent studies have identified a specific region in the auditory cortex that contains a cortical representation of pitch.

Methods

RIN auditory evoked potentials to: 1) determine over what range of temporal periods of RIN evoked a pitch onset response (POR); 2) determine how many cycles of RIN are required for its detection; 3) investigate inter-hemispheric predominance of the POR; 4) investigate if different periodicities are represented in different cortical regions; 5) determine if the degree of refractoriness of the POR is similar when the same pitches occur at regular, short intervals, as when infrequent occurrences of a particular pitch are interspersed with other pitches at the same short intervals.

Results

1) a pitch onset response is evoked over the whole range of periodicities giving rise to a sense of musical pitch; 2) at most periodicities less than 2 cycles of RIN are required in order to evoke a response; 3) no marked interhemispheric differences were revealed; 4) evidence for a “periodotopic” distribution of responses was inconclusive; 5) the refractory properties of the response mainly suggested a common neuronal population, but with small degree of pitch specificity.

Conclusion

There is a striking apparent disparity between human studies, which suggest a large increase in both metabolic and electrophysiological activity when random noise becomes periodic, at whatever pitch, and the animal literature which suggests only a few pitch-selective neurons are excited. We propose a model that explains the observed differences: that the large neuronal population response represents widespread inhibition.

Significance

The investigation of patients with auditory processing disorders.