Spectral processing does not give rise to behaviorally relevant cues for “pitch” perception in mammals.

Abstract

Mechanisms giving rise to pitch reflect spectral or temporal processing is still equivocal, because sounds having strong harmonic structures also have strong temporal structures. When a harmonic tone complex is passed through a noise vocoder, the resulting sound can have a harmonic structure with large peak-to-valley ratios, but little or no temporal structure. To test the role of harmonic structure in mammals, we measured behavioral responses to vocoded tone complexes in chinchillas using a stimulus generalization paradigm. Animals discriminated a harmonic tone complex from a one-channel vocoded version of the complex. When tested with vocoded versions generated with 8–128 channels, animals generalized to the one-channel version and showed no gradient in their behavioral responses. This suggests that spectral structure was not the cue for the behavioral response. To further test this, chinchillas discriminated an iterated rippled noise from the one-channel vocoded tone complex. When tested using vocoded tone complexes having harmonic peak-to-valley ratios that were larger than or similar to the rippled noise, animals again generalized to the 1-channel version rather than the to rippled noise. The results suggest that mammalian “pitch” attributes arise through temporal, not spectral, processing mechanisms. [Work supported by NIDCD R01 DC005596.]