Processing Pitch in a Nonhuman Mammal

Whether the mechanisms giving rise to pitch reflect spectral or temporal processing has long been debated. Generally, sounds having strong harmonic structures in their spectra have strong periodicities in their temporal structures. We found that when a wideband harmonic tone complex is passed through a noise vocoder, the resulting sound can have a harmonic structure with a large peak-to-valley ratio, but with little or no periodicity in the temporal structure. To test the role of harmonic structure in pitch perception for a nonhuman mammal, we measured behavioral responses to noise-vocoded tone complexes in chinchillas (Chinchilla laniger) using a stimulus generalization paradigm. Chinchillas discriminated either a harmonic tone complex or an iterated rippled noise from a 1-channel vocoded version of the tone complex. When tested with vocoded versions generated with 8, 16, 32, 64, and 128 channels, responses were similar to those of the 1-channel version. Behavioral responses could not be accounted for based on harmonic peak-to-valley ratio as the acoustic cue, but could be accounted for based on temporal properties of the autocorrelation functions such as periodicity strength or the height of the first peak. The results suggest that pitch perception does not arise through spectral processing in nonhuman mammals but rather through temporal processing. The conclusion that spectral processing contributes little to pitch in nonhuman mammals may reflect broader cochlear tuning than that described in humans.