Perception of the periodicity strength of complex sounds by the chinchilla.

The perception of periodicity strength was studied in chinchillas using a stimulus generalization paradigm in an operant-conditioning, positive reinforcement behavioral task. Stimuli consisted of cosine-phase and random-phase harmonic complex tones, infinitely iterated rippled noises, and wideband noise. These stimuli vary in periodicity strength as measured by autocorrelation functions and are known to generate a continuum in the perception of pitch strength in human listeners. Chinchillas were trained to discriminate a cosine-phase harmonic tone complex from wideband noise and tested in the generalization paradigm using random-phase tone complexes and iterated rippled noises as probe stimuli. Chinchillas were tested in three different conditions in which the periods of the fundamental frequencies of the tone complexes were fixed at 2 ms, 4 ms, or 8 ms. Behavioral responses obtained from chinchillas were related to stimulus periodicity strength. For most animals, the behavioral responses to random-phase tone complexes were smaller than those to cosine-phase tone complexes. The behavioral responses were analyzed in terms of the Auditory Image Model of Patterson et al. [Patterson, R.D., Allerhand, M.H., Giguère, C., J. Acoust. Soc. Am. 98 (1995) 1890-1894], and the results suggest that the periodicity information in the stimulus envelope has a large influence in controlling the behavioral response of the chinchilla. Comparison of the generalization data obtained in the present study to magnitude estimation data obtained previously in human subjects suggests a greater influence of stimulus envelope for the perception of periodicity strength in chinchillas than for the perception of pitch strength in human listeners.