Perception of degraded speech sounds differs in chinchilla and human listeners.

The behavioral responses of chinchillas to noise-vocoded versions of naturally spoken speech sounds were measured using stimulus generalization and operant conditioning. Behavioral performance for speech generalization by chinchillas is compared to recognition by a group of human listeners for the identical speech sounds. The ability of chinchillas to generalize the vocoded versions as tokens of the natural speech sounds is far less than recognition by human listeners. In many cases, responses of chinchillas to noise-vocoded speech sounds were more similar to responses to band limited noise than to the responses to natural speech sounds. Chinchillas were also tested with a middle C musical note as played on a piano. Comparison of the responses of chinchillas for the middle C condition to the responses obtained for the speech conditions suggest that chinchillas may be more influenced by fundamental frequency than by formant structure. The differences between vocoded speech perception in chinchillas and human listeners may reflect differences in their abilities to resolve the formants along the cochlea. It is argued that lengthening of the cochlea during human evolution may have provided one of the auditory mechanisms that influenced the evolution of speech-specific mechanisms.