Discrimination of speech by nonhuman animals: Basic auditory sensitivities conducive to the perception of speech-sound categories

Chinchillas (Chinchilla laniger) were tested in a same–different task to determine the location of greatest sensitivity along a continuum of voice-onset-time (VOT). The procedure used was an up–down staircase technique which allowed the determination of the just-noticeable-difference in VOT (Δ VOT) for VOT’s on a continuum ranging from dα to tα. Results demonstrated that the animals were most sensitive to change (i.e., produced the smallest Δ VOT values) in the region of the phonetic boundary dividing voiceless-unaspirated and voiceless-aspirated sounds, in good agreement with the boundary value previously obtained in an identification task with this same species Kuhl and Miller, J. Acoust. Soc. Am. 63, 905 (1978). The results support the notion that the mammalian auditory system provided a selective pressure on the choice of acoustic cues to represent the phonetic oppositions employed by the world’s languages. The data are discussed in terms of the original definition of categorical perception and current psychoacoustic explanations of the peak in sensitivity for two-component stimuli varying in onset time.