Studies on speech perception by primates

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Using a reaction time task, auditory sensitivity, 'loudness' and perception of speech sounds by Japanese macaques, chimpanzees and humans were studied. Auditory sensitivity and 'loudness' were measured using pure tones (62.5 - 32k Hz, -10 - 90 dB SPL). Based on reaction time data, equal loudness contours were obtained. Humans showed U-shaped curves and were most sensitive to 500-4k Hz tones. Chimpanzees showed W-shaped curves, which were similar to those of Old- and New-world monkeys. Chimanzees were sensitive to 1k and 8k Hz tones. However, a decrease in auditory sensitivity was observed at 2k-4k Hz.Then the discrimination of vowels was investigated. Natural and synthesized Japanese and French vowels were used. Longer reaction times were expected when members of a vowel pair were similar to each other. Reaction time data were analyzed by MDSCAL and cluster analyses. The main difference between chimpanzees and humans was that the reaction times for discriminating /i/ from /u/ and /e/ f … More rom /o/ were longer for chimpanzees. Thus, it is suggested that chimpanzees require longer reaction times for discrimination of vowels based on differenses in the frequency of the second formant. The species difference between humans and chimpanzees may be partially explained by the W-shaped chimpanzees' auditory sensitivity functions.Perception of consonant(mainly stops)+vowel(/a/) syllables was studied. Synthesized stop consonants with different values of voice onset time and 2nd and 3rd formant transient frequencies were also used for studying categorical perception of voicing (/ga/-/ka/) and place of articulation (/ba/-/da/) features. Following results were obtained. Although discrimination between voiced and unvoiced stops was easy, discrimination of stops based on the place of articulation was difficult for chimpanzees. Chimpanzees showed categorical discrimination of voicing and place features of stop consonants. The position of each boundary was not so different from that of humans. Less