Studies on the Vocal-Auditory Functions of Primates

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The following four aspects of the vocal-auditory functions of the chimpanzee and the Japanese monkey were investigated : Basic auditory functions, the perception of human speech and conspecific vocal sounds, ear (hemisphere) dominance in the perception of speech and vocal sounds and early vocal development.The chimpanzee and the Japanese monkey showed a W-shaped auditory sensitivity function. Although they are more sensitive to high frequencies (higher than 8 kHz), they are less sensitive to low (lower than 250 Hz) and middle (2-6 kHz) frequencies than human. The origin(s) of the sensitivity loss at mid-frequencies were suggested to exist somewhere between the eardrum and hair cells in the cochlea. Difference thresholds for frequency and intensity were investigated. They were the smallest for human.The chimpanzee required long reaction times to discriminate between the synthetic Japanese vowels [i] and [u] and between [e] and [o]. The chimpanzee discriminates vowels based on the frequency of the first formant. These results were supported by the perception of conspecific vowel-like vocal sounds. The relation between the W-shaped sensitivity and the vowel perception was discussed. For the perception of consonants, the manner of articulation was the most important determinant. For the perception of stop consonants, the voicing was more important than the place feature. The chimpanzee and the Japanese monkey showed the ability of vocal tract normalization and the categorical perception of stop consonants.The chimpanzee and the Japanese monkey did not show the right ear (left hemisphere) dominance in the perception of human speech and conspecific vocal sounds. The early vocal development of a chimpanzee baby was compared with that of human babies. Although the chimpanzee baby had cooing, she did not show babbling. This result indicates the importance of babbling for the development of human speech.