Species differences in neural lateralization of species-specific calls

Abstract

Petersen et al. [Science 202, 324–327 (1978)] reported that Japanese macaques exhibited a right ear advantage (REA} when listening to a semantically distinctive feature of their vocalizatioos. Several heterospecific monkeys tested in identical conditions with the Japanese monkeys' calls failed to show any reliable ear advantage. However, recent studies with humans have shown that the size and/or direction of ear advantages vary with the physical feature to which subjects attend. This raised the question of whether the species differences in lateralization obtained in our setting represented (1) a processing difference for the same signal feature or (2) a difference between species in the signal feature to which they attended. To choose between those alternatives we ascertained whether Japanese and heterospecific monkeys were in fact attending to the same acoustic feature when they showed their respective patterns of lateralization. Two Japanese and two heterospecific monkeys received extensive training on a discrimination task that required them to classify 15 vocalizations into two categories. During this time the Japanese monkeys exhibited REA's but the heterospecifics showed no ear advantage. The animals were then tested for generalization to 25 novel, natural vocalizations and six synthetic calls. These tests revealed that all four animals were attending to the same feature of the Japanese monkey calls. The animals were then returned to the original training stimuli and they exhibited their earlier patterns of lateralization. Thus this study shows that even when attending to the same physical feature, Japanese and heterospecific monkeys employ different neural processing strategies. This suggests that the communicative valence of the signals used in testing may account for the lateralization differences observed between species. [Work supported by NSF. Deafness Research Foundation and Sloan Foundation.]