Neural lateralization of vocalizations by Japanese macaques: communicative significance is more important than acoustic structure.

The study was designed to determine whether the neural lateralization of vocal perception in Japanese macaques depends on the acoustic properties of the calls used or their communicative significance. Four monkeys--two Japanese macaques and two comparison macaques--were trained to discriminate among monaurally presented exemplars of two classes of vocalizations from the Japanese macaque's repertoire. Once the subjects mastered the discrimination, they performed at equivalent accuracy levels for 150 sessions. However, during this time the Japanese monkeys showed a right ear performance advantage, whereas the comparison monkeys showed no ear advantage. In order to assess whether the comparison and Japanese monkeys were attending to the same acoustic cue when performing the discrimination, a generalization test was conducted with 27 novel vocalizations. The individual monkeys' generalization gradients were highly similar and revealed that all subjects were in fact listening to the same feature of the calls. These findings, coupled with the fact that the calls were of biological significance to the Japanese monkeys alone, suggest that the laterality effect is related, in some fashion, to the communicative valence of the signals rather than their purely physical characteristics.