The ontogeny and phylogeny of bimodal primate vocal communication

The two primary channels of social communication in primate species are the face and the voice and there is no doubt that each alone can provide perceptually, cognitively and socially meaningful information. When combined, however, the communicative value of the information conveyed through each of these two channels can be greatly enhanced as many stimulus attributes in each channel are invariant and redundant. To be specifi c, the sender of a communicative signal usually provides the recipient with spatially synchronous facial and vocal signals that also correspond in terms of their duration, tempo, and rhythmic patterning and that usually convey the same linguistic information, affective meaning, and intentions. It is probably precisely because of the fact that the the multisensory signals specifying a vocalizing face are so much more salient than unisensory signals that the primate nervous system has evolved a variety of multisensory integration mechanisms (Ghazanfar and Schroeder 2006; Stein and Meredith 1993). Such mechanisms enable organisms to take full advantage of this increased salience and, in the process, provide for more effective detection, learning, discrimination, and interpretation of perceptual input (Marks 1978; Rowe 1999).