Summary

Development is a progressive process that results in the growth and proliferation of motor, perceptual and cognitive skills. A growing body of evidence shows, however, that seemingly paradoxical regressive processes also contribute to perceptual development and to the emergence of specialization. This evidence shows that unisensory perceptual sensitivity in early infancy is so broadly tuned that young infants respond to, and discriminate, native sensory inputs (e.g. speech sounds in their own language and faces from their own species and race) as well as non-native sensory inputs (e.g. speech sounds from other languages and faces from other species and other races). In contrast, older infants only respond to native inputs. For example, younger but not older infants discriminate monkey, human, and other-race faces, native and foreign speech contrasts, and musical rhythms from different cultures. Here, we review new findings indicating that perceptual narrowing is not just a unisensory developmental process, but a general, pan-sensory one. These new data reveal that young infants can perceive non-native (monkey) faces and vocalizations as well as non-native speech gestures and vocalizations as coherent multisensory events, and that this broad multisensory perceptual tuning is present at birth. These data also reveal that this broad tuning narrows by the end of the first year of life, leaving older infants only with the ability to perceive the multisensory coherence of native sensory inputs.

Introduction

In a recent review article, Corballis (1998) states that perhaps ‘the major question confronting research on cerebral asymmetry is whether it will survive into the new millennium’. This volume certainly shows that the field has a strong basis, and that the contributions emerging from studies of animals are providing an increasingly precise picture of how cerebral asymmetries have evolved. Although the field has come a long way since the days when humans were considered uniquely lateralized, there are still many gaps in our knowledge. In this chapter, we attempt to fill in a portion of this gap, focusing explicitly on the non-human primates (hereafter referred to as ‘primates’) and the specialized mechanisms underlying the production and perception of their facial and vocal expressions.

We begin our review by discussing the logic underlying the search for neural specializations, and then briefly discuss a selective set of problems associated with the comparative method. We then discuss current evidence for specialized processing mechanisms, focusing on the perception of faces and facial expressions, the perception of vocalizations, and the production of facial and vocal expressions. We conclude the chapter with a few comments on how future studies of hemispheric specialization must integrate behavioural studies of wild and captive animals with laboratory studies of neurophysiology.

Why Should We Expect Neural Specializations?

Like other species, including humans, it seems reasonable to expect primates to have a suite of specialized brain structures dedicated to processing ethologically relevant behaviours.