Heterogeneity and heterochrony in the development of intersensory perception.

It is now well established that a variety of intersensory perceptual skills emerge in early human development. Empirical evidence from studies in the author's as well as other laboratories charting the developmental emergence of these abilities is reviewed. The evidence is considered in terms of the currently dominant theoretical view of intersensory development that assigns the detection of amodal invariants a primary and foundational role. It is argued that this view is inadequate because the detection of amodal invariants is only one of three distinct intersensory integration processes. It is noted that the other two processes, namely, intersensory association of modality-specific cues and non-specific effects of stimulation in one modality on responsiveness to stimulation in another modality, are equally important and that the operation of all three and, in particular, the relation between them, must be studied to attain a complete understanding of intersensory perceptual development. It is suggested that the theoretical approach to the development of intersensory perception should be broadened to include all three types of processes and that developmental studies must respect basic facts and principles of development. To this end, a developmental systems approach is proposed that holds that the development of intersensory integration consists of the heterochronous emergence of heterogeneous perceptual skills.