Introduction

Most objects and events present a mix of visual, auditory, tactile, and olfactory stimulation simultaneously. How do young infants come to perceive and derive meaning from this array of multimodal stimulation? How do young infants determine which sights and sounds constitute unitary objects and events and which patterns of stimulation are unrelated to one another?

Historically, two prevailing theoretical views, known as the integration view and the differentiation view, have dominated attempts to address these important questions regarding the development of intersensory perception (see Bahrick & Pickens, 1994; Gibson & Pick, 2000; Lewkowicz, 1994). Generally speaking, the integration view proposes that the different sensory modalities function as separate sensory systems during the initial stages of postnatal development and gradually become integrated during the course of development through the infant's activity and repeated experience with concurrent information provided to the different sensory modalities (Birch & Lefford, 1963; Friedes, 1974; Piaget, 1952). According to this view, young organisms must learn to coordinate and integrate the separate senses during early development. The differentiation view holds that the different senses form a primitive unity early in development, and as the infant develops, the sensory modalities differentiate from one another. In this view, the senses are initially unified, and infants differentiate finer and more complex multimodal relationships through their experience over the course of development (Bower, 1974; Gibson, 1969; Marks, 1978).

As a result of these opposing views, the most prominent question guiding behavioral research on early intersensory development over the past several decades has focused on whether intersensory development proceeds from initially separate senses that become increasingly integrated through the infant's ongoing experience, eventually resulting in coordinated multimodal perception, or whether the development of intersensory perception is a process of differentiation and increasing specificity (Kellman & Arterberry, 1998; Lewkowicz & Lickliter, 1994a; Rose & Ruff, 1987).

In recent years the discussion has become less polarized, due in large part to the adoption of a more systems-based approach to the development of perception, according to which any given perceptual skill or ability is generated by a network of multiple, cocontributing neural, physiological, and behavioral factors (Gottlieb, Wahlsten, & Lickliter, 1998; Lewkowicz, 2000; Lickliter & Bahrick, 2000; Thelen & Smith, 1994). Although some controversy remains as to whether perceptual development proceeds from a wholistic unity to differentiated sensory modalities or from separated senses to coordinated multimodal experience (i.e., Bushnell, 1994; Maurer, 1993), the dominant view at present argues against an all-or-none dichotomy between integration and differentiation views of perceptual development. The increasing research focus on the processes and mechanisms underlying human and animal infant intersensory perception over the past several decades has provided mounting evidence that the separate senses are not so separate, highlighting the importance of differentiation in early development. Moreover, both differentiation and integration processes appear to be involved in perceptual development and function in an intercoordinated manner. In this chapter, we briefly review converging evidence across species, developmental periods, and properties of objects and events suggesting a general developmental trajectory in which differentiation of amodal and modality-specific stimulus properties emerges in a coordinated and interdependent manner, with the detection of more global, amodal stimulus properties leading and constraining perceptual responsiveness to more specific properties of objects and events.