Introduction

We live in a multisensory world in which we are continually bombarded with information conveyed via the different sensory modalities. One important and frequently unappreciated role of the brain is to synthesize this melange of sensory information into an adaptive and coherent perceptual Gestalt. Simply stated, information from the different senses that is associated with a single object or event (e.g., the sight and sound of an automobile) must be interpreted as such, whereas information from the different senses that is not associated with an object or event must be interpreted in a very different manner. As indicated by the wealth of literature on cross-modal interactions in human perception (see Stein & Meredith, 1993; Welch & Warren, 1986), this sensory synthesis is a constantly occurring phenomenon that is continually shaping our view of the world. Not surprisingly, not only do these interactions modify our perceptions, they also play an important role in influencing our reactions to sensory stimuli. For example, simple reaction times are often made faster by multisensory stimuli (Andreassi & Greco, 1975; Bernstein, Clark, & Edelstein, 1969; Gielen, Schmidt, & Van den Heuvel, 1983; Hershenson, 1962; Hughes, Reuter-Lorenz, Vozawa, & Fendrich, 1994; Morrell, 1968), and a similar speeding is seen for simple motor acts such as rapid eye movements (i.e., saccades; see Frens, Van Opstal, & Van der Willigen, 1995; Goldring, Dorris, Corneil, Ballantyne, & Munoz, 1996; Harrington & Peck, 1998; Hughes et al., 1994; Nozawa, Reuter-Lorenz, & Hughes, 1994; Perrott, Saberi, Brown, & Strybel, 1990).

Much work has gone into understanding the neural underpinnings of these cross-modal processes. Numerous sites of multisensory convergence and integration have been identified in the brain, and the principles according to which individual multisensory neurons integrate their various inputs have been elucidated in several model populations (see Stein & Meredith, 1993). This work has made clear that the spatial, temporal, and physical characteristics of the stimuli that are combined critically determine how they are synthesized. Thus, multisensory stimuli that are in close physical proximity, that occur at or near the same moment in time, that are weakly effective on their own, and that are contextually similar result in enhanced neural activity. In contrast, stimuli that are spatially or temporally distant or incongruent result in depressed neural activity. Such changes in neuronal response have striking parallels in the behavioral realm, where they likely play an important deterministic role in these overt responses (Stein, Huneycutt, & Meredith, 1988; Stein, Meredith, Huneycutt, & McDade, 1989).

Despite the recent explosion of interest in studying multisensory interactions at the neuronal and behavioral levels, little research effort has been directed toward understanding the ontogeny of these cross-modal processes. This lack is quite surprising, given the long history of research attempting to elucidate the basic principles of sensory development in both humans and animals. The importance of gaining a better understanding of these questions is best framed by two radically different views of human multisensory development (for a more in-depth discussion of this debate, see Lewkowicz & Kraebel, Chap. 41, this volume). On the one side are those who view this process as a chronological progression from the development of modality-specific responsiveness to multisensory responsiveness. According to this view, the young infant has segregated channels for processing information arriving via the different senses, and only develops multisensory capacities with the associations between the senses that occur during postnatal experience. Alternatively, some investigators believe the process moves in the opposite direction, with the young infant being extraordinarily multisensory and only later learning to segregate information on a sense-by-sense basis.

Studies of human sensory development are extremely difficult to conduct and interpret, although their results are fascinating and quite important. A different approach to understanding the genesis of multisensory development is to conduct such studies in animal models, where it is possible to examine the appearance and functional maturation of elements within the nervous system as development progresses.