Introduction

The activity of a multisensory neuron, like that of any other neuron, is the result of the physical properties of its membrane and the collective effect of its inputs. Although little is known about the membrane properties of multisensory neurons per se and how they might differ from the properties of unimodal neurons, multisensory neurons are defined by their multiple sensory inputs. Beyond this essential feature, however, little detailed information is known about the organization of inputs to identified multisensory neurons. From what has been reported to date, it has been generalized that multisensory neurons are generated by the convergence of basically excitatory inputs from different sensory modalities (see, e.g., Stein & Meredith, 1993). Inhibition, when it occurs, has been observed as an accompaniment to complex receptive field properties such as spatial inhibition and postexcitatory inhibition. Thus, the well-known designations of bimodal and trimodal aptly apply to neurons responsive to two or three different sensory modalities, respectively. How these properties arise and are distributed within a given area (e.g., areal convergence) has been the subject of a number of investigations not only of the well-known multisensory layers of the superior colliculus but also of “polymodal” or multisensory regions of cortex (see, e.g., Jones & Powell, 1970; Seltzer & Pandya, 1994). Although the documentation of multiple sensory inputs to lower-level, “unimodal” cortical areas has received a great deal of recent attention (Falchier, Clavagnier, Barone, & Kennedy, 2002; Schroeder & Foxe, 2002; Schroeder et al., 2001), the patterns of convergence that generate multisensory neuronal properties in any region largely remain unexplored. This chapter examines the areal and circuit-level relationships of cortical association areas representing different sensory modalities with the intention of revealing organizational patterns that expand our current appreciation of the structure of multisensory convergence at the neuronal level.