My colleagues and I use the superior colliculus (SC) as a model to understand how individual neurons in the brain synthesize cross-modal cues, how this capacity develops and how this synthesis affects overt behavior. The SC, a midbrain structure, plays an integral role in orientating to visual, auditory and somatosensory cues, and we have found that the majority of its multisensory neurons are able to synthesize cross-modal cues and thereby produce responses greater (or less) than those elicited by the most effective single modality stimulus and, sometimes, greater than those predicted by the arithmetic sum of their modality-specific responses. Enhanced is elicited by spatially-coincident cross-modal cues and depression is elicited by spatially-disparate cross-modal cues. This synthetic ability of SC neurons is dependent on unisensory cortical inputs that converge on SC neurons from two areas: the anterior ectosylvian sulcus (AES) and the rostral aspect of the lateral suprasylvian sulcus (rLS). Deactivation of AES and rLS eliminates multisensory integration in SC neurons, but generally has little or no effect on a neuron's responses to the individual unisensory stimuli. These cortical influences develop only gradually during postnatal maturation, occurring during the period in which animals obtain experience with multisensory stimuli. Early experience has proved to be crucial to the development of normal multisensory integration in SC neurons and may be mediated via these cortical projections.
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