Just how important is spatial coincidence to multisensory integration? Evaluating the spatial rule.

This review addresses the question of when spatial coincidence facilitates multisensory integration in humans. According to the spatial rule (which was first formulated on the basis of neurophysiological data in anesthetized animals), multisensory integration is enhanced when stimuli in different sensory modalities are presented from the same spatial location. While the spatial rule fits with the available data from studies of overt and covert spatial attentional orienting, and from the majority of those studies in which space has been somehow relevant to the participant's task, it is inconsistent with the evidence that has emerged from the majority of multisensory studies of stimulus identification and temporal perception. Such a mixed pattern of behavioral results suggests that the spatial rule does not represent a general constraint on multisensory integration in humans. Instead, it would appear to be a much more task-dependent phenomenon than is often realized. These results, however, are broadly consistent with a distinction between the processing of "where" and "what" (or "how") information processing in the human brain.