Comparisons of cross-modality integration in midbrain and cortex.

Multisensory neurons are abundant in the superior colliculus and anterior ectosylvian cortex of the cat. Despite the fact that these areas receive inputs from different regions, and are likely to be involved in different functional roles, there multisensory neurons have many fundamental similarities. They all have multiple receptive fields, one for each sensory input, and these receptive fields overlap one another. It is this spatial correspondence among receptive fields that determines the manner in which both populations of neurons integrate the inputs they receive from different sensory channels. Several principles of integration characterize both cortical and midbrain multisensory neurons, and these constancies in the fundamentals of cross-modality integration are likely to provide a basis for coherence at different levels of the neuraxis. Yet there are also obvious differences in these populations of multisensory neurons. Cortical receptive fields are significantly larger than those in the midbrain, have a lower incidence of suppressive surrounds, and exhibit less cross-modality inhibitory interactions than in the midbrain. Presumably, these differences reflect a greater emphasis on non-spatial aspects of cross-modality integration in cortex than is required by the orientation and localization functions mediated by the superior colliculus.