The visuotopic component of the multisensory map in the deep laminae of the cat superior colliculus.

A well-defined map of visual space is located in the deep laminae of the cat superior colliculus. The horizontal meridian is oriented rostral-caudal, while the vertical meridian is oriented perpendicular to it in the rostral third of the structure. This map represents the entire contralateral visual field and extends approximately 40 degrees into ipsilateral visual space. Although the deep-laminae visuotopy is similar to that found in the superficial laminae of the same structure, the topographic register among these maps is most secure rostrally but becomes increasingly poorer at more caudal and lateral locations. The combination of 2 features distinguish the deep-layer visual representation from that found in the superficial laminae and in geniculocortical systems: (1) the constituent visual receptive fields are very large (mean diameter, 66.9 degrees), and (2) the majority (greater than 70%) of the neurons composing it receive nonvisual inputs. Because the visual receptive fields of visual-multisensory neurons are significantly larger than those of neighboring neurons that respond only to visual stimuli, far more visual-multisensory neurons are activated by any given visual stimulus. These data, when coupled with those from previous studies, suggest that, from a functional perspective, deep-laminae visual neurons form one component of an integrated multisensory map, and that their topographic organization is essential for the normal dynamics of multisensory integration.