Elasmobranch vision: multimodal integration in the brain.

Multimodal sensory areas that include vision have been identified physiologically in two separate pallial areas in the telencephalon and in the tectum of the mesencephalon. Multisensory integration occurs in the medial pallium of the little skate, Raja erinacea, and a primitive squalomorph shark, Squalus acanthias, whereas in the advanced galeomorph shark, Ginglymostoma cirratum, a major multimodal area is in the dorsal pallium pars centralis. Pars centralis has undergone extensive hypertrophy in the evolution of advanced batoids and galeomorph sharks. More complete studies are required on individual species to assess the possibility that there has been an evolutionary shift in major sensory processing areas from medial to dorsal pallium among the elasmobranchs. Most retinofugal fibers in elasmobranchs project spatiotopically to the tectum, the central zone of which is an area of multimodal integration. The spatiotopic tectal map of the electrosense in the little skate includes only that part of the electrosensory field that is within the visual field, and individual points on the tectum represent the same spatial location in each sense. In both maps the region of space near the horizon is greatly overrepresented. For vision this corresponds to a band of increased retinal ganglion cell density, and for both senses the overrepresentation may be related to the importance of this region of space in the skate's natural orienting. Spatial congruence of visual and electrosensory maps should ensure that individual tectal cells integrate multimodal information in a space-specific fashion.