Differential formation of topographic maps on the cerebral cortex and superior colliculus of the mouse by temporally correlated tactile- tactile and tactile-visual inputs.

Coincident electrical activity of nerve fibres seems to play a fundamental role in the development of ordered connections in the CNS. To test this hypothesis on the formation of topographic maps we connected two cutaneous regions of the body of newborn mice by implanting an artificial bridge of pig hair. Through this procedure we produced the mechanical fusion of the ear with either the shoulder or the nose. In these conditions the ear not only was connected with shoulder or nose, but was also in relation with the nasal or the inferior portion of visual space. Therefore the probability of temporally correlated tactile-tactile inputs (ear-shoulder or ear-nose) as well as tactile-visual inputs (ear-inferior or ear-nasal visual space) increased. By recording from the primary somatosensory cortex and superior colliculus, we found that the formation of topographic maps was based on different principles. The somatosensory cortex developed in terms of tactile-tactile correlated inputs, showing somatosensory neurons with receptive fields extending through the fused parts of the body. Conversely, the superior colliculus processed tactile-visual correlated inputs; we found somatosensory-visual bimodal neurons with visual receptive fields extending into the portion of visual space where the artificial bridge was directed. These results suggest that the fusion of two body parts is represented in terms of cutaneous coordinates in the cortex and external world (visual) coordinates in the superior colliculus. Therefore the differential tactile-tactile and tactile-visual coincident activity seems to be correlated to the different meaning of information processing of these two brain regions.