Optical imaging of rat somatosensory cortex reveals representational overlap as topographic principle.

We measured reflectance changes by means of optical imaging of intrinsic signals to study the topography of the paw representations in rat somatosensory cortex. Following circumscribed tactile stimulation of single digits or pads, we found large and partially overlapping areas of reflectance changes (delta R). The diameters of their focal zones defined at 75% maximal delta R were in the range of 150 microns and preserved all details of the underlying maps. Zones of overlap were in the range 15-25% measured at half-maximal delta R. In contrast, we found sharp boundaries with no overlap between the fore- and hindpaw representations. The data suggest that large and overlapping cortical maps constitute a normal type of neural representation supporting the idea of a distributed neural processing scheme.