OPTICAL IMAGING AND ELECTROPHYSIOLOGY OF RAT BARREL CORTEX - I - RESPONSES TO SMALL SINGLE-VIBRISSA DEFLECTIONS

A study was undertaken to investigate the response of the rodent somat osensory barrel cortex to single-whisker, near-threshold vibrissal sti muli. Cortical responses to controlled whisker deflections were record ed by (i) conventional multi unit extracellular recording within the c ytochrome oxidase rich barrels centers and the interbarrel septa, and (ii) intrinsic signal optical imaging, a technique that provides a spa tial view of cortical activation thought to be related to the deoxygen ation of hemoglobin in activated areas, Barrel cortex neurons responde d weakly to whisker deflections of 0.04 degrees, Their response to a s eries of small stimuli of increasing amplitude was well fitted by a lo garithmic function, Responses to larger stimuli declined monotonically with distance from the center of the barrel column, and were characte rized by greater onset and offset firing rates, by greater post-excita tory reduction of firing to below spontaneous levels, and by shorter r esponse latency. In comparison to measurements taken previously from p rimary vibrissal afferent fibers, we conclude that cortical cells can respond to activity in a very small fraction of first-order sensory ne urons.