Cortical Columnar Processing in the Rat Whisker-to-Barrel System

Abstract

Controlled whisker stimulation and single-unit recordings were used to elucidate response transformations that occur during the processing of tactile information from ventral posterior medial thalamus (VPM) through cortical columns in the rat whisker/barrel cortex. Whiskers were either deflected alone, using punctate ramp-and-hold stimuli, or in combination with a random noise vibration applied simultaneously to two or more neighboring whiskers. Quantitative data were obtained from five anatomically defined groups of neurons based on their being located in: VPM, layer IV barrels, layer IV septa, supragranular laminae, and infragranular laminae. Neurons in each of these populations displayed characteristic properties related to their response latency and time course, relative magnitudes of responses evoked by stimulus onset versus offset, strength of excitatory responses evoked by the noise stimulus, and/or the degree to which the noise stimulus, when applied to neighboring whiskers, suppressed or facilitated responses evoked by the columnar whisker. Results indicate that within layer IV itself there are at least two anatomically distinct networks, barrel and septum, that independently process afferent information, transforming thalamic input in similar but quantitatively distinguishable ways. Transformed signals are passed on to circuits in supragranular and infragranular laminae. In the case of supragranular neurons, evidence suggests that circuits there function in a qualitatively different fashion from those in layer IV, diminishing response differentials between weak and strong inputs, rather than enhancing them. Compared to layer IV, the greater heterogeneity of receptive field properties in nongranular layers suggests the existence of multiple, operationally distinct local circuits in the output layers of the cortical column.