Orientation detectors in the primary somatosensory neocortex of the raccoon.

An analysis of response properties of single neurons in the forepaw region of the primary somatosensory neocortex (SI) of the raccoon was undertaken to gain a better understanding of the neural bases of tactile form discrimination. In the course of this investigation a new type of feature detector neuron which responded preferentially to tactile stimulus orientation was discovered. Within a sample of 110 neurons responding to light touch, two categories of cells were noted in terms of the type of stimulation best able to excite them. One category (linear units) was preferentially responsive to indentations of the skin using a narrow elongated stimulus probe applied in a particular orientation. The other category (round field units) gave a maximum response to stimulation with a round stimulus probe applied anywhere within a roughly circular receptive field. The response pattern of some SI neurons to a maintained indentation of the skin consisted of early and late excitatory responses, separated by a depression in responding. The second excitatory response appeared to be more influenced by variations in stimulus parameters than was the first.