MODALITY AND TOPOGRAPHIC PROPERTIES OF SINGLE NEURONS OF CAT'S SOMATIC SENSORY CORTEX

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Abstract
The modality and topographical attributes of the neurons of the somatic sensory cortex were studied by the method of single unit analysis. All the neurons observed were activated by mechanical deformation of some peripheral tissue. Within this broad modality class 3 distinct subgroups were seen: cells activated by movement of hairs; by pressure upon the skin; and by mechanical deformation of deep tissues. No neuron was ever seen which could be classified in more than 1 subgroup. All were activated by stimulation of the contralateral side of the body only. The 2 groups of neurons related to the skin showed different discharge properties: those responsive to hair movement adapting quickly; those driven by pressure upon the skin continuing to discharge steadily to a steady stimulus. Cells belonging to each subgroup were found in all of the cellular layers. In 84% of penetrations across the cellular layers which were directed perpendicularly, all the neurons encountered belonged to either cutaneous or deep subgroups. These modality-specific vertical columns of cells are intermingled for any given topographical region. Cells related to deep structures were activated from deep fascia and connective tissue and the regions of the joints and joint capsules, but never from muscle. Those driven by joint movement signal the steady position and phasic changes in positions of joints, and are suitably arranged to subserve position sense. Pairs of closely adjacent cells were seen to respond reciprocally to alternating joint movements, a form of reciprocal afferent innervation. Direct measurements of the size of the peripheral receptive fields playing upon cortical neurons were made. They are comparable in size to those related to dorsal root fibers, cells of the dorsal column nuclei, or the thalamic neurons. A form of afferent inhibition of cortical neurons is described, the peripheral inhibitory field surrounding the peripheral excitatory field of cortical neurons. These data, together with those on the discharge properties of cortical cells (19), are used in a discussion of the neural correlates of sensory localization, 2-point discrimination and pattern and contour recognition, on the basis of partially shifted reciprocal overlap rather than on the basis of a patterned sensory mosaic, with insulated central projections. The neurons encountered in a perpendicular traverse of the cortex are activated from almost identical peripheral receptive fields at latencies which on the average are not a function of the position of the cell in depth within the cortex. These facts support an hypothesis that the elementary pattern of organization in the cerebral cortex is a vertically oriented column or cylinder of cells capable of input-output functions of considerable complexity, independent of horizontal intragriseal spread of activity.