CUTANEOUS MASKING .2. GEOMETRY OF EXCITATORY AND INHIBITORY RECEPTIVE-FIELDS OF SINGLE UNITS IN SOMATOSENSORY CORTEX OF THE CAT

The responses of a single neuron in the primary somatosensory cortex of the cat to brief air-pulse stimuli were quantitatively examined. These controlled natural stimuli activated almost exclusively rapidly adapting hair units which, on systematic movement of the stimulus through the receptive field, gave unit-response profiles that showed the classical unimodal tent-shaped distribution. Conditioning stimulus-induced inhibition of a response evoked by systematically moving the conditioning stimulus through the receptive field. The spatial distribution of in-field inhibitory activity was unimodal and highly covariant with that of the conditioning excitation, the peak inhibition corresponding to the functional center of the excitatory receptive field. Nearly one-half of the units studied evidenced inhibition extending beyond the excitatory receptive field, forming a 'surround' inhibitory region; but these were usually restricted areas with rather weak inhibitory effects. Time-course measurements revealed, on the average, inhibition effects measureable from 10 ms before to some 70 ms following conditioning stimulation, with peak inhibition delayed some 10-15 ms from the conditioning stimulus onset. We showed the backward inhibition, occurring with the test stimulus delivered before the onset of the conditioning stimulus, to be a property of the test response duration. Inhibition measured in the surround areas had essentially the same time course as the inhibition calculated from measurements made within the receptive fields. The spatial and temporal profiles of the excitatory and inhibitory cortical unitary activity are thus very similar to the parametric features of psychophysical enhancement and masking. These findings suggest that the excitatory and inhibitory activities related to individual stimuli interact in multipoint stimulus paradigms so that simple unimodal composite profiles are synthesized.