The responses of rapidly adapting (RA) neurons to constant-velocity ramp stimulation were studied in the forepaw representation of primary somatosensory cortex (SI) of anesthetized cats. Single probe stimuli were used to indent the skin or to move hair parallel to the skin surface. The velocity of the moving stimulus probe was varied to determine the rate sensitivity of the neurons. 2. The cortical RA neurons were classified into four categories identified as G1/F1, Gint/Fint, G2/F2, and complex classes. The primary bases for classification in the present experiments were the pattern of response during ramp stimulation, velocity threshold, and directional sensitivity. 3. Of the RA neurons recorded in SI, 84% (49/58) could be assigned to one of the three response classes with little ambiguity. The remaining neurons showed more complex responses. The form of the complex responses suggested that they arose from a combination of inputs of different response classes. Some of these appeared to arise from a combination of different RA input classes, whereas others had components that resembled responses previously described for C mechanoreceptors. 4. Increased ramp velocity resulted in increased average firing frequency in 87% of the RA neurons. This relationship, which could be fitted with a power function, varied with response class. G1/F1 neurons were more sensitive to stimulus rate than G2/F2 neurons. Significant differences between response classes also were seen in the relationship between ramp velocity and their number of evoked action potentials and in their spontaneous firing rates. 5. The results demonstrate that a discrete SI neuron population is sensitive to the rate of stimulus movement. This observation is consistent with psychophysical studies reporting effects of stimulus indentation rates on perception of single probe stimuli. The appearance of complex responses in a small proportion of SI neurons provides evidence of convergence in somatosensory pathways to SI.
