Responses of primary somatosensory cortical neurons to controlled mechanical stimulation.

The results of psychophysical studies suggest that displacement velocity may contribute significantly to the sensation of subcortical somatosensory neurons. The cortical correlates of these phenomena, however, are not known. In the present study the responses of rapidly adapting (RA) neurons in the forelimb region of cat primary somatosensory cortex (SI) to controlled displacement of skin and hair was studied. The cortical RA neurons were grouped according to their response patterns to constant-velocity ramp stimuli. Firing frequency, spontaneous activity, receptive field organization, and cortical laminar location of the RA neurons were studied and compared between the groups. There was a continuum in the response pattern of RA neurons to constant -velocity displacement. In one extent of the continuum, (G1IF1, n = 16) neurons typically responded to constant-velocity ramp stimuli with 1-3 impulses at the beginning of the ramp they had little or no spontaneous activity and responded only to fast stimuli. In the other extent of the continuum, (G2IF2, n = 15) neurons responded throughout the ramp and sometimes during the initial phase of sustained stimuli. These neurons usually had higher spontaneous activities than G1IF1 neurons and responded to relatively slower stimuli. Gint/Fint (n = 18) neurons fell between the two types: they had an initial brief response which was sometimes followed, after a period of low or no activity, by a discharge towards the end of the ramp. Nine neurons had response patterns suggesting the existence of either convergence between these types or input from other types of mechanoreceptors. The average and instantaneous firing frequency of the majority of RA neurons increased as the ramp velocity was increased. The variation in the firing frequency of G2IF2 RA neurons had the widest range of firing frequencies and, in average, generated higher firing frequencies to both low and high displacement velocities. Coefficients of regression lines for linear, logarithmic and power functions were not significantly different among RA types. No correlation was found between cortical laminar location and RA types.