Response of monkey glabrous skin mechanoreceptors to random noise sequences: II. Dynamic stimulus state analysis.

The responses of monkey glabrous skin slowly adapting (SAI, Merkel cell), rapidly adapting (RA, Meissner) and Pacinian corpuscle (PC) mechanoreceptors were analyzed as a function of the instantaneous position, velocity, and acceleration of a dynamic stimulus. For these experiments, a vibrotactile punctate stimulator was driven by a non-repeating-noise sequence. The resulting data (sampled stimulus waveform, windowed impulse) were processed in several ways. Initially, input-output correlation analysis was implemented to generate spike-aligned averages of the stimulus waveform preceding and following impulse initiation. From this analysis, it was determined that dual-responding RA and PC afferents--that is, those afferents that responded to both indenting and extracting stimulus movements--universally responded in a nearly perfectly symmetrical manner to the stimulus. Subsequently, two-dimensional (position, velocity) state histograms were generated and used to assess mechanoreceptor dynamic stimulus sensitivities. From these state histograms, it was determined that the threshold for impulse initiation by SAI afferents was preferentially sensitive to the indentation position of the stimulus, with only a minor sensitivity to stimulus velocity. RA afferent thresholds were sensitive to a continuum of dynamic stimulus velocities and positions. At the extremes, RA afferent impulses could be initiated by either a highly indented, low-velocity stimulus or high-velocity stimulus with a limited indentation position. PC afferents appeared to be preferentially sensitive to a combination of stimulus velocity and acceleration, but the data-sampling interval was too coarse to adequately resolve the full range of dynamic stimulus sensitivities.