Single quickly adapting mechanoreceptive afferents innervating monkey glabrous skin: response to two vibrating probes.

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Abstract
The stimulus response of single, quickly adapting, cutaneous, mechanoreceptive afferents (QA) innervating the glabrous skin were studied in the monkey [M. nemestrina]. The form of the amplitude-response function relating vibratory amplitude to QA response was always a piecewise linear succession of ramps and plateaus; this form held regardless of the probe separation, the position of the probes in the receptive field and the vibratory frequency. The effect of these stimulus variables could be characterized by their effect on the parameters of the 1st ramp of the amplitude-response function, i.e., its slope and intercept. For each QA the ramp intercept remained approximately constant regardless of changes in the stimulus. There was a wide variation of the value of the intercept among different QA. For each QA the ramp slope varied systematically with changes in the stimulus parameters. For a fixed set of stimulus conditions this slope varied widely from QA to QA. The response to a QA was determined by the stimulus configuration and the characteristics of the particular fiber. The contribution of these 2 factors could be separated. The systematic dependence of a QA response on the position of the 2 probes in the receptive field was characterized by the effect on the ramp slope. This defined 2-dimensional field factor profiles, which were double-peaked functions. The separation between peaks depended on the separation between the probes. Profiles at a stimulus frequency of 200 Hz were similar to those at 40 Hz. These profiles are determined by the skin mechanics and by the stimulus geometry. The observations allowed a simple but complete quantitative description, in terms of the stimulus variables, of the response of any QA to 2-point stimuli, making a complete population reconstruction feasible.