Population of quickly adapting mechanoreceptive afferents innervating monkey glabrous skin: representation of two vibrating probes.

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Abstract
Data describing the response of single quickly adapting cutaneous mechanoreceptive afterents (QA) of M. nemestrina to 2 probes vibrating in phase were used to reconstruct the response of a population of QA innervating the region of glabrous skin surrounding the double-probe stimulus. The parameters of the stimulus were: position of the probes on the skin, probe separation (1 or 2 mm), vibratory amplitude and vibratory frequency (40 or 200 Hz). Responses of individual QA were measured with the probes placed at different positions in the receptive field. These position-response profiles had a similar shape for all QA but varied widely in amplitude as different QA differ greatly in sensitivity. The profiles were effectively normalized and averaged, yielding the profile for an average QA. The distribution of amplitudes provided an estimate of the distribution of sensitivities in the natural QA population. The response of the average QA was calculated for various positions of the probes in the receptive field and at various vibratory amplitudes. This response profile was viewed as the ideal population response to 2 probes. The profile was double peaked, with a peak of activity at QA under each probe and a trough of activity between the probes; topographically the profile describes a saddle. Representation of the saddle is optimum at a particular amplitude; both higher and lower amplitudes flatten the saddle. The optimum amplitude at 200 Hz is different from that at 40 Hz. Information conveyed by single QA about spatial and intensive parameters of the stimulus was ambiguous. The response of the whole population of QA contained an independent representation of each stimulus parameter and could transmit unambiguous information about these parameters to the brain. The population was now made more realistic by allowing individual QA to vary in sensitivity. Small variations distorted the profile considerably. The effect of innervation density was assessed and it was shown that, by current estimates, density on the fingertip is adequate to resolve the stimulus. Results were extrapolated to examine the nature of the population response to vibrating probes moved tangentially across the skin.