Neural correlates of tactile roughness perception

Tactile perception of surface roughness was investigated in a combined psychophysical and neurophysiological study using 18 embossed, tetragonal dot patterns with varying dot spacings (1.3 to 6.2 mm) and sizes (0.4‐ to 1.2‐mm diameter). Psychophysical subjects explored these surfaces with the pad of the index finger and reported their estimates of subjective roughness. The results were inverted U‐shaped functions of spatial frequency. In neurophysiological experiments, exactly the same surfaces were scanned across the distal pads of macaque monkeys while recording from peripheral mechanoreceptive afferents (SA, QA, and PC afferents). Neural coding hypotheses were tested by comparing various features of the responses with the psychophysical results. Mean rate codes fared poorly as did codes based on “rate fluctuation” (summed absolute values of differences between rate maxima and minima). However, the second central moment of firing rate (variance) for the SA afferents matched the psychophysical data close...