Neural mechanisms of tactual roughness perception

Abstract

The neural mechanisms underlying tactual roughness were investigated in a combined psychophysical and neurophysiological study. Stimuli consisted of surfaces embossed with dot arrays of varying dot diameter and spacing. Human subjects scanned the surfaces tactually and responded with numerical magnitudes proportional to their sense of roughness magnitude. The same surfaces were scanned across the receptive fields of cutaneous mechanoreceptive afferents in monkeys while recording the evoked action potentials. Hypothetical neural codes for roughness magnitude were computed from the neural response patterns and tested for their ability to account for the psychophysical data. Four types of neural coding mechanisms were considered: (1) mean firing rate; (2) general variation in firing rate; (3) short‐term temporal variation in firing rate; and (4) local spatial variation in firing rate. Mean firing rate failed to explain the psychophysical results: Surfaces that evoked the same firing rate evoked very different roughness judgments. In contrast, neural codes based on spatial firing rate variation, especially in slowly adapting afferents, account for the psychophysical results [Connors et al., J. Neurosci. 10, 3823–3826 (1990)]. [Work supported by NIH.]