Temporal cues contribute to tactile perception of roughness

Optimal perception of surface roughness requires lateral movement between skin and surface, suggesting the importance of temporal cues. The roughness of periodic gratings is affected by changing either inter-element spacing (groove width, G) or element width (ridge width, R). Peripheral neural responses to gratings depend quantitatively on a spatial variable, G, and a temporal variable, grating temporal frequency (F_t), with changes in R acting indirectly through concomitant changes in F_t. We investigated, psychophysically, the contribution of temporal cues to human tactile perception of roughness, using gratings varying in either R or G. Gratings were scanned across the immobile fingerpad with controlled movement speed (S) and contact force. In one experiment, we found that roughness magnitude estimates depended on both G and F_t. In a second experiment, discrimination of the roughness of gratings varying in either R or G was affected by manipulating F_t. Overall, the effect of G on roughness judgments was much stronger than that of F_t, probably explaining why many previous studies using surfaces that varied only in inter-element spacing led to the conclusion that temporal factors play no role in roughness perception. However, the perceived roughness of R-varying gratings was determined by F_t and not spatial variables. Roughness judgments were influenced by G and F_t in a manner entirely consistent with predicted afferent response rates. Thus perceived roughness, like peripheral afferent responses, depends in part on temporal variables.