Tactile perception of roughness: raised-dot spacing, density and disposition

Recently, we showed that tactile speed estimates are modified by the spatial parameters of moving raised-dot surfaces, specifically dot spacing but not dot disposition (regular, irregular) or density. The purpose of this study was to determine the extent to which tactile roughness perception resembles tactile speed with respect to its dependence and/or independence of the spatial properties of raised-dot surfaces. Subjects scaled the roughness of surfaces displaced under the finger. Dot spacing (centre-to-centre) ranged from 1.5 to 8.5 mm in the direction of the scan (longitudinal). Mean dot density varied from 2.2 to 46.2 dots/cm². Dot disposition was varied: repeating rows (periodic) or quasi-random (non-periodic). In the first experiment (n = 8), the periodic and non-periodic surfaces were matched for mean dot density. Roughness showed a monotonic increase with 1/dot density, but non-periodic surfaces were judged to be smoother than the periodic surfaces. Subjective equality was obtained when the data were re-expressed relative to longitudinal SP. In the second experiment (n = 7), the periodic and non-periodic surfaces were matched for longitudinal dot spacing. Perceptual equivalence was observed when the results were plotted relative to dot spacing, but not 1/dot density. Dot spacing in the orthogonal direction (transverse) was excluded as a contributing factor. Thus, as found for tactile speed scaling, roughness is critically dependent on longitudinal dot spacing, but independent of dot disposition and dot density (over much of the tested range). These results provide a set of predictions to identify cortical neurones that play critical roles in roughness appreciation.