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

Abstract

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.

Tactile texture signals in primate primary somatosensory cortex and their relation to subjective roughness intensity

“…For the P1 series, the first half (5 cm) of the segment had a longitudinal SP of 1.5 mm; dot spacing was either the same (segment 1) or increased on the second half of the surface (segments 2-4: a, 3.5, 5.5, 7.5 mm; b, 4.5, 6.5, 8.5 mm). These surfaces covered a wide range of SPs across which humans show a linear increase in roughness (Dépeault et al 2009;Meftah et al 2000;Sutu et al 2013) and extend the ranges studied previously, both in this laboratory (Chapman and Meftah 2005;Jiang et al 1997;Meftah et al 2002Meftah et al , 2009Tremblay et al 1996) and elsewhere (Darian-Smith et al 1982;Burton 1988, 1991a).…”

“…1 (Meftah et al 2000): subjective roughness showed a monotonic increase across a wide range of dot spacings (1.5-8.5 mm SP), with identical results obtained when scanning speed was doubled on a trialby-trial basis. Similar monotonic relations between roughness and SP are obtained using nonperiodically disposed raised dots (Dépeault et al 2009) and tetragonal arrays of raised dots (Sutu et al 2013). Taken together, the critical factor for subjective roughness intensity is the spacing between raised dots in the direction of the scan, and any neuronal code needs to reflect this.…”

“…The exception would have been cells with a smaller RF restricted to the distal part of the phalanx. For comparison, measures were taken in two representative human subjects, in this case using a single digit, D3, as in our laboratory's psychophysical studies (Dépeault et al 2009;Meftah et al 2000;Sutu et al 2013): the area of contact was 14 ϫ 10 mm (dimensions of distal phalanx, respectively, 23 ϫ 16 mm). The human digit was thus also contacted with two rows or more at all times.…”

“…10B. The latter data come from earlier experiments using the same types of raised-dot surfaces (data from Dépeault et al 2009;Meftah et al 2000). For the human data, the estimates of each subject were normalized by dividing all values by the grand mean for the session to take into account the fact that different subjects used different rating scales.…”

“…Although the nature of the relationship between subjective roughness and texture remains controversial, the bulk of the evidence suggests that, if the tactile elements are sufficiently high (Ն1 mm), then there is a linear relation between subjective roughness and tactile element spacing [spatial period (SP), distance center-to-center between adjacent elements] for surfaces composed of gratings and raised dots (Dépeault et al 2009;Lederman 1974;Meftah et al 2000;Morley et al 1983;Sathian et al 1989;Sutu et al 2013;Yoshioka et al 2001;cf, Connor et al 1990). A representative example of such a psychophysical curve is shown in Fig.…”