A major challenge for the brain is to extract precise information about the attributes of tactile stimuli from signals that co-vary with multiple parameters, e.g., speed and texture in the case of scanning movements. We determined the ability of humans to estimate the tangential speed of surfaces moved under the stationary fingertip and the extent to which the physical characteristics of the surfaces modify speed perception. Scanning speed ranged from 33 to 110 mm/s (duration of motion constant). Subjects could scale tactile scanning speed, but surface structure was essential because the subjects were poor at scaling the speed of a moving smooth surface. For textured surfaces, subjective magnitude estimates increased linearly across the range of speeds tested. The spatial characteristics of the surfaces influenced speed perception, with the roughest surface (8 mm spatial period, SP) being perceived as moving 15% slower than the smoother, textured surfaces (2-3 mm SP). Neither dot disposition (periodic, non periodic) nor dot density contributed to the results, suggesting that the critical factor was dot spacing in the direction of the scan. A single monotonic relation between subjective speed and temporal frequency (speed/SP) was obtained when the ratings were normalized for SP. This provides clear predictions for identifying those cortical neurons that play a critical role in tactile motion perception and the underlying neuronal code. Finally, the results were consistent with observations in the visual system (decreased subjective speed with a decrease in spatial frequency, 1/SP), suggesting that stimulus motion is processed similarly in both sensory systems.

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.

Dépeault A, Meftah EM, Chapman CE. Neuronal correlates of tactile speed in primary somatosensory cortex. J Neurophysiol 110: 1554-1566, 2013. First published July 10, 2013 doi:10.1152/jn.00675.2012.-Moving stimuli activate all of the mechanoreceptive afferents involved in discriminative touch, but their signals covary with several parameters, including texture. Despite this, the brain extracts precise information about tactile speed, and humans can scale the tangential speed of moving surfaces as long as they have some surface texture. Speed estimates, however, vary with texture: lower estimates for rougher surfaces (increased spatial period, SP). We hypothesized that the discharge of cortical neurons playing a role in scaling tactile speed should covary with speed and SP in the same manner. Single-cell recordings (n ϭ 119) were made in the hand region of primary somatosensory cortex (S1) of awake monkeys while raised-dot surfaces (longitudinal SPs, 2-8 mm; periodic or nonperiodic) were displaced under their fingertips at speeds of 40 -105 mm/s. Speed sensitivity was widely distributed (area 3b, 13/25; area 1, 32/51; area 2, 31/43) and almost invariably combined with texture sensitivity (82% of cells). A subset of cells (27/64 fully tested speed-sensitive cells) showed a graded increase in discharge with increasing speed for testing with both sets of surfaces (periodic, nonperiodic), consistent with a role in tactile speed scaling. These cells were almost entirely confined to caudal S1 (areas 1 and 2). None of the speed-sensitive cells, however, showed a pattern of decreased discharge with increased SP, as found for subjective speed estimates in humans. Thus further processing of tactile motion signals, presumably in higherorder areas, is required to explain human tactile speed scaling.

Bourgeon S, Dépeault A, Meftah EM, Chapman CE. Tactile texture signals in primate primary somatosensory cortex and their relation to subjective roughness intensity. J Neurophysiol 115: 1767-1785, 2016. First published February 10, 2016 doi:10.1152/jn.00303.2015.-This study investigated the hypothesis that a simple intensive code, based on mean firing rate, could explain the cortical representation of subjective roughness intensity and its invariance with scanning speed. We examined the sensitivity of neurons in the cutaneous, finger representation of primary somatosensory cortex (S1) to a wide range of textures [1 mm high, raised-dot surfaces; spatial periods (SPs), 1.5-8.5 mm], scanned under the digit tips at different speeds (40 -115 mm/s). Since subjective roughness estimates show a monotonic increase over this range and are independent of speed, we predicted that the mean firing rate of a subgroup of S1 neurons would share these properties. Single-unit recordings were made in four alert macaques (areas 3b, 1 and 2). Cells whose discharge rate showed a monotonic increase with SP, independent of speed, were particularly concentrated in area 3b. Area 2 was characterized by a high proportion of cells sensitive to speed, with or without texture sensitivity. Area 1 had intermediate properties. We suggest that area 3b and most likely area 1 play a key role in signaling roughness intensity, and that a mean rate code, signaled by both slowly and rapidly adapting neurons, is present at the level of area 3b. Finally, the substantial proportion of neurons that showed a monotonic change in discharge limited to a small range of SPs (often independent of response saturation) could play a role in discriminating smaller changes in SP. tactile roughness; neuronal coding; passive touch; hierarchical processing WHEN WE TOUCH A SURFACE, THE tactile impression of smoothness/roughness is influenced by the physical structure of the surface. A number of factors contribute, including the number of projections (or tactile elements) that make up the surface, their spacing, density, size, form, degree of unevenness and the material from which they are constructed. Any tactile exploration of a surface necessarily activates all of the largediameter cutaneous mechanoreceptive afferents that are involved in discriminative touch, including (in the monkey) slowly adapting type I (SA), rapidly adapting (RA) and Pacinian (PC) afferents. The discharge evoked by textured surfaces is complex, reflecting not only the surface structure, but also factors intrinsic to the exploration process, including scanning speed and contact force.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 sur-

We present a case of a 64-year-old man who presented with a rapidly growing tumor in the left buttock and intergluteal cleft area, which was affected by hidradenitis suppurativa. The patient was on tumor necrosis factor-alpha inhibitors for hidradenitis suppurativa for 2 years prior to the development of the mass. Initial biopsy of the mass showed a well-differentiated squamous cell carcinoma with spindle cells and positive epithelial immunomarkers. Subsequent excisional biopsy of the tumor showed an infiltrating poorly differentiated squamous cell carcinoma composed of islands of atypical sarcomatoid spindle cells. Squamous cell carcinoma arising in hidradenitis suppurativa is a rare complication which may occur secondary to chronic inflammation and epidermal hyperproliferation in hidradenitis suppurativa–affected areas.