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-

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The aim of this study was to define the relative contribution of self-generated cutaneous and proprioceptive feedback to haptic shape discrimination by systematically constraining the exploratory strategy. Subjects (n = 23) explored pairs of two-dimensional (2-D) angles (standard angle, 90 degrees; comparison angles, 91 degrees -103 degrees) placed at arm's length from the subject, and identified the larger angle of each pair. The exploratory strategies included a reference condition, dynamic scan of the index finger over the entire object [combined cutaneous and proprioceptive (shoulder) feedback], and modified conditions, static touch of the intersection of the two bars that formed the angle using the index finger (cutaneous feedback) and dynamic scans of the object using a hand-held tool (proprioceptive feedback, shoulder). Discrimination thresholds (75% correct) were very similar for dynamic and static touch with the index finger. Thresholds varied as a function of the static contact duration (<1 s, 7.2 degrees +/- 0.6 degrees; approximately 3 s, 4.2 degrees +/- 0.5 degrees), but were not different from the reference condition (6.0 degrees +/- 0.9 degrees). The higher threshold with short static touch likely reflects movement-related gating of self-generated tactile inputs. Together, the results suggested that cutaneous feedback alone may be sufficient to explain 2-D angle discrimination, because the added proprioceptive feedback did not improve performance. Also, threshold did not vary with the number of dynamic scans (one or two), suggesting that the critical information was gathered on the first pass over the angle. In contrast, when the angles were explored with the tool, the threshold increased relative to the corresponding reference condition from the same session (tool, 9.6 degrees +/- 0.9 degrees; dynamic scan with the finger, 6.2 degrees +/- 1.0 degrees). Thus, performance was poorer with proprioceptive feedback alone, suggesting that cutaneous feedback was relatively more important for 2-D haptic angle discrimination in the present experiment.

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