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 neurones in the cutaneous, finger representation of primary somatosensory cortex (S1) to a wide range of textures (1mm high, raised-dot surfaces; spatial periods (SPs), 1.5 - 8.5mm), scanned under the digit tips at different speeds (40-115mm/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 neurones would share these properties. Single unit recordings were made in 4 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 signalling roughness intensity, and that a mean rate code, signalled by both slowly and rapidly adapting neurones, is present at the level of area 3b. Finally, the substantial proportion of neurones 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
- Copyright © 2015, Journal of Neurophysiology