Discriminative and affective touch in human experimental tactile allodynia

Recently, several studies have suggested a role for unmyelinated (C-tactile, CT) low-threshold mechanoreceptive afferents in the allodynic condition. In this psychophysical study we explored the integrity of both A beta and CT afferent processing following application of the heat capsaicin model of tactile allodynia on the left forearm in healthy subjects (n = 40). We measured tactile direction discrimination (TDD) to target the integrity of A beta processing (n = 20). The TDD accuracy was significantly lower in the allodynic compared to a control zone. In addition, we measured the perceived pleasantness and pain of brush stroking at CT targeted (slow) and CT sub-optimal (fast) stroking velocities to investigate the integrity of CT processing (n = 20). When comparing touch pleasantness in the allodynic and control zone, there was a significantly larger difference in ratings for CT targeted compared to CT suboptimal stimulation. The results suggest a disturbance in both A beta-mediated discriminative and CT-mediated affective touch processing in human experimental tactile allodynia. Our findings support the canonical view that tactile allodynia is signaled by A beta afferents but that CTs seem to contribute by the loss of a pain inhibiting role. (C) 2014 Elsevier Ireland Ltd. All rights reserved.