Location specificity of tactile aftereffects

Adaptation aftereffects can reveal how the nervous system encodes sensory features. We have recently demonstrated that the distance between two tactile events is a property of somatosensation susceptible to adaptation. The reported aftereffects shared several characteristics with low-level visual aftereffects, including orientation and location specificity, pointing to a locus of tactile distance representation early in a bottom-hierarchy of somatosensory processing. Here we focus on this location specificity feature, to better characterize the process by which tactile distance aftereffects operate. In a series of experiments, we applied pairs of pointed tactile stimuli separated by defined distances to one adapting skin region, either on the hand, or on one finger, and tested the magnitude of adaptation aftereffects across adjacent skin regions that did, or did not, cross joint boundaries (i.e., the wrist), or separate body parts (i.e., the fingers). We found a spatial gradient in the magnitude of adaptation aftereffects, both in the mediolateral and proximodistal hand axes, with stronger adaptation aftereffects in the vicinity of the adapting region. Crossing joint boundaries from hand to wrist did not change the magnitude and extent of this gradient. Interestingly, however, the effect of adaptation did not transfer across fingers, even when adapting and tested skin regions had somatotopically adjacent representations (e.g., little and ring fingers). Similar effects were observed during adaptation to textures. Tactile aftereffects thus might provide a psychophysical window onto the distribution, organization, and overlap of tactile receptive fields, specifically with respect to anatomical or functional boundaries between body parts.