Structural representation of the body differs for glabrous and hairy skin surfaces

Knowledge about body representation is drawn from different sensory modalities but relies strongly on tactile information. The body structural representation (BSR) is a topological map of the body in which the spatial configuration of different body parts is defined. Recent evidence has demonstrated that the BSR is not fixed but can be dynamically updated by external factors such as changes in body posture. However, the extent to which access to the structural representation of the hand differs for different skin regions (glabrous vs. hairy) remains unclear. To address this question, we conducted two experiments using an adapted version of the "in-between" test, where healthy individuals received tactile stimulations on the fingertips and estimated the number of unstimulated fingers between the two touched. In Experiment 1, the skin regions (glabrous vs. hairy) were varied across conditions. In Experiment 2, the stimulated skin region was manipulated while hand posture was held constant (palm down). Results showed a significant difference in finger numerosity estimation between the glabrous and hairy skin regions. Specifically, participants estimated greater numerosity with glabrous skin stimulation regardless of hand posture, but this effect was only evident when non-adjacent fingers were stimulated. This suggests that access to the BSR of the hand depends on the skin surface stimulated and varies as a function of the anatomical distance between different body parts.