Graphesthesia on human fingernails.

The fingernails have been considered passive support structures that enhance tactile sensitivity by providing a rigid background, preventing the fingertips from slipping when interacting with objects. Recent studies have provided evidence that the people can perform basic tactile judgments of stimuli applied to the fingernails. In this study, we investigate whether fingernails also contribute to more complex, higher-order spatial processing. Specifically, we examine graphesthesia - the ability to recognise shapes or letters traced onto the skin - and assess whether this tactile capacity extends to the fingernail. Participants were asked to discriminate between the letters b, d, p, and q drawn either on the left middle fingertip or fingernail. Results showed that graphesthetic performance was lower on the fingernail compared to the fingertip. However, participants were still able to classify letters drawn on the fingernail at levels significantly above chance. These findings align with theoretical claims that the fingernail plays a role in perception and expand the growing body of evidence supporting its perceptual functions highlighting the fingernail as an active contributor to complex spatial processing in touch perception.