Transfer of tactile perceptual learning to untrained neighbouring fingers reflects natural use relationships.

Tactile learning transfers from trained to untrained fingers in a pattern that reflects overlap between the representations of fingers in the somatosensory system (e.g. neurons with multi-finger receptive fields). While physical proximity on the body is known to determine the topography of somatosensory representations, tactile co-activation is also an established organising principle of somatosensory topography. Here we investigated whether tactile co-activation, induced by habitual inter-finger cooperative use (use pattern), shapes inter-finger overlap. To this end, we used psychophysics to compare the transfer of tactile learning from the middle finger to its adjacent fingers. This allowed us to compare transfer to two fingers that are both physically and cortically adjacent to the middle finger, but have differing use-patterns. Specifically, the middle finger is used more frequently with the ring than with the index finger. We predicted this should lead to greater representational overlap between the former than the latter pair. Further, this difference in overlap should be reflected in differential learning transfer from the middle to index versus ring fingers. Subsequently, we predicted temporary learning-related changes in the middle finger's representation (e.g. cortical magnification) would cause transient interference in perceptual thresholds of the ring, but not the index finger. Supporting this, longitudinal analysis revealed a divergence where learning transfer was fast to the index finger, but relatively delayed to the ring finger. Our results support the theory that tactile co-activation patterns between digits affect their topographic relationships. Our findings emphasise how action shapes perception and somatosensory organisation