Hand use affects somatotopy, but does somatotopy affect hand performance?

Cortical somatotopy, the point-for-point mapping between the body and its cortical representation, is a prominent feature of the primary somatosensory cortex, where nearby body parts are represented adjacently. The cortical map of the hands has been studied for decades, but our comprehension of the interplay between map organisation and hand use remains limited. In this thesis, we first examine a theory regarding the behavioural relevance of somatotopy, namely the intracortical inhibition theory. This theory states that engagement of the hand somatotopy in choice reaction time (RT) tasks leads to longer RTs for specific digits. Instead, by manipulating the visual representation of the stimuli and the digits involved in the tasks, our results argue that RT differences across digits are predominantly influenced by stimulus presentation rather than cortical somatotopy. While digit-specific RT patterns, as implied by intracortical inhibition theory, are observed with a novel effector set to an extent, we attribute this observation to the anatomical identity of the digits rather than their somatotopic positions. Moreover, we explore other possible factors with further experiments, which reveal the facilitating roles of the hand figures and spatial clues in the stimuli. In the second part of this thesis, we investigate the other aspect of the interplay – how hand use may affect the organisation of hand somatotopy. Previous studies assume that Hebbian plasticity mechanisms drive the somatotopic reorganisation following alterations in hand use such as amputation, while newer theorists argue for the key role of homeostatic mechanisms. We implement computational models to simulate the separate contributions of these mechanisms in somatotopic reorganisation, which supports the hypothesis of homeostatic mechanisms as regulators against Hebbian plasticity. Overall, this thesis rejects the theory that somatotopic organisation affects the choice RT of the digits while computationally reproducing somatotopic reorganisation after changes in hand use.