ELSC Seminar Series

Humans skillfully use their fingers to perform most daily tasks, more so than any other animal species. This unique dexterous ability, primarily involving the flexion and extension directions, is a product of the complex anatomical properties of the human hand and the neural mechanisms that control it. Although several specialized neural circuits for control of aspects of dexterity have been identified, the neural basis that underlies flexion vs. extension dexterous movement remains unclear. Here, in a study on both healthy subjects and stroke patients, we first characterized the main components of finger motor function in terms of dexterity (i.e., individuation) and strength, isolated the passive peripheral mechanical coupling component from the central neuromuscular origin involved in dexterity and then applied lesion-based mapping in sub-acute stroke patients to investigate the neural origin leading to the control deficit in each of the behavioral core components. We found that across all digit combinations (single and multi-finger movements) and a range of force levels, finger independence during extension was substantially limited compared to finger flexion. We ruled out the possibility that mechanical coupling of the digit extensors explains the reduced individuation in the extension direction compared to flexion. Lastly, we identified selective cortical structures, originating from descending tracts of the primary motor cortex and premotor areas, involved in independence during finger flexion and extension. Despite the remarkable overlap across digits between flexion and extension within these structures, we found that independence of finger movements in the flexion direction is associated with larger, yet distinct, cortical substrates, mostly in premotor areas. This flexion-based differential premotor cortical organization was exclusively associated with the finger individuation component, but not with finger strength. From these results, we inferred a potential neuroanatomical structure that underlies the dissimilar control between finger flexion and extension in human dexterity.