Finger interactions studied with transcranial magnetic stimulation during multi-finger force production tasks

Objective: We used transcranial magnetic stimulation (TMS) to investigate interactions among neuromuscular complexes involved in force production by individual fingers of a hand during single- and multi-finger tasks. Methods: Subjects were asked to press with the fingertips at various levels of force using different finger combinations. TMS was applied over the M1 cortical hand area during constant force production. TMS-induced increments in fingertip forces were analyzed, as well as motor evoked potentials (MEPs) in flexor digitorum superficialis. Results: Both MEP size and individual force increments had inverted U-shaped dependences on the background force (peak responses were seen at about 50% of the maximal force). Similar relationships were obtained when subjects were asked to produce different forces with the same finger combination or the same total force with different finger combinations. The relationships were similar when the force was produced by explicitly instructed fingers or by other fingers of the hand. Effects of TMS on the force of a finger showed a strong dependence on the background force produced by this finger and minimal or no dependence on forces produced by other fingers of the hand. Conclusions: Overall, TMS applied over M1 revealed little interaction among fingers. This supports the notion of digit-specific compartments in multi-digit extrinsic muscles of the hand and suggests that these compartments possess a high degree of physiological independence.