Cerebral circuits involved in preferential hand posture representation for action

Objective: The thumb-down/fingers-up (standard) posture was shown to be the configuration in which the hand is preferentially represented by the brain. This supra-modal representation results in improved sensorimotor performances. This study aims at identifying the cortical circuit responsible for this advantage. Methods: Different TMS coil orientations (antero-posterior, latero-medial, postero-anterior) − activating different cortical circuits- have been used to test cortico-spinal and cortico-cortical connections in different postures. Additionally, pair-pulse TMS protocols were used to explore inhibitory and facilitatory mechanisms within the primary motor cortex (M1) that might contribute to the observed postural advantage. Results: A significant corticospinal excitability difference in nMEPs between standard and inverse posture was observed with antero-posterior TMS (p = 0.003), activating cortico-cortical connections from premotor cortex (PM) and with postero-anterior TMS (p = 0.042), favoring local cortical connections. Latero-medial stimulation, activating direct corticospinal connections, did not show a preference. A MEPs analysis further revealed a significant effect exclusively for antero-posterior stimulation (p < 0.003). Intracortical facilitation and inhibition were unaffected by hand postures. Conclusions: The postural advantage of the thumb-down/fingers-up hand configuration is associated with a neural circuit involving PM cortex projecting to M1, activated by antero-posterior and postero-anterior TMS. Significance: The corticospinal evoked activity to different TMS protocols gave cues on the neural basis of body representation and favored posture.