Altered Motor Resonance Effects during Action Observation in Functional Weakness: A Transcranial Magnetic Stimulation Study

Background: Motor resonance, driven by the mirror neuron system, activates brain regions during action observation similarly to execution. Pattern of activation has been shown to be driven by the motor repertoire in conditions like Parkinson's disease and dystonia, but remains unexamined in functional weakness, a common phenotype of functional motor disorder. Objectives: To investigate motor resonance by assessing corticospinal excitability modulation in patients with upper limb functional weakness during the observation of actions within their motor repertoire versus actions no longer executable due to their disorder. Methods: In this exploratory study, 19 adult patients with functional weakness and 21 healthy controls observed videos of hand movements-index finger abduction and power grip-performed with strong or weak force. Single-pulse transcranial magnetic stimulation targeted the primary motor cortex, with motor-evoked potentials recorded from first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles. Results: In healthy controls, corticospinal excitability increased significantly in the FDI during strong abduction compared to "static" (P = 0.013) and "weak" (P = 0.027) conditions, and during both "weak" (P = 0.002) and "strong" (P = 0.032) power grips compared to baseline. Conversely, patients with functional weakness showed no significant modulation across any condition. Compared to controls, patients exhibited reduced excitability during strong actions in both FDI (abduction; P = 0.03) and ADM (power grip; P = 0.029). Conclusions: Functional weakness patients demonstrated altered force-dependent motor resonance, suggesting a disruption in the motor system's ability to internally simulate actions, particularly those requiring high muscular contraction. These findings may inform future rehabilitation strategies leveraging action observation in functional motor disorders.