Shadows in the mirror.

Mirror neurons are a class of visuo-motor neurons activated by both the execution and passive observation of object-related actions. Evidence for the existence of mirror neurons in the human brain comes in part from transcranial magnetic stimulation studies showing that observation of an action causes subliminal activation of corresponding corticospinal pathways within the motor system. During daylight and lighted conditions movement is nearly always preceded, accompanied, and followed by shadows. Shadows that are cast as someone observes a biological movement could potentially provide information for action recognition. The objective of this study was to assess the mirror system's ability to resonate with shadowed movements. Primary motor cortex excitability was evaluated here by motor-evoked potentials elicited during single-pulse transcranial magnetic stimulation and recorded from two hand muscles as participants observed a prehensile action performed in two illumination conditions: one in which the observed action was fully illuminated and one in which a moving body part was partially shadowed. It will be shown that overall modulation of the primary motor cortex excitability during action observation is significantly lower for the shadowed with respect to the fully illuminated condition. Processing shadows determines a modulation of corticospinal excitability, suggesting that the mirror system is finely tuned to that visual aspect of biological movements.