FINE-GRAINED ANALYSIS OF FUNCTIONAL ACTIVATION OF SUBCORTICAL STRUCTURES DURING ACTION OBSERVATION, MOTOR IMAGERY AND EXECUTION

Action observation, motor imagery and action execution activate a common premotor-parietal network. Recent studies suggested that also subcortical structures are activated during these tasks. However, few fMRI studies included all three conditions within the same paradigm, employing acquisition with a voxel size ≥3mm. This 3T fMRI study was aimed at investigating, in the same participants, using a fine-grained analysis of functional activation (high spatial resolution: voxel size=1.75 mm), which subcortical structures are active during observation, imagery, and execution of goal-directed grasping actions. Twenty-five participants underwent six conditions: passive observation of goal-directed reaching-grasping actions (OBS ACTION); passive observation of static hand-object images; motor imagery of a reaching-grasping action on the objects (IMA MOT); passive objects observation; execution of reaching-grasping of a sphere (EXE GRASP); execution of hand opening-closing movement. Subcortical activations were assessed in thalamus, basal ganglia, red nucleus (RN) and dentate nucleus (DN). OBS ACTION engaged bilateral pulvinar and left MD/VLP/VPL thalamic nuclei, left putamen and GPe, and bilateral DN. IMA MOT activated bilateral caudate, putamen and left GPe, with left pulvinar involvement. EXE GRASP engaged bilateral pulvinar and CM/MD/VA/VLP/VPL nuclei, with right caudate, bilateral putamen, GPe, left substantia nigra, right DN, left RN. In conclusion, motor imagery engages basal ganglia, whereas action observation and execution additionally recruit specific sectors of thalamus, DN and RN. This evidence supports the hypothesis of different cortico-subcortical networks involved in performing motor simulation during action observation (DN), processing visual information/involving motor resonance mechanisms (thalamus), and inhibiting unwanted motor outcomes during motor imagery (basal ganglia).