Mapping the involvement of BA 4a and 4p during Motor Imagery.

Motor Imagery (MI) is an attractive but intriguing means to access the motor network. There are marked inconsistencies in the functional imaging literature regarding the degree, extent and distribution of the primary motor cortex (BA 4) involvement during MI as compared to Executed Movement (EM), which may in part be related to the diverse role of BA 4 and its two subdivisions (i.e., 4a and 4p) in motor processes as well as to methodological issues. Here we used fMRI with monitoring of compliance to show that in healthy volunteers optimally screened for their ability to perform MI the contralateral BA 4 is involved during MI of a finger opposition sequence (2, 3, 4, 5; paced at 1 Hz), albeit less than during EM of the same sequence, and in a location sparing the hand area. Furthermore, both 4a and 4p subdivisions were found to be involved in MI, but the relative involvement of BA 4p appeared more robust and closer to that seen with EM. We suggest that during MI the role of BA 4 and its subdivisions may be non-executive, perhaps related to spatial encoding, though clearly further studies are needed. Finally, we report a similar hemispheric activation balance within BA 4 with both tasks, which extends the commonalities between EM and MI.