Primary Motor Cortex Involvement in a Handedness Recognition Task – Evidence from Transcranial Magnetic Stimulation

Abstract

Motor imagery and motor execution share common neural substrates. While the primary motor cortex is crucial for motor execution, its contribution to motor imagery is still debated. We applied transcranial magnetic stimulation (TMS) to the primary motor hand area (M1-HAND) during a hand recognition task to assess the involvement of M1-HAND in motor imagery of body parts. A schematic picture of the back or palm of a human hand was presented at various stimulus orientations. Subjects were required to indicate whether they thought it was a right or a left hand by pressing a button with their right or left toe as fast as possible. Single-pulse TMS at 120% of resting motor threshold was given to left M1-HAND 0, 200, 400, 600, 800 or 1000 ms after onset of stimulus presentation. Reaction times (RTs) and error rates were measured as was the size of the motor-evoked potential (MEPs) evoked by the TMS pulse in the relaxed right first dorsal interosseous (FDI) muscle. Mean RTs and error rates increased with angle of rotation depending on the actual biomechanical constraints of the hand. TMS had no effect on reaction times or error rates regardless of the relative timing of stimulation. However, the size of the MEPs that it evoked in the right hand was modulated during the reaction. Stimuli that depicted the left hand caused MEP amplitudes to decrease 300–100 ms prior to the response, whereas stimuli indicating the right hand caused MEP amplitudes to increase immediately before the response. These effects were the same for pictures of backs and palms and independent of the angle of rotation. The failure of TMS to affect task performance suggests that M1-HAND is not critically involved in motor imagery of body parts. However, the fact that the amplitude of evoked MEPs varied according to the laterality of the stimulus is compatible with a secondary effect of the task on motor excitability.