Activation of frontal premotor areas during suprathreshold transcranial magnetic stimulation of the left primary sensorimotor cortex: A glucose metabolic PET study

We employed cerebral ¹⁸Fluoro-deoxyglucose positron emission tomography ([¹⁸F]FDG-PET) to visualize neuronal activation of the frontal motor and premotor cortex during suprathreshold repetitive transcranial magnetic stimulation (rTMS) applied to the left primary sensorimotor hand area (SM1_HAND). Twelve right-handed normal subjects underwent two [¹⁸F]FDG-PET measurements at baseline without rTMS and during suprathreshold 2 Hz rTMS of the left SM1_HAND. In the rTMS condition, 1,800 magnetic stimuli at an intensity of 140% of motor-resting threshold were delivered immediately after intravenous injection of [¹⁸F]FDG. Relative differences in the normalized regional cerebral metabolic rate for glucose (rCMRglc) between the rTMS condition and baseline were determined using a voxel-by-voxel Student's t-test and a volume-of-interest analysis. Data analysis was a priori restricted to primary motor and premotor areas in the frontal cortex, namely the SM1, the supplementary motor area (SMA), the lateral premotor cortex (PMC), and the caudal anterior cingulate cortex (ACC) of either hemisphere. In addition to a relative increase in normalized rCMRglc in the stimulated SM1_HAND, suprathreshold rTMS was associated with well-localized increases in normalized rCMRglc in the caudal SMA and ACC on the medial wall of the frontal cortex and in the right precentral gyrus in the lateral PMC rostrally to the SM1. These data demonstrate that a selective activation of the SM1_HAND is paralleled by an activation of a distinct set of remote premotor areas, suggesting a functional interaction between the primary motor and premotor cortex in humans.