Contributions of the mesial frontal cortex to the premovement potentials associated with intermittent hand movements in humans.

Two premovement potentials, the bereitschaftspotential (BP) and negative slope (NS'), can be recorded prior to the execution of self-paced hand movements using back-averaging of scalp electrical recordings. The contributions of the contralateral and ipsilateral primary motor cortex (M1) and the mesial dorsal frontal cortex (MFC) to the generation of the potentials were examined by simultaneously collecting positron emission tomography (PET) scans and scalp recorded electrical activity for dipole source analysis in eight right-handed normal subjects. Subjects performed simple unilateral thumb-finger opposition movements intermittently with an average inter-movement interval of 7.4 s. PET was also collected for the same movement performed repetitively with inter-movement intervals of 0.5 s such that finger movements were nearly continuous. PET studies of the intermittent movement revealed marked activation of the MFC in the region of the rostral supplementary motor area (SMA) and cingulate motor area, contralateral sensorimotor cortex and no activation of the ipsilateral sensorimotor cortex. When the same movements were performed in a continuous repetitive manner, PET revealed strong contralateral sensorimotor and caudal MFC activation, and no ipsilateral sensorimotor or rostral MFC activation. Dipole source solutions of the back-averaged potentials for the intermittent movements were analyzed by testing dipole vectors placed into the regions of PET activation. The premovement potentials were dominated by dipoles in the region of the MFC, with minimal contribution from either the contralateral or ipsilateral M1. Activation in the region of the contralateral M1 began near the onset of muscle activity. The orientation and timing of the MFC dipoles were consistent with both the BP and NS' potentials originating from neurons in the rostral SMA and dorsal tier of the cingulate sulcus and were appropriate for MFC activity to contribute to both the preparation for movement and the descending activation of spinal motor networks.