Posterior parietal negativity preceding self-paced praxis movements.

Studies of movement-related cortical potentials (MRCPs) for simple movements have shown a slowly rising negativity (Bereitschaftspotential, or BP) about 2 s prior to movement onset, centered in the bilateral sensorimotor area. However, complex movements may elicit a different temporal and spatial distribution of this pre-movement activity. In this study, 64-channel electroencephalography (EEG) was recorded while normal volunteers were asked to perform a simple thumb adduction once every 10--15 s for three 10--15 min blocks. Following this, they were asked to make tool-use movements (hammer, scissor, and screwdriver pantomime) in the same manner. Surface electromyography (EMG) was recorded on the thumb adductor and forearm flexor. MRCP was analyzed for the beginning part of the epoch (from 3.5 s to 1.5 s before EMG onset, with 0.5 s time bins) for differences in the amplitude and spatial distribution of the BP. Significant differences were seen from 3.0 s to 2.0 s before EMG onset, where the amplitude was greater for the more complex movements. On average, negativity began at 3.0 s before onset for praxis movements, and only 1.7 s before onset for thumb adduction. Additionally, the negativity seen for the complex movements had a distribution beginning over the left hemisphere posterior parietal area, whereas, thumb adduction movements had a more anterior distribution, over the bilateral sensorimotor area. The posterior parietal negativity (PPN) suggests that early parietal activity is essential for tool-use movements and is not a part of preparing simple movements.