Cerebral averaged potentials preceding oral movement.
The "readiness potential" is an event-related potential that shows increasing negativity at vertex and motor strip scalp recording sites prior to voluntary, unilateral limb movements. Though speech involves movement on both sides of the midline, recent recordings of prespeech potentials suggest a pattern of bilateral activation that lateralizes to the dominant hemisphere just prior to the onset of articulatory movement. To determine whether this pattern of dominant hemisphere activation is present prior to a stereotyped, nonspeech movement of the mouth, the averaged potentials preceding a lip protrusion task were recorded at the cranial vertex and over the right and left motor cortex. Results were compared to potentials preceding a right finger extension task performed by the same subjects. Both the finger and the lip movements were initially preceded by slow negative potentials. Prior to the finger extension task, the negative amplitude became greatest over the left motor cortex, contralateral to the side of movement. Prior to the lip protrusion task, the amplitude of the potential remained even over the right and left motor cortices. The results suggest that, for this nonspeech movement of a midline structure, bilateral cortical control takes place. Control of lip movement is apparently not necessarily a dominant hemisphere function, though dominance may become part of the motor control strategy for more complex movements such as those used during speech.