Sensory and movement-related cortical potentials in nociceptive and auditory reaction time tasks.

The processing of a sensory stimulus leading to a simple motor command was studied with scalp-recorded long latency cortical potentials in humans. Two sensory modalities were tested in their ability to activate descending motor pathways: auditory stimuli and painful cutaneous stimuli produced by a CO2 laser. Subjects were asked to react to stimuli with voluntary index finger movements. The stimulus-related and movement-related cortical potentials were recorded simultaneously with five midline electrodes on the scalp. The auditory reaction time, measured from the stimulus to the onset of electromyogram (EMG), was faster (150 ms) than the laser reaction time (350 ms). The onset of EMG of finger movements occurred only after the first negative components following auditory or laser stimuli but before the positive components. The latency from the auditory negativity to the onset of EMG was about 50 ms and the latency from the laser negativity to the onset of EMG was about 110 ms. This finding indicates that not only the peripheral afferent conduction but also central processing takes longer in a pain-related somatosensory task than in an auditory task. The frontal peak of Motor Potential (fpMP), a cortical potential related to the sensory feedback from movement, occurred with a constant latency after the onset of EMG (100 ms) and was unaffected by the task.