Laser pain fails to inhibit innocuous-related activity in the central somatosensory pathways

Our recent work suggests that innocuous somatosensory activity elicited by a brief electrical stimulus is inhibited by pain evoked by the same electrical stimulus but not by pain evoked by continuous heat. These results led to the hypothesis, tested in the present experiment, namely that pain only inhibits innocuous somatosensation when the painful and innocuous stimuli have short durations and close temporal and spatial overlap. A painful 200-ms laser pulse did not produce a decrease in the perceived magnitude of the innocuous electrical stimulus or in the amplitude of a scalp potential that earlier work suggested is generated by neurons involved in innocuous somatosensation. Hence, the pain-related inhibition of innocuous somatosensation observed in these electrical studies cannot be attributed solely to the duration or to the temporal and spatial overlap of the innocuous and painful stimuli. Laser pain did produce a small increase in the subjective magnitude of the innocuous electrical stimulus, which the electrophysiological evidence suggests might be due to a change in the late- but not the midlatency responses in the central innocuous somatosensory pathways.