Effects of interstimulus interval on cortical responses to painful laser stimulation.

Short laser pulses applied to the skin are used increasingly in both clinical and basic assessment of nociceptive brain mechanisms. The authors aimed to characterize further the cortical responses to noxious laser stimuli and to define the interstimulus interval (ISI) for the optimum signal-to-noise ratio during a fixed measurement time. Three hundred six-channel whole-scalp magnetoencephalographic (MEG) and midline EEG signals were recorded from nine healthy adults during painful thulium laser stimulation. The stimuli were delivered on the dorsum of the left hand at ISIs of 0.5, 1, 2, 4, 8, and 16 seconds. The MEG responses peaked at 160 to 195 msec around the contralateral primary somatosensory (SI) cortex, at 150 to 190 msec in the contralateral secondary somatosensory (SII) cortex, and at 160 to 205 msec in the ipsilateral SII cortex. The simultaneously measured electrical vertex potentials peaked at 190 to 230 msec and 310 to 330 msec (N200-P300). All these responses showed rather similar refractory times: The amplitudes increased strongly from 0.5 to 4-second ISIs and thereafter saturated at ISIs of 8 to 16 seconds. On the basis of the time constants of the recovery cycles, the optimum ISI for obtaining the best signal-to-noise ratio for laser-evoked MEG and EEG responses during a fixed measurement interval is 4 to 5 seconds.