Mapping somatosensory evoked potentials to finger stimulation at intervals of 450 to 4000 msec and the issue of habituation when assessing early cognitive components.

Somatosensory evoked potentials (SEPs) to mild electric stimulation of two fingers of the left hand were studied at regular interstimulus intervals (ISIs) of 450, 800, 1400, 2500 and 4000 msec. Habituation was evaluated while the subject was reading a novel so as to virtually ignore the finger stimuli while maintaining steady vigilance levels. Brain SEPs recorded from 25 scalp electrodes were assessed by scatter displays, electronic subtraction, bit-mapped potential fields, and by calculating the Z estimator and dilation factor. Similar results were obtained with randomly varying ISIs. The P14 farfield and cortical N20 did not change with ISIs. The parietal P27-P45 decreased at ISIs of 800 and 450 msec, but showed no significant habituation at ISIs of 1400, 2500 or 4000 msec. This validated the control conditions used for assessing the early cognitive P30 and P40 to attended target stimuli. The frontal N30 also decremented the shorter ISIs but still habituated up to ISIs of 2500 msec. The clear dissociation between frontal N30 and parietal P27 at the larger ISIs suggests that they involve at least in part distinct neural generators.