Twenty experiments involving an intramodality somatosensory selective attention tasks were carried out in 12 normal adult motivated subjects. A random sequence of subjectively equivalent electrical stimuli was delivered to 4 fingers in the two hands. The intervals between shocks varied at random between 0.5 and 15 sec (mean 6 sec) in any one finger. The subject was instructed to mentally count the target shocks in a designated finger which was different in alternate runs. The target shocks about 1 mA above subjective threshold elicited large P400 components in the SEP, whereas non-target shocks elicited little, if any, P400. The somewhat more complex findings when strong shocks were delivered according to the same random programme have been discussed. With sequences of weaker shocks, the unitary P400 phenomenon could be studied in single trial SEPs in the best subjects. The early components N20 and P45 of the SEP were not affected by the task conditions, and this was also true for the N140 component. The large P400 to target shocks were of the same voltage at symmetrical parietal locations corresponding to the hand projections.
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