Effects of choice complexity on different subcomponents of the late positive complex of the event-related potential.

The effects of choice complexity on different subcomponents of the late positive complex were investigated. In a previous choice reaction study, two subcomponents of this complex were identified, called P-SR and P-CR, which seem to be related to stimulus evaluation and response selection, respectively. The present study attempts to show the dependence of the P-CR (and the independence of the P-SR) on response selection by manipulating response selection complexity. This was done by having the subjects perform either 2-way or 4-way choice reactions to single letter stimuli. To enhance the discriminability of P-SR and P-CR, visual and auditory stimuli were used, since the P-SR is modality-dependent. Moreover, the stimulus modalities were mixed ("divided attention paradigm"), which was expected to lead to a dissociation of P-SR and P-CR, especially after auditory stimuli. The choice reaction times were about 100 msec longer for difficult than for easy choices. The main ERP result was a 65 msec increase of the P-CR latency for the difficult as compared to the easy choice, while the P-SR latency remained constant. The P-CR latency difference precisely matched the onset difference of the lateralized readiness potential. The P-SR showed a modality-dependent latency and topography, while the P-CR did not. The present data confirm the close relation of one subcomponent of the late positive complex, the P-CR, to the cognitive response-selection process.