Stimulus and response related components of the late positive complex in visual discrimination tasks.

Visual event-related potentials (ERP) were recorded from 6 subjects aged 10--15 in response to target stimuli in 2 vigilance tasks which differed in their processing requirements. Stimulus- and response-synchronized averages were computed for each subject for each quartile (Q) of that subject's reaction time (RT) distribution. Late positive complexes (LPC) of identical morphology were seen in the brain potentials from each task. In Q1, the LPC was an undifferentiated, large amplitude, positive-going potential with a parietal-maximum topography. As RT increased, the LPC became increasingly differentiated so that by Q3 3 components in the latency range of P300 were visible: two constant-latency peaks, P286 (fronto-central), P341 (parietal); and one variable-latency deflection, P539 (parietal), whose peak latency lengthened as RT increased. An additional constant-latency peak, P198, was also recorded. Principal component analyses of the averaged wave forms confirmed the visual identification of these components. The fixed-latency peaks identified in the brain potentials of these subjects were similar in timing and topography to previously reported late positive components. This study demonstrated. however, that the LPC can be segregated into stimulus-related and response-related components.