Functionally independent components of the late positive event-related potential during visual spatial attention

Human event-related potentials (ERPs) were recorded from 10 subjects presen ted with visual target and nontarget stimuli at five screen locations and r esponding to targets presented at one of the locations. The late positive r esponse complexes of 25-75 ERP average waveforms from the two task conditio ns were simultaneously analyzed with Independent Component Analysis, a new computational method for blindly separating linearly mixed signals. Three s patially fixed, temporally independent, behaviorally relevant, and physiolo gically plausible components were identified without reference to peaks in single-channel waveforms. A novel frontoparietal component (P3f) began at s imilar to 140 msec and peaked, in faster responders, at the onset of the mo tor command. The scalp distribution of P3f appeared consistent with brain r egions activated during spatial orienting in functional imaging experiments . A longer-latency large component (P3b), positive over parietal cortex, wa s followed by a postmotor potential (Pmp) component that peaked 200 msec af ter the button press and reversed polarity near the central sulcus. A fourt h component associated with a left frontocentral nontarget positivity (Pnt) was evoked primarily by target-like distracters presented in the attended location. When no distracters were presented, responses of five faster-resp onding subjects contained largest P3f and smallest Pmp components; when dis tracters were included, a Pmp component appeared only in responses of the f ive slower-responding subjects. Direct relationships between component ampl itudes, latencies, and behavioral responses, plus similarities between comp onent scalp distributions and regional activations reported in functional b rain imaging experiments suggest that P3f, Pmp, and Pnt measure the time co urse and strength of functionally distinct brain processes.