Reduced P3b category response in prefrontal patients.

Several studies have implicated a frontal/prefrontal contributor to P3b. We hypothesized that prefrontal involvement may be necessary to the processing of novel contextual information, as suggested by selective reduction of P3a in prefrontal patients. ERPs were recorded from 12 controls and 12 unilateral prefrontal lesion patients. In two target-detection tasks, subjects/patients viewed randomly-ordered stimuli drawn from differentially probable categories: numbers (standard, 90%) and letters (deviant, 10%). Topographic and midline analyses were applied to examine target and category effects on P3b. Low-probability targets evoked distinct P3bs in each group. Control subjects' P3bs were additionally enhanced to deviant targets and nontargets. Patients' P3s were enhanced to deviant nontargets but were unaffected by deviant targets. This is consistent with the view that target- and nontarget-evoked P300s are functionally distinct, and suggests that a prefrontal contributor to P3b supports attention to contextual attributes among task-relevant stimuli.