Modulation of P300

As is typical in oddball paradigms, all target stimuli in the present study elicited a late positive potential that was identified as P300 on the basis of task characteristics.

Several studies have emphasized the distinction between a fronto-central (“P3a”) and a parietal (“P3b”) component of the P300. The P3a is believed to reflect the activation of brain reactions to unexpected events (“processing of surprise”) and P3b appears to be associated with the task-relevant categorization of oddball stimuli (Donchin, 1981; Verleger, Jaskowski, & Wauschkuhn, 1994). In the present study, the centro-parietal distribution of the P300 identified it as a P3b component. MAny studies suggested that the P3b peak latency may be used as a temporal metric for stimulus evaluation (e.g., McCarthy & Donchin, 1981) and that it is sensitive to categorical decision strategies as well as the difficulty of discriminating targets from nontargets (e.g., Kutas, McCarthy, & Donchin, 1977). Its amplitude is determined by the task difficulty and the variance in the response latency in single trials, the amount of attention resources invested in the task, and design parameters such as the relative frequency of the target or its physical salience (Ducan-Johnson & Donchin, 1982). With this background in mind, we will examine the characteristics of our four tasks as reflected by the latency and amplitude of the P300 elicited by each target type.