Courchesne, Hillyard, and Galambos (1975) introduced a new class of infrequent, unexpected, and task-irrelevant stimuli into a visual oddball task (recognizable geometric shapes and novel, unfamiliar patterns). Although targets and irrelevant shapes both resulted in a posterior P3, novel stimuli produced a prominent, early frontal P3, preceded by a large N2. Within the context of an auditory oddball task, an increase in the salience of a class of stimuli (e.g., incidence probability or uniqueness) also elicits an early (233–278ms), frontocentral positivity (P3a), even if the stimuli are ignored (Squires et al., 1975). Knight (1984) used a variation in which an irrelevant novel sound (a dog bark) was presented, and observed and enhanced, shorter-latency N2 (Cz maximum), followed by a frontocentral P3 that decreased over the first five presentations. Patients with frontal lesions showed neither the enhanced N2 nor the frontocentral P3.
