RARE EVENTS AND THE CNV - THE ODDBALL CNV

Event-related brain potentials (ERPs), such as Nd and P300, change as a function of the proportion of time if a rare, significant, stimulus occurs. This 'oddball' paradigm has had a significant influence on the interpretation of ERPs in terms of thc psychological and information-processing functions they reflect. Interpretations of transient components have differed considerably from that of the sustained potential known as the Contingent Negative Variation (CNV), the latter appearing to reflect proactive rather than reactive brain processes. However, experiments in monkeys indicated that the CNV was also sensitive to the oddball effect. The purpose of this experiment was to replicate that finding in humans. The CNVs and P300s of five male and five female college students were studied under three conditions involving different proportions of two types ot warning stimuli in a cued reaction time task (paired tone and light). The proportions of one of the warning stimuli, with respect lo the total number of trials, were 0.10, 0.30 and 0.50. P300s were larger when elicited by rare warning tones only in the 0.10 condition with maximum P300 amplitude at the parietal site. CNVs at frontal and central areas were larger when the warning cue was the rare event, but there was no effect at the parietal site. These findings may require a rethinking of specific information processing interpretations ot other endogenous ERPs, although the results also indicate that the 'oddball' effect on the P300 and CNV was distinctive in terms of scalp distributions and sensitivity to the manipulation.