ERPs during continuous recognition memory for words.

Event-related brain potentials (ERPs) were recorded from 10 young adults during a version of the continuous recognition memory paradigm. Words were presented after lags of either 2, 8 or 32 intervening items (equiprobable) following their first presentation, and subjects were required on each trial to make a choice: new (never presented previously) or old (previously presented) response. To assess the effect of probability of new to old items, words were presented in separate blocks with ratios of new to old of 2:1 and 1:1. Reaction time increased and successful recognition decreased systematically as the lag between first and second presentations of an item increased, supporting the distinction between primary (immediate memory) and secondary memory for verbal material. However, there were no systematic effects of item lag on the ERP components. ERPs to new items were characterized by larger N300 and smaller P300 amplitudes (from about 250 to 700 ms) than those to old items. These amplitude differences between old and new ERPs were interpreted as primarily reflecting repetition as opposed to semantic priming effects. These old/new effects did not interact with probability, suggesting that frequency of occurrence is not a major determinant the ERP old/new difference. Old items elicited a late negativity following the behavioral response that was interpreted as due to the presence of a "positive slow wave," with a frontally oriented distribution to new words that was absent in the ERPs to old words. Similarly, subtraction of ERPs elicited by new items that were subsequently unrecognized from those subsequently recognized, showed that underlying the ERP subsequent "memory effect" was a "frontal positive slow wave," dissociable from P300 on the basis of differences in scalp distribution. Since positive slow wave has been interpreted as reflecting "further processing," the data suggest that such processing, possibly similar to elaboration (Graf & Mandler, 1984), enhanced the probability of subsequent recognition.