Evoked brain potentials and memory: more positivity in response to forgotten items

Brain evoked potentials (EPs) were recorded in human subjects participating in a free recall memory task involving retroactive interference. A learning list was presented five times. The fifth repetition was followed by an interference list. Both lists were composed of either words or abstract figures, and each subject experienced each of four possible combinations. Analysis of items recalled during the learning phase revealed larger N400 and P600 amplitudes for those items that were later forgotten vs remembered following the interference. This contradicts the usual finding that more positivity is associated with better memory. However, both the present as well as the extant findings can be explained in terms of cognitive resource allocation. Specifically, items receiving greater allocations are more likely to be immediately recalled. However, as the number of items working memory increases, the allocation required to add new items also increases. Thus, items learned on later trials would receive larger allocations (i.e., larger positivities) than items learned earlier, yet would be more likely forgotten following the interference because their presence in memory would not be reinforced during later trials, as is the case with items learned earlier.