Asymmetric hemispheric contribution to ERPs in associative memory indexes goal relevance and quantity of information

Explicit encoding requires humans to select the information relevant to their goals, yet not all irrelevant information is discarded. The present study addressed how different quantity and relevance of information modulate the electrophysiological activity during the encoding and retrieval phases of a recognition memory task. Subjects learned associations between two semantically unrelated pictures, and then performed a recognition judgment. After recognition, subjects were asked to recall the associated picture by using the recognized image as a cue. Cues yielded either high quantity of information (the cue evoked two associations, only one of them being relevant to the task), or low quantity of information (the cue evoked a single, relevant association). At encoding, a negative peak (400 ms) showed reduced negativity at left sites for the associative trials compared to the non-associative ones, while at right frontal sites the peak was more negative for goal-unrelated associations, compared to goal-related ones. Late right negativity during the test phase (800-1000 ms) discriminated hits followed either by correct or by no recall, but only when the cue evoked multiple associations. Frontal electrophysiological asymmetry at encoding was affected by the behavioral goal, i.e. activity reflected goal-related encoding on the left and goal-unrelated encoding on the right. The late right effect at retrieval suggests a link between this activity during encoding and the evaluation of the higher quantity of information in light of the behavioral goal during retrieval. Overall, the results indicate that different mechanisms and/or neuronal populations are involved in goal-related versus goal-unrelated association.