The current study investigates whether changes in scalp electroencephalographic activity over time reflect the effects of target detection and divided attention on memory encoding. We recorded electroencephalographic activity in 61 young adults as they memorized lists of words either under full attention (single-task) or while performing a secondary task (dual task). In both cases, colored squares appeared with each word. However, in the dual-task condition, participants also pressed a button when the colored squares were in a predefined color (target) but made no response when the squares were in a different color (distractor). Subsequent memory effects in the alpha (8–12 Hz) and high gamma (50–100 Hz) frequency bands changed throughout the trial, and these effects differed across conditions. Before word presentation, high gamma activity was associated with encoding success in the target and single-task conditions, but not in the distractor conditions. In contrast, alpha band activity decreased following word presentation, and these decreases were greater for successfully encoded words in the target condition than in the distractor or single-task conditions. The results are consistent with the view that alpha and gamma activity reflect distinct neural processes, which both contribute to memory formation, but are differentially sensitive to task demands and momentary shifts in attention.

Observers spontaneously segment larger activities into smaller events. For example, “washing a car” might be segmented into “scrubbing,” “rinsing,” and “drying” the car. This process, called event segmentation, separates “what is happening now” from “what just happened.” In this study, we show that event segmentation predicts activity in the hippocampus when people access recent information. Participants watched narrative film and occasionally attempted to retrieve from memory objects that recently appeared in the film. The delay between object presentation and test was always 5 sec. Critically, for some of the objects, the event changed during the delay whereas for others the event continued. Using fMRI, we examined whether retrieval-related brain activity differed when the event changed during the delay. Brain regions involved in remembering past experiences over long periods, including the hippocampus, were more active during retrieval when the event changed during the delay. Thus, the way an object encountered just 5 sec ago is retrieved from memory appears to depend in part on what happened in those 5 sec. These data strongly suggest that the segmentation of ongoing activity into events is a control process that regulates when memory for events is updated.