Effects of Ultra-short Offline States on Item-level Verbal Memory

Memory is consolidated during "offline" states of reduced attention to the external stimulus environment, including sleep and resting wakefulness. While consolidation is usually studied on a timescale of hours to days, accumulating evidence suggests that organisms also transiently go offline for very short durations during wakefulness, on a timescale of seconds. Here, we asked how going offline during the intertrial intervals of a verbal memory task affects memory. Participants encoded eight lists of 20 words, during which we continuously recorded high-density EEG, pupillometry, RTs, and occasional reports of participants' subjective focus of attention. Using these multimodal data, we applied our previously published methods to define whether participants were "online" or "offline" just prior to and following the time each individual word was presented. As in prior studies, we found evidence that participants fluctuated between three distinct states during the task, which we term "online" (high EEG delta and theta, larger pupil diameter), "offline1" (high EEG alpha, smaller pupil diameter), and "offline2" (high EEG slow oscillation, smaller pupil diameter). As anticipated, encoding was superior when participants were online just prior to word presentation. But contrary to our hypotheses, entering an offline state just after word presentation did not improve memory. These observations are discussed in light of memory consolidation theory, including recent evidence of "micro-offline" consolidation in the motor domain.