Recent evidence indicates that the processing of a stimulus can be influenced by preceding patterns of brain activity. Here we examine whether prestimulus oscillatory brain activity can influence the ability to retrieve episodic memories. Neural activity in the theta-frequency band (4-8 Hz) was enhanced before presentation of test items which elicited accurate recollection of contextual details of the prior study episode ("source retrieval"), relative to trials for which item recognition was successful but source retrieval failed. Poststimulus theta activity was also related to source retrieval, and the magnitude of poststimulus theta was predicted by the magnitude of the prestimulus theta effects. The results suggest that ongoing neural processes occurring before stimulus onset might play a critical role in readying the brain for successful memory retrieval.EEG | oscillation R esearch on the neural basis of human memory has generally proceeded from the assumption that memory retrieval is driven by incoming stimuli that cue recovery of past experiences. However, recent research suggests that fluctuations in neural activity prior to stimulus presentation can also play a critical role in determining how a stimulus will be processed (1-4). For example, synchronous electrophysiological activity observed just before stimulus presentation has been found to modulate behavior on psychophysical, conditioning, motor, and attentional tasks (2,(5)(6)(7)(8). It is unclear, however, whether other cognitive functions such as episodic memory retrieval are influenced by prestimulus neural activity.Previous research has shown that cueing subjects to retrieve from memory leads to sustained changes in neural activity (9-14). These results indicate that brain networks are sensitive to the intention to retrieve from memory, but it is not known whether this neural activity has any impact on memory retrieval. Evidence from event-related potential (ERP) studies indicates that the intention to retrieve might not be related to successful memory retrieval (13,15). However, an examination of oscillatory brain activity, rather than sustained activity, may prove more useful in revealing such retrieval effects. Computational models suggest an important role for theta oscillations (4-8 Hz) in memory functions (16), particularly in the rodent hippocampus (17-19), a region which has been shown to also be particularly important for humans in source recognition (20). In addition, results from studies using scalp and intracranial electroencephalography (EEG) suggest that cortical theta activity during stimulus presentation may be correlated with successful episodic memory (21-23), and that theta oscillations before initial stimulus presentation are associated with more effective encoding (24).Here we examine whether neural activity that occurs before the presentation of a retrieval cue would predict whether the item would elicit successful episodic retrieval (Fig. 1). Seventeen healthy college-aged subjects encoded a series of words by making pleasa...

Recollection is typically associated with high recognition confidence and accurate source memory. However, subjects sometimes make accurate source memory judgments even for items that are not confidently recognized, and it is not known whether these responses are based on recollection or some other memory process. In the current study, we measured event related potentials (ERPs) while subjects made item and source memory confidence judgments in order to determine whether recollection supported accurate source recognition responses for items that were not confidently recognized. In line with previous studies, we found that recognition memory was associated with two ERP effects: an early on-setting FN400 effect, and a later parietal old-new effect [Late Positive Component (LPC)], which have been associated with familiarity and recollection, respectively. The FN400 increased gradually with item recognition confidence, whereas the LPC was only observed for highly confident recognition responses. The LPC was also related to source accuracy, but only for items that had received a high confidence item recognition response; accurate source judgments to items that were less confidently recognized did not exhibit the typical ERP correlate of recollection or familiarity, but rather showed a late, broadly distributed negative ERP difference. The results indicate that accurate source judgments of episodic context can occur even when recollection fails.

In several previous behavioral studies, we have identified a group of amnestic patients that, behaviorally, appear to exhibit severe deficits in recollection with relative preservation of familiarity-based recognition. However, these studies have relied exclusively on behavioral measures, rather than direct measures of physiology. Event-related potentials (ERPs) have been used to identify putative neural correlates of familiarity- and recollection-based recognition memory, but little work has been done to determine the extent to which these ERP correlates are spared in patients with relatively specific memory disorders. ERP studies of recognition in healthy subjects have indicated that recollection and familiarity are related to a parietal old-new effect characterized as a late positive component (LPC) and an earlier mid-frontal old-new effect referred to as an ‘FN400’, respectively. Here, we sought to determine the extent to which the putative ERP correlates of recollection and familiarity are intact or impaired in these patients. We recorded ERPs in three amnestic patients and six age matched controls while they made item recognition and source recognition judgments. The current patients were able to discriminate between old and new items fairly well, but showed nearly chance-level performance at source recognition. Moreover, whereas control subjects exhibited ERP correlates of memory that have been linked to recollection and familiarity, the patients only exhibited the mid-frontal FN400 ERP effect related to familiarity-based recognition. The results show that recollection can be severely impaired in amnesia even when familiarity-related processing is relatively spared, and they also provide further evidence that ERPs can be used to distinguish between neural correlates of familiarity and recollection.

The hippocampus has traditionally been thought to be critical for conscious explicit memory but not necessary for unconscious implicit memory processing. In a recent study of a group of mild amnesia patients with evidence of MTL damage limited to the hippocampus, subjects were tested on a direct test of item recognition confidence while electroencephalogram (EEG) was acquired, and revealed intact measures of explicit memory from 400–600ms (mid-frontal old-new effect, FN400). The current investigation re-analyzed this data to study event-related potentials (ERPs) of implicit memory, using a recently developed procedure that eliminated declarative memory differences. Prior ERP findings from this technique were first replicated in two independent matched control groups, which exhibited reliable implicit memory effects in posterior scalp regions from 400–600 msec, which were topographically dissociated from the explicit memory effects of familiarity. However, patients were found to be dramatically impaired in implicit memory effects relative to control subjects, as quantified by a reliable condition × group interaction. Several control analysis were conducted to consider alternative factors that could account for the results, including outliers, sample size, age, or contamination by explicit memory, and each of these factors were systematically ruled out. Results suggest that the hippocampus plays a fundamental role in aspects of memory processing that is beyond conscious awareness. The current findings therefore indicate that both memory systems of implicit and explicit memory may rely upon the same neural structures – but function in different physiological ways.