Time-resolved neural reinstatement and pattern separation during memory decisions in human hippocampus.

Mnemonic decision-making has long been hypothesized to rely on hippocampal dynamics that bias memory processing toward the formation of new memories or the retrieval of old ones. Successful memory encoding may be best optimized by pattern separation, whereby two highly similar experiences can be represented by underlying neural populations in an orthogonal manner. By contrast, successful memory retrieval is thought to be supported by a recovery of the same neural pattern laid down during encoding. Here we examined how hippocampal pattern completion and separation emerge over time during memory decisions. We measured electrocorticography activity in the human hippocampus and posterior occipitotemporal cortex (OTC) while participants performed continuous recognition of items that were new, repeated (old), or highly similar to a prior item (similar). During retrieval decisions of old items, both regions exhibited significant reinstatement of multivariate high-frequency activity (HFA) associated with encoding. Further, the extent of reinstatement of encoding patterns during retrieval was correlated with the strength (HFA power) of hippocampal encoding. Evidence for encoding pattern reinstatement was also seen in OTC on trials requiring fine-grained discrimination of similar items. By contrast, hippocampal activity showed evidence for pattern separation during these trials. Together, these results underscore the critical role of the hippocampus in supporting both reinstatement of overlapping information and separation of similar events.

Neural temporal context reinstatement of event structure during memory recall.

The transformation of experiences into meaningful events and memories is intertwined with the notion of time. Temporal perception can influence, and be influenced by, segmenting continuous experience into meaningful events. Episodic memories formed from these events become associated with temporal information as well. However, it is less clear how temporal perception contributes to structuring events and organizing memory: whether it plays a more active or passive role, and whether this temporal information is encoded initially during perception or influenced by retrieval processes. To address these questions, we examined how event segmentation influences temporal representations during initial perception and memory retrieval, without testing temporal information explicitly. Using a neural measure of temporal context extracted from scalp electroencephalography in human participants (N = 170), we found reduced temporal context similarity between studied items separated by an event boundary when compared to items from the same event. Furthermore, while participants freely recalled list items, neural activity reflected reinstatement of temporal context representations from the study phase, including temporal disruption. A computational model of episodic memory, the context maintenance and retrieval (CMR) model, predicted these results, and made novel predictions regarding the influence of temporal disruption on recall order. These findings implicate the impact of event structure on memory organization via temporal representations, underscoring the role of temporal information in event segmentation and episodic memory. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Neural fatigue influences memory encoding in the human hippocampus.

Here we examine the variability underlying successful memory encoding. Successful encoding of successive study items may fatigue encoding resources, thus decreasing the ability to encode subsequent items (Tulving and Rosenbaum, 2006); alternatively, successful encoding may be persistent, leading to more successful encoding (Kahana, Aggarwal, and Phan, 2018). Analyzing intracranial electroencephalographic activity while subjects studied lists of words for subsequent free recall, we examined high-frequency activity (HFA) in hippocampus and dorsolateral prefrontal cortex (DLPFC), as HFA was greater for subsequently recalled than non-recalled items in these regions. We compared non-recalled items with good encoding history (i.e. one of the two preceding items was recalled) with non-recalled items with poor encoding history (i.e. neither prior item was recalled). In the hippocampus, good encoding history led to reduced HFA, whereas in the DLPFC, good encoding history led to enhanced HFA. Hippocampal findings appear consistent with the neural fatigue hypothesis, whereas the DLPFC results appear consistent with persistent encoding states.

A retrieved context model of the emotional modulation of memory.

Emotion enhances episodic memory, an effect thought to be an adaptation to prioritize the memories that best serve evolutionary fitness. However, viewing this effect largely in terms of prioritizing what to encode or consolidate neglects broader rational considerations about what sorts of associations should be formed at encoding, and which should be retrieved later. Although neurobiological investigations have provided many mechanistic clues about how emotional arousal modulates item memory, these effects have not been wholly integrated with the cognitive and computational neuroscience of memory more generally. Here we apply the Context Maintenance and Retrieval Model (CMR; Polyn, Norman, & Kahana, 2009) to this problem by extending it to describe the way people may represent and process emotional information. A number of ways to operationalize the effect of emotion were tested. The winning emotional CMR (eCMR) model conceptualizes emotional memory effects as arising from the modulation of a process by which memories become bound to ever-changing temporal and emotional contexts. eCMR provides a good qualitative fit for the emotional list-composition effect and the emotional oddball effect, illuminating how these effects are jointly determined by the interplay of encoding and retrieval processes. eCMR can account for the increased advantage of emotional memories in delayed memory tests by assuming a limited ability to reinstate the temporal context of encoding after a delay. By leveraging the rich tradition of temporal context models, eCMR helps integrate existing effects of emotion and provides a powerful tool to test mechanisms by which emotion affects memory in a broad range of paradigms. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

A spacing account of negative recency in final free recall.

The well-known recency effect in immediate free recall reverses when subjects attempt to recall items studied and tested on a series of prior lists, as in the final-free-recall procedure (Craik, 1970). In this case, the last few items on each list are actually remembered less well than are the midlist items. Because dual-store theories of recall naturally predict negative recency, this phenomenon has long been cited as evidence favoring these models. In a final-free-recall study, we replicate the negative-recency effect for the within-list serial position curve and the positive-recency effect for the between-list serial position curve. Whereas we find prominent negative recency for items recalled early in the initial recall period, this effect is markedly reduced for items recalled later in the recall period. When considering initial recall as a second presentation of studied items, we find that the probability of final free recall increases as the number of items between initial presentation and initial recall increases. These results suggest that negative recency may reflect the beneficial effects of spaced practice, in which end-of-list items recalled early constitute massed repetitions and end-of-list items recalled late are spaced repetitions. To help distinguish between the spacing account and the prevailing dual-store, rehearsal-based account, we examined negative recency in continual-distractor free recall. Contrary to the dual-store account, but in accord with the spacing account, we find robust negative recency in continual-distractor free recall, which is greater for those items recalled early in output. (PsycINFO Database Record

Expanding the scope of memory search: Modeling intralist and interlist effects in free recall.

The human memory system is remarkable in its capacity to focus its search on items learned in a given context. This capacity can be so precise that many leading models of human memory assume that only those items learned in the context of a recently studied list compete for recall. We sought to extend the explanatory scope of these models to include not only intralist phenomena, such as primacy and recency effects, but also interlist phenomena such as proactive and retroactive interference. Building on retrieved temporal context models of memory search (e.g., Polyn, Norman, & Kahana, 2009), we present a substantially revised theory in which memory accumulates across multiple experimental lists, and temporal context is used both to focus retrieval on a target list, and to censor retrieved information when its match to the current context indicates that it was learned in a nontarget list. We show how the resulting model can simultaneously account for a wide range of intralist and interlist phenomena, including the pattern of prior-list intrusions observed in free recall, build-up of and release from proactive interference, and the ability to selectively target retrieval of items on specific prior lists (Jang & Huber, 2008; Shiffrin, 1970). In a new experiment, we verify that subjects' error monitoring processes are consistent with those predicted by the model.

Compound cuing in free recall.

According to the retrieved context theory of episodic memory, the cue for recall of an item is a weighted sum of recently activated cognitive states, including previously recalled and studied items as well as their associations. We show that this theory predicts there should be compound cuing in free recall. Specifically, the temporal contiguity effect should be greater when the 2 most recently recalled items were studied in contiguous list positions. A meta-analysis of published free recall experiments demonstrates evidence for compound cuing in both conditional response probabilities and interresponse times. To help rule out a rehearsal-based account of these compound cuing effects, we conducted an experiment with immediate, delayed, and continual-distractor free recall conditions. Consistent with retrieved context theory but not with a rehearsal-based account, compound cuing was present in all conditions, and was not significantly influenced by the presence of interitem distractors.

Parametric effects of word frequency in memory for mixed frequency lists.

[Correction Notice: An Erratum for this article was reported in Vol 39(6) of Journal of Experimental Psychology: Learning, Memory, and Cognition (see record 2013-27860-001). In the article there were omissions in Figure 1. All versions of this article have been corrected.] The word frequency paradox refers to the finding that low frequency words are better recognized than high frequency words yet high frequency words are better recalled than low frequency words. Rather than comparing separate groups of low and high frequency words, we sought to quantify the functional relation between word frequency and memory performance across the broad range of frequencies typically used in episodic memory experiments. Here we report that both low frequency and high frequency words are better recalled than midfrequency words. In contrast, we only observe a low frequency advantage when participants were given a subsequent item recognition test. The U-shaped relation between word frequency and recall probability may help to explain inconsistent results in studies using mixed lists with separate groups of high and low frequency words.

Contextual Variability in Free Recall.

According to contextual-variability theory, experiences encoded at different times tend to be associated with different contextual states. The gradual evolution of context implies that spaced items will be associated with more distinct contextual states, and thus have more unique retrieval cues, than items presented in proximity. Ross and Landauer (1978) tested this theory by examining whether the probability of recalling at least one of two studied items should increase as a function of the items' spacing. Their failure to observe this result was taken as strong evidence against contextual variability theory. We replicated their analysis on six recall datasets. For all of these datasets we found the pattern of results predicted by contextual-variability theory. These findings provide critical support for contextual-variability theories of episodic memory.