The hippocampus as the switchboard between perception and memory

Adaptive memory recall requires a rapid and flexible switch from external perceptual reminders to internal mnemonic representations. However, owing to the limited temporal or spatial resolution of brain imaging modalities used in isolation, the hippocampal–cortical dynamics supporting this process remain unknown. We thus employed an object-scene cued recall paradigm across two studies, including intracranial electroencephalography (iEEG) and high-density scalp EEG. First, a sustained increase in hippocampal high gamma power (55 to 110 Hz) emerged 500 ms after cue onset and distinguished successful vs. unsuccessful recall. This increase in gamma power for successful recall was followed by a decrease in hippocampal alpha power (8 to 12 Hz). Intriguingly, the hippocampal gamma power increase marked the moment at which extrahippocampal activation patterns shifted from perceptual cue toward mnemonic target representations. In parallel, source-localized EEG alpha power revealed that the recall signal progresses from hippocampus to posterior parietal cortex and then to medial prefrontal cortex. Together, these results identify the hippocampus as the switchboard between perception and memory and elucidate the ensuing hippocampal–cortical dynamics supporting the recall process.

A long time ago, in a galaxy far, far away: How temporal are episodic contents?

A prominent feature of mental event (i.e. ‘episodic’) simulations is their temporality: human adults can generate episodic representations directed towards the past or the future. The ability to entertain event representations with different temporal orientations allows these representations to play various cognitive roles. Here, we investigated how the temporal orientation of imagined events relates to the contents (i.e. ‘what is happening’) of these events. Is the temporal orientation of an episode part of its contents? Or are the processes for assigning temporality to an event representation distinct from those generating its contents? In three experiments (N = 360), we asked participants to generate and later recall a series of imagined events differing in (1) location (indoors vs. outdoors), (2) time of day (daytime vs. nighttime), (3) temporal orientation (past vs. future), and (4) weekday (Monday vs. Friday). We then tested to what extent successful recall of episodic content (i.e. (1) and (2)) would predict recall of temporality and/or weekday information. Results showed that while recall of temporal orientation was predicted by content recall, weekday recall was not. However, temporal orientation was only weakly integrated with episodic contents. This finding suggests that episodic simulations are unlikely to be intrinsically temporal in nature. Instead, similar to other forms of temporal information, temporal orientation might be determined from such contents by reconstructive post-retrieval processes. These results have implications for how the human ability to ‘mentally travel’ in time is cognitively implemented.

Schema representations in distinct brain networks support narrative memory during encoding and retrieval

Schematic prior knowledge can scaffold the construction of event memories during perception and also provide structured cues to guide memory search during retrieval. We measured the activation of story-specific and schematic representations using fMRI while participants were presented with 16 stories and then recalled each of the narratives, and related these activations to memory for specific story details. We predicted that schema representations in mPFC would be correlated with successful recall of story details. In keeping with this prediction, an anterior mPFC region showed a significant correlation between activation of schema representations at encoding and subsequent behavioral recall performance; however, this mPFC region was not implicated in schema representation during retrieval. More generally, our analyses revealed largely distinct brain networks at encoding and retrieval in which schema activation was related to successful recall. These results provide new insight into when and where event knowledge can support narrative memory.

EEG biomarkers of free recall

Brain activity in the moments leading up to successful verbal recall provide a window into the cognitive processes underlying memory retrieval. But these same recordings also subsume neural signals unrelated to mnemonic retrieval, such as response-related motor activity. Here we examined spectral EEG biomarkers of successful recall under an extreme manipulation of mnemonic demands: subjects either recalled items after a few seconds or several days. This manipulation isolated EEG components specifically re- lated to episodic retrieval. Theta and gamma power (4-8 Hz and 40-128 Hz respectively) increased immediately prior to long-delay recall, whereas 8-20 Hz power decreased. A direct comparison of long-delay and immediate recall revealed a nearly identical pattern, indicating that these spectral biomarkers of successful retrieval reflect memory-specific processes. Ruling out a confound of motor related activity, these results identify theta and gamma activity as biomarkers of successful episodic memory retrieval.

Modeling Working Memory in a Spiking Neuron Network Accompanied by Astrocytes

We propose a novel biologically plausible computational model of working memory (WM) implemented by a spiking neuron network (SNN) interacting with a network of astrocytes. The SNN is modeled by synaptically coupled Izhikevich neurons with a non-specific architecture connection topology. Astrocytes generating calcium signals are connected by local gap junction diffusive couplings and interact with neurons via chemicals diffused in the extracellular space. Calcium elevations occur in response to the increased concentration of the neurotransmitter released by spiking neurons when a group of them fire coherently. In turn, gliotransmitters are released by activated astrocytes modulating the strength of the synaptic connections in the corresponding neuronal group. Input information is encoded as two-dimensional patterns of short applied current pulses stimulating neurons. The output is taken from frequencies of transient discharges of corresponding neurons. We show how a set of information patterns with quite significant overlapping areas can be uploaded into the neuron-astrocyte network and stored for several seconds. Information retrieval is organized by the application of a cue pattern representing one from the memory set distorted by noise. We found that successful retrieval with the level of the correlation between the recalled pattern and ideal pattern exceeding 90% is possible for the multi-item WM task. Having analyzed the dynamical mechanism of WM formation, we discovered that astrocytes operating at a time scale of a dozen of seconds can successfully store traces of neuronal activations corresponding to information patterns. In the retrieval stage, the astrocytic network selectively modulates synaptic connections in the SNN leading to successful recall. Information and dynamical characteristics of the proposed WM model agrees with classical concepts and other WM models.

Situation-bound utterances in Chinese as a foreign language: the effectiveness of the identification task

This paper investigates the effectiveness of the identification task on the retention of situation-bound utterances (SBUs) in Chinese as a foreign language (CFL). The participants were Italian CFL learners with different lengths of learning experience, divided into an experimental and a control group. The target SBUs were selected by means of a discourse completion questionnaire previously submitted to Italian and Chinese native speakers and Italian CFL learners. During the treatment, the experimental group was provided with six communicative functions and was asked to identify the corresponding SBUs from eight short dialogues. Subsequently, a posttest was administered to both the experimental and the control groups. Overall, the statistical analysis of the test results suggests a significant effect of the task on the participants’ ability to recall the target SBUs, both from an immediate and a longitudinal perspective. However, the data also suggest that, while the participants’ length of learning experience and minor linguistic or cultural specificities of the target formulae did not significantly influence the effectiveness of this specific vocabulary learning task, the presence vs. absence of the SBUs in either the L1 or the L2 might be a crucial factor for their successful recall.

Age differences in the precision of memory at short and long delays

Age differences are well established for many memory tasks assessing both short-term and long-term memory. However, how age differences in performance vary with increasing delay between study and test is less clear. Here we report two experiments in which participants studied a continuous sequence of object-location pairings. Test events were intermixed such that participants were asked to recall the precise location of an object following a variable delay. Older adults exhibit a greater degree of error (distance between studied and recalled locations) relative to younger adults at short (0-2 intervening events) and longer delays (10-25 intervening events). Mixture modeling of the distribution of recall error suggests that older adults do not fail to recall information at a significantly higher rate than younger adults. Instead, what they do recall appears to be less precise. Follow up analyses demonstrate that this age difference emerges following only one or two intervening events between study and test. These findings are consistent with the suggestion that aging does not greatly impair recall from the focus of attention but age differences emerge once information is displaced from this highly accessible state. Further, we suggest that age differences in the precision of memory, but not the probability of successful recall, may be due to the use of more gist-like representations in this task.

Content tuning in the medial temporal lobe cortex: Voxels that perceive, retrieve

How do we recall vivid details from our past based only on sparse cues? Research suggests that the phenomenological reinstatement of past experiences is accompanied by neural reinstatement of the original percept. This process critically depends on the medial temporal lobe (MTL). Within the MTL, perirhinal cortex (PRC) and parahippocampal cortex (PHC) are thought to support encoding and recall of objects and scenes, respectively, with the hippocampus (HC) serving as a content-independent hub. If the fidelity of recall indeed arises from neural reinstatement of perceptual activity, then successful recall should preferentially draw upon those neural populations within content-sensitive MTL cortex that are tuned to the same content during perception. We tested this hypothesis by having eighteen human participants undergo functional magnetic resonance imaging (fMRI) while they encoded and recalled objects and scenes paired with words. Critically, recall was cued with the words only. While HC distinguished successful from unsuccessful recall of both objects and scenes, PRC and PHC were preferentially engaged during successful vs. unsuccessful object and scene recall, respectively. Importantly, within PRC and PHC, this content-sensitive recall was predicted by content tuning during perception: Across PRC voxels, we observed a positive linear relationship between object tuning during perception and successful object recall, while across PHC voxels, we observed a positive linear relationship between scene tuning during perception and successful scene recall. Our results thus highlight content-based roles of MTL cortical regions for episodic memory and reveal a direct mapping between content-specific tuning during perception and successful recall.

Can sleep protect memories from catastrophic forgetting?

Continual learning remains to be an unsolved problem in artificial neural networks. Biological systems have evolved mechanisms by which they can prevent catastrophic forgetting of old knowledge during new training and allow lifelong learning. Building upon data suggesting the importance of sleep in learning and memory, here we test a hypothesis that sleep protects memories from catastrophic forgetting. We found that training in a thalamocortical network model of a “new” memory that interferes with previously stored “old” memory may result in degradation and forgetting of the old memory trace. Simulating NREM sleep immediately after new learning leads to replay, which reverses the damage and ultimately enhances both old and new memory traces. Surprisingly, we found that sleep replay goes beyond recovering old memory traces that were damaged by new learning. When a new memory competes for the neuronal/synaptic resources previously allocated to the old memory, sleep replay changes the synaptic footprint of the old memory trace to allow for the overlapping populations of neurons to store multiple memories. Different neurons become preferentially supporting different memory traces to allow successful recall. We compared synaptic weight dynamics during sleep replay with that during interleaved training – a common approach to overcome catastrophic forgetting in artificial networks – and found that interleaved training promotes synaptic competition and weakening of reciprocal synapses, effectively reducing an ensemble of neurons contributing to memory recall. This leads to suboptimal recall performance compared to that after sleep. Together, our results suggest that sleep provides a powerful mechanism to achieve continual learning by combining consolidation of new memory traces with reconsolidation of old memory traces to minimize memory interference.

Opportunities for Corrective Feedback During Study Abroad: A Mixed Methods Approach

ABSTRACTThe provision of corrective feedback during oral interaction has been deemed an essential element for successful second language acquisition (Gass & Mackey, 2015a). However, corrective feedback—especially corrective feedback provided by peer interlocutors—remains understudied in naturalistic settings. The present mixed methods study aimed to identify the target and type of corrective feedback provided by both native-speaker and peer interlocutors during conversation groups while abroad. U.S. study abroad students (N= 19) recorded group conversations with native speakers (N= 10) at the beginning, middle, and end of a 6-week stay in Barcelona, Spain. Results indicate a significant decrease in the provision of corrective feedback by both native speakers and peer learners over the course of the program. Qualitative analyses revealed that both learners and natives alike engage in negotiations for meaning throughout the program, which for learners resulted in successful recall on tailor-made quizzes. The use of the first language by both the study abroad students and the native speakers promoted these opportunities in some instances. Results are discussed in terms of their contribution to the study abroad literature as well as to research into the effects of feedback on second language development.