The spectro-contextual encoding and retrieval theory of episodic memory.

The spectral fingerprint hypothesis, which posits that different frequencies of oscillations underlie different cognitive operations, provides one account for how interactions between brain regions support perceptual and attentive processes (Siegel etal., 2012). Here, we explore and extend this idea to the domain of human episodic memory encoding and retrieval. Incorporating findings from the synaptic to cognitive levels of organization, we argue that spectrally precise cross-frequency coupling and phase-synchronization promote the formation of hippocampal-neocortical cell assemblies that form the basis for episodic memory. We suggest that both cell assembly firing patterns as well as the global pattern of brain oscillatory activity within hippocampal-neocortical networks represents the contents of a particular memory. Drawing upon the ideas of context reinstatement and multiple trace theory, we argue that memory retrieval is driven by internal and/or external factors which recreate these frequency-specific oscillatory patterns which occur during episodic encoding. These ideas are synthesized into a novel model of episodic memory (the spectro-contextual encoding and retrieval theory, or "SCERT") that provides several testable predictions for future research

Episodic encoding is more than the sum of its parts: An fMRI investigation of multifeatural contextual encoding

Episodic memories are characterized by their contextual richness, yet little is known about how the various features comprising an episode are brought together in memory. Here we employed fMRI and a multidimensional source memory procedure to investigate processes supporting the mnemonic binding of item and contextual information. Volunteers were scanned while encoding items for which the contextual features (color and location) varied independently, allowing activity elicited at the time of study to be segregated according to whether both, one, or neither feature was successfully retrieved on a later memory test. Activity uniquely associated with successful encoding of both features was identified in the intra-parietal sulcus, a region strongly implicated in the support of attentionally mediated perceptual binding. The findings suggest that the encoding of disparate features of an episode into a common memory representation requires that the features be conjoined in a common perceptual representation when the episode is initially experienced

Refresh my memory: Episodic memory reinstatements intrude on working memory maintenance

A fundamental question in memory research is how different forms of memory interact. Previous research has shown that people rely on working memory (WM) in short-term recognition tasks; a common view is that episodic memory (EM) only influences performance on these tasks when WM maintenance is disrupted. However, retrieval of memories from EM has been widely observed during brief periods of quiescence, raising the possibility that EM retrievals during maintenance-critically, before a response can be prepared-might affect short-term recognition memory performance even in the absence of distraction. We hypothesized that this influence would be mediated by the lingering presence of reactivated EM content in WM. We obtained support for this hypothesis in three experiments, showing that delay-period EM reactivation introduces incidentally-associated information ( context ) into WM, and that these retrieved associations negatively impact subsequent recognition, leading to substitution errors (Experiment 1) and slowing of accurate responses (Experiment 2). fMRI pattern analysis showed that slowing is mediated by the content of EM reinstatement (Experiment 3). These results expose a previously hidden influence of EM on WM, raising new questions about the adaptive nature of their interaction

Semantic memory

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The very same thing: Extending the object token concept to incorporate causal constraints on individual identity

The contributions of feature recognition, object categorization, and recollection of episodic memories to the re-identification of a perceived object as the very same thing encountered in a previous perceptual episode are well understood in terms of both cognitive-behavioral phenomenology and neurofunctional implementation. Human beings do not, however, rely solely on features and context to re-identify individuals; in the presence of featural change and similarly-featured distractors, people routinely employ causal constraints to establish object identities. Based on available cognitive and neurofunctional data, the standard object-token based model of individual re-identification is extended to incorporate the construction of unobserved and hence fictive causal histories (FCHs) of observed objects by the pre-motor action planning system. Cognitive-behavioral and implementation-level predictions of this extended model and methods for testing them are outlined. It is suggested that functional deficits in the construction of FCHs are associated with clinical outcomes in both Autism Spectrum Disorders and later-stage stage Alzheimer's disease.\u

Integrating across memory episodes: Developmental trends

Memory enables us to use information from our past experiences to guide new behaviours, calling for the need to integrate or form inference across multiple distinct episodic experiences. Here, we compared children (aged 9-10 years), adolescents (aged 12-13 years), and young adults (aged 19-25 years) on their ability to form integration across overlapping associations in memory. Participants first encoded a set of overlapping, direct AB- and BC-associations (object-face and face-object pairs) as well as non-overlapping, unique DE-associations. They were then tested on these associations and inferential AC-associations. The experiment consisted of four such encoding/retrieval cycles, each consisting of different stimuli set. For accuracy on both unique and inferential associations, young adults were found to outperform teenagers, who in turn outperformed children. However, children were particularly slower than teenagers and young adults in making judgements during inferential than during unique associations. This suggests that children may rely more on making inferences during retrieval, by first retrieving the direct associations, followed by making the inferential judgement. Furthermore, young adults showed a higher correlation between accuracy in direct (AB, BC) and inferential AC-associations than children. This suggests that, young adults relied closely on AB- and BC-associations for making AC decisions, potentially by forming integrated ABC-triplets during encoding or retrieval. Taken together, our findings suggest that there may be an age-related shift in how information is integrated across experienced episodes, namely from relying on making inferences at retrieval during middle childhood to forming integrated representations at different memory processing stages in adulthood

Reinstatement of memory representations for lifelike events over the course of a week

When we remember an event, the content of that memory is represented across the brain. Detailed memory retrieval is thought to involve the reinstatement of those representations. Functional MRI combined with representational similarity analyses (RSA) of spatial patterns of brain activity has revealed reinstatement of recently-experienced events throughout a core memory retrieval network. In the present study, participants were scanned while they watched, immediately retrieved and then retrieved after a week, 24 short videos. Following the delayed retrieval, they freely recalled all videos outside of the scanner. We observed widespread within- and between-subject reinstatement effects within a posterior midline core memory retrieval network during all phases of the experiment. Within precuneus, bilateral middle temporal gyrus and the left hippocampus, reinstatement effects between the retrieval phases correlated with memory performance. These findings extend previous studies that have only employed short retention periods or highly rehearsed materials, demonstrating that memory representations for unique events are reliably reinstated over longer timeframes that are meaningful in the context of real-world episodic memory

Shared mechanisms support controlled retrieval from semantic and episodic memory: Evidence from semantic aphasia

Semantic cognition is supported by at least two interactive components: semantic representations and control mechanisms that shape retrieval to suit the circumstances. Semantic and episodic memory draw on largely distinguishable stores, yet it is unclear whether controlled retrieval from these representational systems is supported by shared mechanisms. Patients with semantic aphasia (SA) show heteromodal semantic control deficits following stroke to left inferior frontal gyrus (LIFG), an area implicated in semantic processing plus the control of memory and language. However, episodic memory has not been examined in these patients and although the role of LIFG in semantics is well-established, neuroimaging cannot ascertain whether this area is directly implicated in episodic control or if its activation reflects semantic processing elicited by the stimuli. Neuropsychology can address this question, revealing whether this area is necessary for both domains. We found that: (i) SA patients showed difficulty discarding dominant yet irrelevant semantic links during semantic and episodic decisions. Similarly, recently encoded events promoted interference during retrieval from both domains. (ii) Deficits were multimodal (i.e. equivalent using words and pictures) in both domains and, in the episodic domain, memory was compromised even when semantic processing required by the stimuli was minimal. (iii) In both domains, deficits were ameliorated when cues reduced the need to internally constrain retrieval. These cues could involve semantic information, self-reference or spatial location, representations all thought to be unaffected by IFG lesions. (iv) Training focussed on promoting flexible retrieval of conceptual knowledge showed generalization to untrained semantic and episodic tasks in some individuals; in others repetition of specific associations gave rise to inflexible retrieval and overgeneralization of trained associations during episodic tasks. Although the neuroanatomical specificity of neuropsychology is limited, this thesis provides evidence that shared mechanisms support the controlled retrieval of episodic and semantic memory