Forgetting across a hierarchy of episodic representations

Abstract

<p>Rich episodic experiences are represented in a hierarchical manner across a diverse network of brain regions, and as such, the way in which episodes are forgotten is likely to be similarly diverse. Using novel experimental approaches and statistical modelling, recent research has suggested that item-based representations, such as ones related to the colour and shape of an object, fragment over time, whereas higher-order event-based representations may be forgotten in a more ‘holistic’ uniform manner. We propose a framework that reconciles these findings, where complex episodes are represented in a hierarchical manner across different brain regions and forgetting is underpinned by different mechanisms at each level in the hierarchy.</p>

A partially nested cortical hierarchy of neural states underlies event segmentation in the human brain

“…This is in line with previous conceptualizations of events as partonomic hierarchies (Zacks et al, 2001a) and with other models of hierarchical neural representations, such as the hub-and-spokes model for semantic representations, which proposes that semantic knowledge is represented by the interaction between modality-specific brain regions and a transmodal semantic representational hub in the anterior temporal lobe (Lambon Ralph et al, 2010;Rogers et al, 2004). It is also in line with a recently proposed hierarchical representation of episodic memories, in which items that are linked within small-scale events are in turn linked within large-scale episodic narratives (Andermane et al, 2021).…”

Feature-specific reaction times reveal a semanticisation of memories over time and with repeated remembering

“…Alternatively, it is possible that the perceptual features of our visual objects were forgotten faster than their conceptual features. The nature of item-based forgetting is still under debate (Andermane, Joensen & Horner, 2020). Some recent work suggests that the forgetting of perceptual features, such as colour, is independent of, and occurs faster than, forgetting of higher-level conceptual features such as item state or exemplar (Brady, Konkle, Alvarez, Oliva, 2013;Utochkin & Brady, 2020).…”

Suppression weakens unwanted memories via a sustained reduction of neural reactivation

“…Accordingly, these representations are largely protected from interference. The disruption of hippocampal representations may thus not manifest as graded forgetting but eventually lead to holistic forgetting in an all-or-none fashion (18,65,67). Indeed, a previous study that did not observe evidence for suppression-induced forgetting also did not obtain evidence for lingering effects on hippocampal representations (69).…”

“…By comparison, hippocampal representations are encoded in a more orthogonal fashion (60,67) and may thus be largely protected from interference. The disruption of these representations would thus not manifest as graded forgetting but eventually lead to holistic forgetting in an all-or-none fashion (23,65,66). Indeed, a previous study did not obtain evidence for lingering effects of suppression on hippocampal reinstatement when the memories could still be recalled (68).…”

“…Indeed, disruptions of cortical memory representations such as in the PhC may particularly lead to these graded effects of forgetting (10,30,65). That is, as cortical representations get progressively weakened, they may become increasingly susceptible to interference from overlapping representations of similar memories (67).…”