Feature-specific reactivation and representational geometries in the temporal structure of episodic memory retrieval

Postzich, Michael Christopher (2025) Feature-specific reactivation and representational geometries in the temporal structure of episodic memory retrieval. PhD thesis, University of Glasgow.

Abstract

Episodic memory describes the ability to recall events and situations we have encountered in the past. These memories usually contain multiple elements with lots of conceptual, gist information as well as perceptual details. During perception of visual objects, perceptual and semantic features are represented at different parts along the ventral visual stream hierarchy resulting in a faster accessibility of perceptual feature information compared to semantic features. Previous research indicates that during object retrieval a reversal of this hierarchy is observable with semantic features being accessible before perceptual ones. The endeavour of the current thesis will be to evaluate this reverse stream effect of memory for its generality and robustness, for underlying cognitive components and spatio-temporal representational formats in the brain.

First, in a set of behavioral studies, feature-specific reaction times are evaluated as a measure of mapping the temporal dependencies of features within an episodic long term memory trace. Second, a detailed analysis of these feature-specific reaction time results with the help of cognitive mathematical models of decision making is presented. Third, the spatiotemporal dynamics of feature representations during the retrieval process are accessed by combining MEG, fMRI and Deep Neural Network encodings of naturalistic images using a Representational Similarity-based Fusion approach.

Results indicate that the reverse stream of feature accessibility during memory retrieval is a robust effect that generalizes over different feature dimensions, cues and attentional states. Reaction time modelling suggests that this effect is explained by differences in evidence sampling speeds from memory for perceptual and semantic features. In the fusion analysis of brain imaging data, successful retrievals showed prolonged activations of high-level areas along the visual stream compared to early visual areas and semantic, gist-like feature compared to low-level feature representations.

Together, these results provide complementary and detailed evidence for reversed hierarchical structure of an episodic long term memory trace. Further implications and future directions are discussed.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: B Philosophy. Psychology. Religion > BF Psychology; R Medicine > RC Internal medicine > RC0321 Neuroscience. Biological psychiatry. Neuropsychiatry
• Colleges/Schools: College of Medical Veterinary and Life Sciences > School of Psychology & Neuroscience
• Supervisor's Name: Wimber, Professor Maria
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85666
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 12 Jan 2026 14:05
• Last Modified: 12 Jan 2026 14:08
• Thesis DOI: 10.5525/gla.thesis.85666
• URI: https://theses.gla.ac.uk/id/eprint/85666