Investigating the spatio-temporal dynamics of human episodic and working memory

Ratcliffe, Oliver (2022). Investigating the spatio-temporal dynamics of human episodic and working memory. University of Birmingham. Ph.D.

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Abstract

The overall aim of this thesis was to better characterise and understand the role of frontal regions in working memory and the role of the theta rhythm in episodic and working memory. Chapter 1 provides a general background of episodic and working memory with a focus on key cortical regions and oscillatory rhythms.

In Chapter 2, an EEG study is presented in which time-frequency and multivariate decoding analyses implicate fronto-medial theta (FMT) oscillatory activity as the supervisor of posteriorly maintained WM content. In accordance with phase-coding accounts, FMT oscillations decreased with increasing WM load. In Chapter 3, a transcranial direct current stimulation (tDCS) study was conducted to better understand the role of the frontal cortex in working memory and ageing. Specifically, high definition tDCS was applied over the left, right, or bilateral prefrontal cortices to interrogate a framework (HAROLD) that predicts a reduction in the laterality of frontal activation in older adults. In agreement with the HAROLD model, the effect sizes suggested the greatest benefit following bilateral stimulation, although this effect was not significant.

In Chapter 4, the role of theta phase was further investigated in an intracranial dataset employing a continuous encoding/retrieval paradigm. A computational model predicts distinct preferred phases for differentiating encoding and retrieval processes in the hippocampus. In general agreement with this model, the onset of memory-relevant stimuli prompted a phase reset and there was some evidence of a phase difference in the theta band. Notably, condition-specific theta-gamma interactions were observed.

Finally, Chapter 5 summarises the main findings of this thesis and discusses its implications. The results here provide novel insight into the importance of theta activity in both working memory and episodic memory processes. The thesis also highlights the key executive role of frontal regions in WM.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):