Investigating the cortico-hippocampal dynamics involved in human episodic memory with neural stimulation

van der Plas, Mircea (2022) Investigating the cortico-hippocampal dynamics involved in human episodic memory with neural stimulation. PhD thesis, University of Glasgow.

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The human episodic memory system depends on specific interactions between the hippocampus and neocortex. The three studies performed as part of this doctoral thesis each sought to improve our understanding of the cortico-hippocampal system in the context of episodic memory. Each study used a different approach to directly manipulate neural activity with the aim of revealing causal relationships between certain patterns of neural activity and behaviour.

In the first study the cortico-hippocampal network was investigated by using occipital transcranial alternating current stimulation (tACS) and auditory sensory stimulation with the aim of altering memory performance during an audio-visual association task. The electrical stimulation was hypothesized to interact with the auditory sensory stimulation after propagating from the neocortex to the hippocampus. This study was unsuccessful in modulating behaviour through stimulation.

In the second study, the left Dorsolateral Prefrontal Cortex (DLPFC) was targeted using 1 Hz repetitive transcranial magnetic stimulation (rTMS) over the course of two experiments, during a set of list learning tasks. This study found a beneficial effect on memory performance when stimulation occurred over the left DLPFC compared to stimulation over the vertex (control site). This behavioural effect was further characterized by a beta-power decrease over parietal sensors as measured by electroencephalography (EEG).

The third study probed the cortico-hippocampal network by directly stimulating the hippocampus and the neocortex, by applying direct electrical stimulation through implanted electrodes in human subjects. This study used measures of population activity as well as single neuron activity to monitor how the brain responds to direct stimulation. This study found that direct stimulation throughout the network produces a neural response that is characterized by short, intense excitation and prolonged follow-up inhibition which has the potential to travel throughout the brain. The ability of the response to travel between the neocortex and hippocampus was leveraged to measure a transduction delay of ~140 ms between the two regions.

Together these findings have advanced our understanding on how different stimulation methods can be used to manipulate neural activity and consequently affect the episodic memory system. Through these methods we might one day be able to aid persons suffering from cognitive impairments or related pathologies.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Colleges/Schools: College of Medical Veterinary and Life Sciences > School of Psychology & Neuroscience
Supervisor's Name: Hanslmayr, Prof. Simon
Date of Award: 2022
Depositing User: Theses Team
Unique ID: glathesis:2022-82698
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 11 Feb 2022 15:00
Last Modified: 08 Apr 2022 16:51
Thesis DOI: 10.5525/gla.thesis.82698

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