The role of alpha oscillations in visual processing

Coldea, Andra (2021) The role of alpha oscillations in visual processing. PhD thesis, University of Glasgow.

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Alpha-band oscillatory activity over occipito-parietal areas is involved in shaping perceptual and cognitive processes. In this thesis, I have attempted to identify whether this alpha oscillatory activity is a neural predictor of subjective versus objective measures of task performance. Using electroencephalography (EEG), I demonstrate in the first experiment that pre-stimulus alpha power is inversely related to perceptual awareness, but is not linked to accuracy, indicating a double dissociation between the subjective and objective measures. In contrast, pre-stimulus alpha phase did not predict either behavioural measure. Following up from these findings, I have explored in the second experiment whether the relationship between pre-stimulus alpha power and perceptual awareness can be causally manipulated via entrainment. To do this, I have employed repetitive transcranial magnetic stimulation (rTMS) at alpha frequency. No evidence was found for an effect of active alpha-rTMS over parietal areas relative to control conditions. However, accuracy was positively correlated to resting individual alpha peak frequency of participants, indicating that alpha frequency modulates perceptual sensitivity. Finally, in the third experiment of the series, I continued to test the causal involvement of alpha oscillations in visual perception, while investigating the replicability of effects obtained using another technique, transcranial alternating current stimulation (tACS). To this end, I aimed to replicate the finding that alpha frequency tACS over parietal areas induces a change in attention bias away from the contra- towards the ipsilateral visual hemifield relative to the stimulated hemisphere. In contrast to several previous reports, I did not find an effect of alpha tACS stimulation on attention deployment as compared to sham. This negative result calls for a more systematic assessment of the factors that drive non-invasive brain stimulation effects, so that the full potential of these techniques is achieved. The experiments presented in this thesis add to our current understanding of the role alpha oscillations have in processing visual input, and particularly in conscious awareness. At the same time, it addresses the effectiveness of neuromodulating brain oscillations via non-invasive brain stimulation techniques and emphasizes the importance of replicability.

Item Type: Thesis (PhD)
Colleges/Schools: College of Science and Engineering
Supervisor's Name: Thut, Prof. Gregor and Harvey, Dr. Monika
Date of Award: 2021
Depositing User: Theses Team
Unique ID: glathesis:2021-82644
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 21 Jan 2022 15:05
Last Modified: 29 Mar 2023 15:51
Thesis DOI: 10.5525/gla.thesis.82644
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