This dissertation explored different facets of high frequency activity in the human brain. The first part investigated the role of gamma activity in the interplay of retina and cortex, the second part concentrated on approaches to low SNR signals like gamma power.
Beyond the scope of the single studies, this work proposed several implications with respect to the origin and role of gamma activity. First, it highlighted how important the understanding of retinocortical interactions is for the comprehension of cortical visual and higher-order processes: cortical high frequency activity seems to be inherited from the retina, manifesting latency differences which might also be inherited from earlier processing stages. Furthermore, visual cortex could possess a corticofugal connection to the retina, possibly exerting feedback influence mirrored by retinal high frequency activity.
Second, this work added to the ongoing discussion about narrowband versus broadband gamma: while both narrowband and broadband visual high frequency activity can be inherited from the retina, the narrowband responses in the visual system could arise from the specific stimulus features evoking the OFF pathway or from low informational content in those stimuli.
Third, concerning the analysis of low SNR signals, this dissertation showed the feasibility of single-trial decoding of gamma power. Furthermore, it described the relation between high sensor count and beamformer performance, showing that weak signals like high frequency activity benefit from a high sensor array density.
Finally, this dissertation elicited novel questions on the role of high frequency activity in the human brain – among them the questions whether the bandwidth of visual gamma activity could reflect informational content, whether the visual narrowband activity in response to grating stimuli is inherited from the retina
or whether it is possible to decode higher-order stimulus characteristics such as object categories from high frequency activity – thereby paving the way for further research on the role of high frequency activity in neural information processing.
The studies reported in this dissertation were co-authored and supported by a num-ber of colleagues. Below, the authors of the reported studies and my own research contributions are listed.
Study 1: Faster than the brain’s speed of light: Retinocortical in-teractions differ in high frequency activity when processing darks and lights
Authors: Britta U. Westner and Sarang S. Dalal
Own contributions: I conceptualized the experiment together with Sarang S. Dalal, I carried out the MEG measurements, analyzed the data, and drafted the manuscript.
Status: Submitted to bioRxiv. doi: https://doi.org/10.1101/153551
Study 2: Does transcranial magnetic stimulation of occipital cortex affect the retina? – A pilot study
Authors: Britta U. Westner, Mathis Kaiser, and Sarang S. Dalal
Own contributions: I designed the experiment together with Sarang S. Dalal, I carried out the transcranial magnetic stimulations, analyzed the data, and drafted the manuscript.
Status: In preparation.
Study 3: Across-subjects classification of stimulus modality from hu-man MEG high frequency activity
Authors: Britta U. Westner, Sarang S. Dalal, Simon Hanslmayr, and Tobias Staudigl
Own contributions: The original study was conceptualized and undertaken by
To-bias Staudigl and Simon Hanslmayr. I developed the decoding study, analyzed the data together with Tobias Staudigl, I applied the decoding analyses, and drafted the manuscript.
Status: Submitted to PLOS Computational Biology.
Study 4: Is more always better? The effect of sensor array density on beamformer performance
Authors: Britta U. Westner, Matthew J. Brookes, and Sarang S. Dalal
Own contributions: I conceptualized the examination, programmed the simula-tions, analyzed the data, and drafted the manuscript.
Status: In preparation.
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