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Physical characteristics of intermittent photic stimulation and usage with magnetoencephalography

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Title: Physical characteristics of intermittent photic stimulation and usage with magnetoencephalography
Author(s): Issakainen, Jani
Contributor: University of Helsinki, Faculty of Science
Degree program: Master's Programme in Materials Research
Specialisation: Medical Physics and Biophysics
Language: English
Acceptance year: 2021
Abstract:
Electroencephalography (EEG) is a non-invasive neurophysiological method for evaluating brain activity by measuring electrical potential at the scalp. The electrical potentials originate mainly from postsynaptic cortical currents created by neuronal activity. It is a valuable tool for both research and clinical practice. EEG can be used e.g. to diagnose epilepsy, focal brain disorders, brain death, and coma. Intermittent photic stimulation (IPS) is an important tool in clinical EEG. Healthcare professionals use it to induce epileptic activity in patients to help diagnose their conditions. In these tests, various IPS frequencies are used with eyes-closed, eyes-open, and eye-closure conditions. IPS test is listed in clinical practice guidelines in EEG globally, and it is mainly used to diagnose photosensitive epilepsy, i.e., to detect epilepsy-related abnormal sensitivity to flickering light. Magnetoencephalography (MEG) is a non-invasive neurophysiological method in which minute magnetic fields — produced by the same postsynaptic currents as in EEG — are measured with special superconductive sensors around the head. MEG is a valuable tool for research and clinical practice with increasing world-wide utilization. The main advantages of MEG over EEG are easier source modelling and higher resolution at cortical areas. IPS has not been introduced to MEG since the IPS stimulators used in EEG are not compatible with MEG. IPS in MEG could improve the analysis of IPS and provide better tools for diagnoses. Currently, data analysis of IPS is typically limited to healthcare professionals examining the visualization of the raw data while looking for induced epileptiform activites and lateralizing them. In this thesis, an MEG-compatible IPS stimulator is introduced and alternative ways of analyzing IPS data for both MEG and EEG are showcased. Although analysis methods were applied with decent signal-to-noise ratios, further research is needed—especially to compare responses between patients with epilepsy and healthy subjects.
Keyword(s): intermittent photic stimulation ips epilepsy meg eeg magnetoencephalography electroencephalography


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