Browsing by Subject "meg"
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(2021)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.
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(2015)The sense of touch is an invaluable way for any organism to get information about itself and the environment. In this advanced studies thesis MEG is used to study the primary somatosensory cortex of 12 healthy children aged approximately 6.5 years. The somatosensory activity is evoked with tactile stimulation of the fingertip on digits II and V. The MEG responses are recorded to stimulation of both hands while the test subject focuses on the tactile stimuli. Similar responses are also recorded to stimulation of the left hand while not being focused on the stimuli. The study focuses on the characteristics of the SEFT50 deflection using ECD multidipole model for data analyses. The location, latency and strength of SEFT50 responses are compared between the different fingers and for the left hand also between the conditions of being focused and not being focused on the stimuli. Statistical testing is done by using Student's paired t-test with two-tailed distribution and with a threshold of p < 0.05 for a statistical significance. No statistically significant differences were found on the results between different fingers. Nor did paying attention to the tactile stimuli make difference on results. These may indicate that the somatotopical organization is still underdeveloped in children and that the primary somatosensory cortex is not strongly associated with higher order information processing. Problems with the quality of the measurement data and small sample size mean that further studies are needed to validate the results.
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