On Neuromagnetic Assessment of the Contingent Negative Variation
Abstract: The contingent negative variation, CNV, is a cognitive process related to anticipation and motor preparation. In the present work the CNV was studied by the use of magnetoencephalography, MEG. MEG can achieve high spatial and temporal information about neuroelectrical activity and is thus a promising tool for gaining a better understanding of the complex neurological processes related to the CNV. The neuromagnetic recording of the CNV is, however, a nontrivial task that requires extensive signal processing and carefully designed experimental protocols. There are further reasons to assume that the neuromagnetic fields related to CNV are generated by a different electrophysiological phenomenon from that of neuromagnetic fields of shorter duration.The contingent magnetic variation, CMV, in a warned choice reaction time task was studied in healthy subjects. Based on the spatial and temporal properties of the recorded neuromagnetic signals, four slow components could be separated from the CMV. Three early slow components peaked in the interval 300 - 700 ms after the warning stimulus. The late component started 500 - 700 ms before the imperative stimulus and ended abruptly at the delivery of the imperative stimuli. Large interindividual differences were seen in the early components of the CMV, and general conclusions in terms of source localizations could not be drawn from the data obtained. The results suggest, however, that the dominant sources of the late component are symmetrically located in the left and right premotor areas. It is proposed that the glial buffering current, GBC, is a potential source of sustained neuromagnetic fields, and that it results in an equivalent current dipole directed perpendicular from the surface of the cortex, the same as could be expected due to post-synaptic currents, PSCs. Some of the main features of neuromagnetic fields generated by GBC, compared to fields of PSC origin, are: temporal and spatial integration, phase reversal at the termination of neural activity, and finally that magnetic fields are evoked also by pre-synaptic activity, inhibitory activity and activity in nonpyramidal neurons. Algorithms are developed for calculations of iso-field maps using a three-dimensional, rule-assisted interpolation algorithm on an ellipsoidal surface. Benefits of the algorithm developed, compared to previously used algorithms, include true three-dimensional interpolation, accurate handling of iso-map boundaries, and fast updating of iso-field maps at time or view-angle changes. This thesis further describes the paradigm, the recording procedures, and the artifact rejection algorithms developed to avoid electric and magnetic artifacts and to minimize the influence of the variance in the alertness of the subject on the result. The reaction time is one important parameter in the determination of the alertness. Algorithms are developed for automatic EMG onset detection using rectified median filtered EMG signals.
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