Source Reconstruction of Evoked Cortical Activity using Current Density Reconstruction within Realistically Shaped Head Models

It is well known that source reconstruction techniques based on EEG or MEG data are inferior to fMRI in respect to spatial resolution, but superior concerning the temporal resolution. The present contribution aims to demonstrate the power of modern source reconstruction techniques using standardized, realistically shaped volume conductor models in the spatio-temporal analysis of evoked potentials. A standardized approach of current density reconstruction within a volume conductor model derived from the so-called Montreal wharp brain as implemented in the SPM software will be demonstrated. This approach has the possibility to consider a priori source information, i.e., from functional MRI, to focus the source reconstruction results in respect to the source locations. Using this method, the following exemplary work on source localization and the temporal dynamics of evoked potentials will be demonstrated. The attention effects on median nerve SEP localizes at the contralateral S1 cortex, bilateral temporal, the cingulum and the left parietal cortex. Changes in the electrical activity induced by attention were shown to begin earlier than 100 ms post stimulus. Source reconstruction of the auditory mismatch negativity (MMN) revealed a cingulate source and bilateral temporal generators underlying the MMN. Temporal analysis showed that source activity started at around 130 ms post stimulus within the bilateral temporal gyrus followed by source activity within the cingulum peaking at 220 ms. Source reconstruction of the cortical activity during line bisection judgement revealed 5 source regions, corresponding to those evaluated by a previous fMRI study: right middle occipital gyrus, right superior posterior parietal cortex; bilateral inferior occipital gyrus, and right inferior posterior parietal cortex. Temporal deconvolution presented a sequential activation of these areas starting at the primary visual cortex, followed by the right superior posterior parietal, bilateral inferior occipital cortex and right inferior parietal cortex. In the identification of a network processing spontaneous perceptive reversals of the rotating Necker cube we find support for synchronized neuronal oscillatory activity in the correlation between visual area V3 and left SMA. In summary, the presented work demonstrates the applicability of analyzing EEG data in actual clinical neurophysiology for „functional imaging“ and timing within complex networks of cortical activation.