Magnetoencephalography

E Düzel

A dynamic approach to characterising functional reorganisation and irritative zones in epilepsy

Magnetoencephalography has a number of advantages over EEG, which make it ideal to study functional reorganisation of distributed networks and to localise the irritative zone on the basis of interictal discharges. To be able to use the full potential of MEG, we are developing new analytical and conceptual approaches.

We have developed analytical and conceptual approaches to using magnetoencephalography (MEG) to study two complementary issues in patients with epilepsy: (1) the role of the hippocampus in actively maintaining information in working memory; (2) the use of MEG to localise interictal epileptic activity and thereby guide invasive presurgical diagnostics. This research is important as it will help us to understand the (i) functional role of the hippocampus, a structure known to be critical for memory, (ii) how epilepsy can affect memory, and (iii) will lead to a reduced need for invasive EEG recordings that carry risk and are very costly.

Research in humans and non-human primates has suggested that a neural circuitry involving both prefrontal cortex (PFC), medial temporal (MTL) and visual cortex (VC) is critical for our ability to actively maintain visual information. We are now investigating how synchrony in this network is disrupted by bilateral hippocampal sclerosis. Such a finding would suggest that hippocampal dysfunction impairs dynamic cortical representations and thus sheds a new light on the memory deficits associated with hippocampal dysfunction. We developed new analytical techniques to measure and localise interregional synchrony.

This year we aim to establish recordings of intracranial data from patients undergoing invasive presurgical monitoring simultaneously with whole-head MEG. This will offer a new methodological perspecitve to relating medial temporal memory processing to distant cortical processing. At the same time, it will allow to establish to what extent epileptiform discharges in the MTL can be detected non-invasively with MEG.

We have scanned MRI-negative patients with focal epilepsy who are resistant to anticonvulsant treatment and in whom the ictal and interictal origin of epileptiform activity could not be localised using EEG, with MEG. This is the beginning of a longer term project to determine to what extent the source localisation of interictal epileptiform discharges (spikes) and slow-wave activity will provide converging information about the irritative zone. Patients who are candidates for epilepsy surgery have will then undergo invasive recordings. Here, the MEG data will be used to guide invasive recordings. It is anticipated that this programme will provide information about the utility of the MEG in guiding invasive presurgical diagnostics. We are currently systemically comparing different analytical approaches to the source analysis of interictal discharges such as beamformer techniques and techniques based on inverse solutions.

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