Magnetic source imaging of slow wave activity in psychiatric samples

Large-amplitude low-frequency activity, as in slow wave sleep or coma, indicates widespread synchronization of many neurons and may characterize brain states with little ongoing information processing. If prominent during the waking state, global slow wave activity are involved in neuropathological conditions, whereas focal slow waves are found in the vicinity of lesions, tumors, or epileptic foci. They have been associated with neuropathological or dysfunctional brain tissue (1). Enhanced activity in the lower frequency bands of the EEG has also been described in psychiatric disorders, e.g., in schizophrenia (2). The present study aimed at exploring whether local foci of slow wave activity can be determined in the MEG and whether the spatial distribution of generators of this brain activity would allow the assessment of dysfunctional brain regions also in psychiatric disorders. The source distribution of focal slow waves in the delta (1.5-4.0 Hz) and the theta (4.0-8.0 Hz) band were determined for patients with psychiatric diagnoses and healthy subjects during resting and mental activity. If slow wave activity were to represent dysfunctional brain processes, it should be prominent in those areas, which are assumed to be involved in the diagnosis-specific dysfunction. The association of the focal slow wave activity and neuropsychological test performance was assessed to further elucidate the functional significance of focal slow wave activity.