Abnormal slow wave mapping (ASWAM)—A tool for the investigation of abnormal slow wave activity in the human brain

Abstract

Slow waves in the delta and theta frequency range, normal signs of deactivated networks in sleep stages, are considered 'abnormal' when prominent in the waking state and when generated in circumscribed brain areas. Structural cortical lesions, e.g. related to stroke, tumors, or scars, generate focal electric and magnetic slow wave activity in the penumbra. Focal concentrations of slow wave activity exceeding those of healthy subjects have also been found in individuals suffering from psychiatric disorders without obvious structural brain damage. Hence, identification and mapping of abnormal slow wave activity might contribute to the investigation of cortical indications of psychopathology.Here I propose a method for abnormal slow wave mapping (ASWAM), based on a 5 min resting magnetoencephalogramm (MEG) and equivalent current dipole fitting to sources in the 1-4 Hz frequency band (delta) in anatomically defined cortical regions. The method was tested in a sample of 116 healthy subjects (59 males), with the aim to provide a basis for later comparison with patient samples. As to be expected, delta dipole density was low in healthy subjects. However, its distribution differed between genders with fronto-central > posterior dipole density in male and posterior dominance in female participants, which was not significantly related to either age or headsize. Results suggest that this method allows the identification of ASWA, so that comparison against Z-scores from a larger normal control group might assist diagnostic purposes in patient groups. As specific distributions seem to reflect differences between genders, this should be considered also in the analysis of patient samples.

Changes in cortical slow wave activity in healthy aging

“…Within these eight regions dipole density was estimated as the average of the voxel-based dipole density. (For a more detailed description of data analysis see Wienbruch (2007))…”

“…Slow waves in the delta frequency range can be detected by electroencephalography (EEG) as well as by magnetoencephalography (MEG). However, since MEG allows the separation of focally generated slow waves from those with widespread sources, MEG appears better suited for the identification of focal slow wave activity than EEG (Wienbruch 2007). An equivalent current dipole model provides an excellent approximation to localize such focal assemblies of active pyramidal cells (Elbert 1998).…”

“…An equivalent current dipole model provides an excellent approximation to localize such focal assemblies of active pyramidal cells (Elbert 1998). Accordingly, in abnormal slow wave activity mapping (ASWAM) (Wienbruch 2007), generators of focal magnetic slow waves in the 1-4 Hz (delta) frequency range are localized by employing a single moving dipole model and by calculating dipole densities in anatomically defined cortical regions (frontal, central, temporal and parieto-occipital). High densities of focal generators of such slow wave activity have been found to be related to brain pathology or dysfunctional brain tissue as a result of metabolism and blood flow changes after an insult (e.g.…”

“…So far, alterations in slow wave density have only been shown in pathological aging, but the distribution of slow waves during the normal, non-pathological aging process remains to be explored. To our knowledge, only Wienbruch (2007) investigated age effects in delta dipole density in healthy individuals. In this sample, only a small correlation between age and slow wave activity was found which led the author to conclude that age per se may not account for an enhancement of slow wave activity often seen in subjects with mild cognitive decline or dementia.…”

“…The present study aimed at exploring age-related alterations in the distribution of generators of focal magnetic slow waves and associations between cognitive performance and sources of low-frequency activity in the brain using magnetic source imaging. For this purpose, we performed the above mentioned abnormal slow wave activity mapping (ASWAM) method that has been shown to be a valid tool in the vicinity of the detection of pathological attributed slow wave activity (Wienbruch 2007). …”