EEG functional connectivity of the intrahemispheric cortico-cortical network of idiopathic generalized epilepsy

Intrahemispheric, cortico-cortical EEG functional connectivity (fC) was investigated in untreated patients with idiopathic generalized epilepsy (IGE) in this explorative study.

Group comparison was carried out between 19, drug-naive IGE patients and 19, matched healthy persons. 90×2s of 19 channels waking, interictal background EEG signal (without epileptiform potentials) were processed to the LORETA (low resolution electromagnetic tomography) software to compute current source density for 2394 voxels representing parcels of the cerebral cortex for 25 very narrow bands of 1Hz bandwidth (VNBs) from 1 to 25Hz. EEG fC was investigated among the already localized sources. Pearson correlation coefficients (R) were computed among the 33 regions of interest (ROI) within the left and within the right hemisphere, separately. Group differences were computed by means of t-statistics. Corrected p<0.05 differences were accepted as statistically significant.

(1) The anatomical patterns of the fC differences showed great frequency-dependency. (2) Hemispheric asymmetry was prominent within most VNBs. (3) Decreased fC in the IGE group was found across all VNBs in the 1-6Hz frequency range as compared to mixed patterns comprising both increased and decreased fC at >6Hz frequencies. (4) In the 5-25Hz range, decreased fC dominated in the anterior, increased fC in the posterior parts of the cortex. (5) The results delineated an anterior and a posterior network.

(1) Decreased fC in the 1-6Hz band might indicate some relationship to yet hidden structure network abnormalities. (2) The anatomical patterns of fC indicate frequency-dependent, pathological coupling and decoupling processes in the interictal state. (3) The two networks might help to understand seizure liability and seizure precipitation in IGE.

This is the first study to explore EEG fC in the interictal condition of IGE patients. The importance of EEG frequencies in evaluating fC in IGE was demonstrated and starting points for further research were given.