fMRI activation during spike and wave discharges evoked by photic stimulation

Photoparoxysmal response (PPR) is an electroencephalographic (EEG) trait characterized by the occurrence of epileptiform discharges in response to visual stimulation. Studying this trait helps to learn about mechanisms of epileptogenicity. While simultaneous recordings of EEG and functional MRI (EEG-fMRI) in patients with spontaneous generalised spike-wave discharges (GSW) have revealed activation of the thalamus and deactivation in frontoparietal areas, EEG-fMRI studies on evoked GSW such as PPR are lacking. In this EEG-fMRI study, 30 subjects with reported generalised PPR underwent intermittent photic stimulation (IPS) in a 3 T MR scanner. PPR was elicited in 6 subjects, four diagnosed with idiopathic generalised epilepsy and two with tension-type headache. Because PPR is preceded by synchronization of cortical gamma oscillations, blood oxygenation level-dependent (BOLD) signal changes were analysed at the onset of the PPR (standard regressor) and 3 s before the onset of PPR (early regressor) in one model. In all subjects, IPS led to a significant activation of the visual cortex. Based on the early regressor, PPR associated activation was found in the parietal cortex adjacent to the intraparietal sulcus in five and in the premotor cortex in all 6 subjects. The standard regressor revealed deactivation in early activated areas in all subjects and thalamic activation in one subject. In contrast to spontaneous GSW, these results suggest that PPR is a cortical phenomenon with an involvement of the parietal and frontal cortices. Pronounced haemodynamic changes seen with the early regressor could mirror gamma activity that is known to precede PPR.