ID 431 – A novel method for the diagnosis of photosensitive epilepsy based on the phase-locking of evoked gamma-band magnetoencephalographic oscillatory brain responses

Abstract

Larger size and higher spatial frequency of grating stimuli have been shown not only to maximally modulate early gamma-band visual cortical responses but also to provoke seizures in photosensitive epilepsy. Occipital brain magnetic field (MEG) oscillatory responses were evoked upon static grating pattern subthreshold stimuli in 12 patients with photosensitive epilepsy and 2 matched control groups, one with epilepsy but no photosensitivity and the other healthy controls. The continuous wavelet transform (CWT) was employed to characterize the time–frequency energy dynamics and phase-locking of the early evoked gamma-band oscillatory responses (eGBR). The photosensitive epilepsy group showed statistically significantly increased phase-locking of the gamma-band responses (eGBR) of the striate visual cortex in the (40–70 Hz) × (25–300 ms) and in the (25–30 Hz) × (300–600 ms) time–frequency windows with respect to the healthy controls. Photosensitive epilepsy seems to be characterized by altered phase synchronization dynamics at a higher frequency range (a-frequency band) compared to healthy controls. Our results indicate that photosensitive epilepsy is driven by the large-scale phase-locking of the underlying striatal visual cortical unit oscillators in specific “photosensitive” frequency components (dynamic phase-attractor theory of epilepsy). Our method could provide a novel diagnostic tool in safely detecting, investigating and assessing response to treatment in photosensitive epilepsy.