Evidence for increased visual gamma responses in photosensitive epilepsy

A sustained gamma (30-70 Hz) oscillation induced in occipital cortex by high-contrast visual stimulation has been well characterised in animal local field potential recordings and in healthy human participants using magnetoencephalography (MEG). The spatial frequency of a static grating stimulus that gives maximal gamma is also that most likely to provoke seizures in photosensitive epilepsy.

We used MEG to study visual responses induced by grating stimuli of varying contrast and size in twelve patients with photosensitive epilepsy and two matched control groups, one with epilepsy but no photosensitivity, the other healthy controls. We used a beamformer approach to localise cortical responses and to characterise the time-frequency dynamics of evoked and induced oscillatory responses.

A greater number of patients with photosensitivity had particularly amplitude gamma responses compared to controls. Formal statistical testing failed to find a group difference. One photosensitive patient, tested before and after sodium valproate, had a peak gamma amplitude when drug naive over four times larger than the group mean for controls; this high amplitude was substantially decreased after treatment with sodium valproate. We found no difference in the frequency of the sustained gamma response between the three groups.

Altered power, but not frequency, in induced cortical responses to a static grating stimulus may be a characteristic of photosensitive epilepsy. Our failure to find a group difference on statistical testing may have been due to a wide intersubject variability and heterogeneity of the photosensitive group. A high amplitude response would be in keeping with previous evidence of altered contrast gain and increased spatial recruitment in photosensitive epilepsy.