Abstracts

Rationale: Photosensitivity is characterised by an abnormal brain response to intermittent photic stimulation. Even though a range of approaches and techniques have been applied to identify its possible pathophysiological mechanisms, an agreed explanation of the phenomenon is still lacking. Some studies have suggested that the electro-clinical manifestations are due to an increased excitability in the visual cortex. We investigated visual habituation, a direct measure of the cortical excitability, in 60 children 19 with Photosensitive Epilepsy (PS), 23 normally developing children (ND) and 18 with Idiophatic Generalized Epilepsy without photosensitivity (IGE) Methods: Stimuli consisted of a full-field black-and-white checkerboard pattern subtending 15’ of arc, reversing at 3 Hz with a 100% contrast presented binocularly for 600 consecutive trials. Trials were averaged off-line in six sequential blocks of 100 responses and analysed separately. Peak latencies of the N75, P100 and N145 components of the visual Evoked Potential (VEP) and N75-P100, P100-N145 inter-peak amplitudes were measured. Habituation was expressed as the percentage change of amplitudes between the first and the subsequent blocks Results: One-way ANOVA revealed differences between the three groups in the slope index of N75-P100 amplitude suggesting increased or constant amplitude of the N75-P100 VEP component in the PS group across the 6 blocks with respect to the IGE and ND groups Conclusions: Photosensitive could be the result of an alteration of the balance between excitatory and inhibitory cortical processes and of compensatory mechanisms of visual gating control.