Two visual mechanisms of photosensitivity

Purpose: Photosensitive epilepsy is the most common of the "reflex" epileps ies. Precipitated by television viewing, flickering light, or specific visu al patterns, it is the cause of seizures in 10% of young people with epilep sy. Photosensitivity is associated with two types of EEG abnormalities: pho toparoxysmal responses (PPRs) and occipital spikes (OSs). It is unclear whe ther these abnormalities are mediated by different mechanisms, and furtherm ore, the clinical significance of OS is unknown. Methods: By using our previously established population of patients with ph otosensitive epilepsy, all showing EEG abnormalities on intermittent photic stimulation or pattern stimulation, we examined the effects of pattern con trast, spatial and counterphase temporal frequency, and colour on these abn ormalities. Results: PPRs and not OSs show linens contrast dependency and are elicited by stationary stimuli and by non-colour-opponent isoluminant stimuli. Conclusions: PPRs and OSs are generated independently by the parvocellular and magnocellular visual systems, respectively. The results add support to the hypothesis that only PPRs and not OSs are clinically significant.