Abstracts

Photosensitive epilepsy is the most common form of reflex epilepsy. Many elements of photosensitive epilepsy are studied, including characteristics of triggering stimuli and relation to epilepsy type. Less well understood is the derivation of the underlying photosensitive condition/state. Genetics or lesions do not explain fully the etiology of photosensitivity, and some clinical and basic science reports raise the question of the role of environmental experience. The goal of the present study was to test previous suggestions that photosensitivity may also arise following repeated exposure to certain visual stimuli. Adult rats of several ordinary strains were implanted with electrodes for chronic electrocorticographic recording. Following a period of recovery and habituation to the recording chamber, the animals were exposed to short trains of intermittent strobe stimulation, usually at 8 Hz. In some studies, stimulating electrodes were placed in the thalamic lateral geniculate nucleus so we could monitor the efficacy of geniculo-cortical transmission. Visual stimulation initially elicited a weak visual response or photic driving in occipital cortex (n=36 rats). However, repeated exposure to strobe stimulation led to the rapid, progressive development of a high amplitude photoparoxysmal response (PPR) in 34/36 rats that could persist 3-4 stimulus cycles after strobe train offset. The epileptiform characteristics of the PPR included spike-wave morphology, generalization, and sensitivity to ethosuximide. Acquisition of photosensitivity required high intensity strobe stimuli, but once acquired, low intensity strobes could trigger a PPR, suggesting that, once photosensitivity has been acquired, the mechanisms that underlie acquisition of photosensitivity differ from those for triggering the PPR. Acquisition of photosensitivity was paralleled by potentiation of the response to electrical stimulation of the LGN. Acquisition was blocked by systemic application of an NMDA antagonist, suggesting involvement of long-term potentiation mechanisms. Photosensitivity in ordinary adult rats can be acquired through repeated exposure to high intensity strobe stimulation. This provides evidence that environmental factors may play some role in the evolution of photosensitivity in humans. (Supported by NIH)