Abstracts

Rationale: Patients with photosensitive epilepsy have abnormal responses to visual stimuli even during interictal periods (Porciatti et al, Nat Neurosci. 2000;3:259-63). Their responses are characterized by abnormalities in contrast gain control mechanisms that may be secondary to an underlying cortical imbalance of excitation and inhibition. We hypothesized that similar deficits should be observed in patients with idiopathic generalized epilepsy (IGE): a disorder associated with prominent albeit not predominant photosensitivity. Methods: We measured steady state visually evoked potentials (ssVEP) to contrast reversing gratings over a wide range of stimulus contrast. ssVEPs were analyzed at the pattern reversal rate using spectral analysis. Subjects were adult patients with idiopathic generalized epilepsy and age-matched subjects without epilepsy. We included epileptic subjects with no known history of photosensitivity to minimize the risk of inducing photoparoxysmal responses or seizures. Results: The visual responses of epileptic subjects did not saturate at high stimulus contrasts, unlike control subjects. This finding was not attributable to an antiepileptic drug effect. Response saturation relies on inhibitory interactions that modulate neural activities as part of a general mechanism of sensitivity regulation (gain control). Analysis using a well-known computational model of gain control (Heeger, Vis Neurosci. 1992;9:181-97) suggests that decreased sensitivity to both inhibitory and excitatory inputs accounts for observed data with inhibitory drive showing the largest relative deficit. Conclusions: IGE patients without photosensitivity have abnormal contrast gain as measured with the ssVEP. This abnormality may reflect a relative imbalance of excitation and inhibition. Visual stimulation paradigms are well-suited for studying a variety of gain control mechanisms and further studies of gain control using the ssVEP may help to elucidate the mechanisms of hyperexcitability underlying IGE.