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Mary M. Conte, Anastasiya Ashurova, Laura J. Ponticello, Erik J. Kobylarz, Douglas R. Labar, Jonathan D. Victor; Changes in VEP indices of cortical lateral interactions with epilepsy treatment. Journal of Vision 2006;6(6):204. doi: 10.1167/6.6.204.
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The steady-state VEP elicited by appropriately-designed stimuli, such as the radial windmill-dartboard, can be parsed to isolate components generated by lateral interactions among cortical neurons. The purpose of this study was to determine whether such indices of lateral interactions are affected by treatments for refractory epilepsy. Steady-state VEPs were recorded in 31 epileptic patients receiving standard anti-epileptic drug (AED) treatment (N=24); or long-term (3.5–10 yrs) vagus nerve stimulation (VNS) therapy (N=7). To determine the acute effects of neurostimulation on the VEP, VNS patients were tested twice on the same day, with the stimulator turned on (STIM-ON) and off (STIM-OFF). Twenty-one age-matched normals served as controls. Stimuli consisted of the radial windmill-dartboard pattern and conventional checkerboards (contrast: 0.3; modulation rate: 4.19 Hz). VEP responses were averaged and Fourier analyzed. First- and second-harmonic response components were used to calculate indices reflecting facilitatory (FI) and inhibitory (SI) cortical interactions, along with confidence limits on these indices. Compared to normals, both patient groups showed no difference in the facilitation index, but significantly (p < .05) less lateral suppression. For the VNS patients, there was no difference (paired t-test) between responses obtained in the STIM-ON and STIM-OFF conditions. VEP responses to the radial windmill/dartboard pattern can be used to measure lateral interactions in normal subjects and epileptic patients, including during administration of VNS. This technique identifies an alteration of cortical interactions associated with VNS and gabapentin treatment. We speculate that these changes are related to the mechanisms of action of these treatments.
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