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D Kim, VM Zemon, E Pinkhasov, J Gordon, D Marks; Lateral Interactions in Cases of Partial-Complex Epilepsy: A Visual Evoked Potential Study . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1809.
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Purpose: To investigate nonlinear lateral interactions in the visual system of patients with partial-complex epilepsy and age-matched controls. Methods: Monocular stimulation was employed: dominant vs. nondominant eye conditions. Sweep VEPs (discrete 1-s steps) were elicited by windmill-dartboard stimuli in which static regions were increased in luminance contrast while contiguous dynamic regions were contrast-reversed at ∼ 4 Hz. Peak contrast of the dynamic regions was set at 32%, and contrast of static regions was: 0, 8, 16, 32, 64, and 100%. Also, the gap between static and dynamic regions was swept in ascending order from 0-15 minutes of arc. Each type of sweep condition was presented 10 times. Conventional VEPs to windmill-dartboard stimuli were also obtained in 1-minute runs. Fourier analysis was used to extract the relevant frequency components in the response. One of two statistics, Tcirc2 or magnitude-squared coherence, was applied to each set of 10 epochs to estimate the noise level in the signal for each frequency component of interest. A facilitation index and a suppression index were derived to quantify short- and long-range interactions, respectively. Results: Results depended on eye condition: deficits in long- and short-range interactions were found in the epilepsy group when the nondominant eye was stimulated. Greater noise levels were found consistently in the epilepsy as compared to the control group. Medication history was a factor in lateral interaction results (e.g., a GABA-mimetic drug, divalproex sodium, was associated with strong short-range interactions). Conclusion: Greater noise in patients appears to reflect higher ongoing cortical activity. This finding in addition to that of deficits in lateral interactions suggests a deficit in intracortical inhibition in this patient group.
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