Visual evoked potentials and seizure disorders
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Epilepsy is a disorder thought to involve a deficiency in inhibitory processing and deficits in GABA-mediated inhibition in particular. Therefore, it is thought that some epileptic patients might exhibit abnormal VEPs. In this study, VEP measures that may reflect inhibitory activity were obtained from 20 patients with partial-complex epilepsy and 20 aged-matched healthy adults. Two types of nonlinear lateral interactions elicited by windmill-dartboard stimuli, presumed to be inhibitory in nature, were expressed in terms of a facilitation index (short-range interactions) and a suppression index (long-range interactions). Transient VEPs to contrast-reversing checkerboards were also recorded because the P1 component appears to be of GABAergic origin. In addition, both swept-parameter and one-minute runs were performed.;Analyses were conducted for responses obtained separately by stimulation of dominant and non-dominant eyes. Subgroups of patients, categorized according to medication, were also analyzed separately. Quantification of responses was based on multivariate statistic, Tcir2, and on a measure of coherence (magnitude-squared coherence, MSC).;Results indicated deficient suppressive interactions in the patient group under the non-dominant eye condition for both one-minute and swept parameter runs. A deficit in these longer-range lateral processes may contribute to uncontrolled excitatory activity. The concept of shunting inhibition was explored as a possible explanation for the loss in the suppressive interaction.;One unexpected finding was the presence in many of the patients of short-range interactions, which are thought to reflect a GABAergic process, as evidenced in the facilitation index. The data obtained from patients on a GABA-enhancing medication (Depakote, divaproex sodium) which exhibited strong facilitation, supported this notion. Transient VEP data also demonstrated differences in the patient and control groups. Comparable amplitude measures in these two groups lead one to conclude that lower noise values are responsible for the higher magnitude-squared coherence (MSC) values in the control group. Greater EEG noise in the patient group may reflect a higher level of uncontrolled cortical activity in the patients. Latency measures from transient VEPs were also different for the two groups.;Thus, a number of findings support select inhibitory dysfunction in partial-complex epilepsy.
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