Assessing seizure susceptibility using visual psychophysical tests

Abstract

The spread of epileptic activity within the cortex is opposed by a powerful inhibitory restraint. We hypothesized that the same inhibitory mechanisms are likely also to underlie the phenomenon of centre-surround suppression. In this thesis, I used different non-invasive visual psychophysical assays of surround suppression to answer whether they can be used as a measurement of network state in epilepsy and as a way of predicting seizures. We recruited 146 healthy volunteer controls and 54 patients with clinically confirmed epilepsy. Three different stimulus paradigms (motion direction discrimination, contrast detection and orientation discrimination tasks) were used to derive surround suppression indices which are believed to reflect the strength of cortical inhibition. Our results suggest that motion and contrast surround suppression phenomena are not related. We found that suppression indices for the different tests in individual participants were not significantly correlated. In addition, multivariate regression analyses showed that motion suppression index was predicted strongly by age and seizure type, but not by seizure frequency. Specifically, we found that patients with exclusively focal epilepsy, and no history of generalization, showed significantly stronger cortical inhibition as measured by the surround suppression index compared to all other groups, including controls. In contrast, patients with focal seizures evolving into generalised seizures, and patients with generalised genetic epilepsy, showed a similar level of cortical inhibition to controls. To answer whether psychophysical tests can be used as a way of predicting seizures, a longitudinal study was designed, deriving repeated measures of suppression indices in individuals. The results indicated no strong link between timing of seizures and suppression indices in patients. iii In conclusion, visual psychophysics provides a simple and non-invasive means of assessing the state of inhibitory networks involved in the pathophysiology of epilepsy. The inability to increase activity in inhibitory networks in response to focal epileptic seizure may predict the risk of generalised seizures, which may in turn allow stratification of SUDEP risk.