A QUANTITATIVE-ANALYSIS OF SMOOTH-PURSUIT EYE TRACKING IN MONOZYGOTICTWINS DISCORDANT FOR SCHIZOPHRENIA

Background: Previous studies of discordant monozygotic (MZ) twins have suggested that abnormal smooth pursuit eye tracking is an indicator o f genetic liability for schizophrenia. We attempted to replicate this in a different sample of twins. Methods: Probands from 12 sets of MZ t wins discordant for schizophrenia who met DSM-III-R criteria for schiz ophrenia or schizoaffective disorder and their co-twins without psychi atric diagnosis (except 2 with a history of substance abuse) and 12 se ts of normal control MZ twins. Psychiatric diagnosis was based on Stru ctured Clinical Interview; monozygosity was based on analysis of 19 re d blood cell antigens. Smooth pursuit eye movement gain (equal to the ratio of eye-target velocity) and numbers, amplitudes, and subtypes of saccadic eye movements were compared. Measures were derived from comp uter analysis of digitized infrared oculographic recordings of constan t velocity (16.67 degrees per second) smooth pursuit eye tracking. Res ults: Quantitative measures of eye tracking for the affected twin were inferior to those of the unaffected co-twin, with affected twins show ing significant decreases in gain and significant increases in numbers and amplitudes of total and intrusive saccades. Moreover, whereas mea ns for the group of affected twins differed significantly from those o f normal controls on measures of gain and total saccades, means for th e group of unaffected co-twins were well within the normal range. Conc lusions: These data are consistent with the hypothesis that abnormal e ye tracking is associated with the expression of illness, or phenotype , in schizophrenia, at least in this twin sample. The data raise quest ions regarding the use of eye tracking measurement for identifying put ative gene carriers among at-risk relatives in genetic linkage studies of schizophrenia.