A quantitative analysis of smooth pursuit eye tracking in monozygotic twins discordant for schizophrenia

Previous studies of discordant monozygotic (MZ) twins have suggested that abnormal smooth pursuit eye tracking is an indicator of genetic liability for schizophrenia. We attempted to replicate this in a different sample of twins.

Probands from 12 sets of MZ twins discordant for schizophrenia who met DSM-III-R criteria for schizophrenia or schizoaffective disorder and their co-twins without psychiatric diagnosis (except 2 with a history of substance abuse) and 12 sets of normal control MZ twins. Psychiatric diagnosis was based on Structured Clinical Interview; monozygosity was based on analysis of 19 red 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 computer analysis of digitized infrared oculographic recordings of constant velocity (16.67 degrees per second) smooth pursuit eye tracking.

Quantitative measures of eye tracking for the affected twin were inferior to those of the unaffected co-twin, with affected twins showing significant decreases in gain and significant increases in numbers and amplitudes of total and intrusive saccades. Moreover, whereas means for the group of affected twins differed significantly from those of normal controls on measures of gain and total saccades, means for the group of unaffected co-twins were well within the normal range.

These data are consistent with the hypothesis that abnormal eye tracking is associated with the expression of illness, or phenotype, in schizophrenia, at least in this twin sample. The data raise questions regarding the use of eye tracking measurement for identifying putative gene carriers among at-risk relatives in genetic linkage studies of schizophrenia.