Mixture analysis of pursuit eye-tracking dysfunction in schizophrenia.

Results of two recent studies suggest that a distinct subgroup of schizophrenic patients and their relatives have particularly deviant eye tracking. Such heterogeneity could be of considerable importance, as it may indicate significant pathophysiologic or etiologic heterogeneity in schizophrenia. An analysis of 101 consecutive-admission schizophrenic patients confirmed the existence of two distinct subgroups of patients with higher and lower levels of spatial [root mean square (RMS)] eye-tracking error. However, there was no heterogeneity in the disturbance of pursuit eye movements. Anticipatory saccades, which by definition add very large amounts of spatial tracking error, were more frequent in the "high" RMS error group. Rates of anticipatory saccades were similar in the "low" RMS error patient group and normal controls, and there was no heterogeneity in the expression of anticipatory saccades. Apparent heterogeneity in global indices of eye-tracking impairment in schizophrenia appears to be a measurement artifact reflecting the powerful influence of anticipatory saccades on global performance indices, rather than true heterogeneity in the expression of any specific eye movement abnormality.