The construct validity of root-mean-square error for quantifying smooth-pursuit eye tracking abnormalities in schizophrenia.

Smooth-pursuit eye tracking abnormalities have been consistently observed among schizophrenia patients and their relatives (Iacono and Clementz 1993; Levy et al 1993). Using a statistical technique called mixture analysis, recent studies have demonstrated that distributions of both the patients' and their relatives' ocular motor data are best characterized by two distinct components (Blackwood et al 1991; Clementz et al 1992; Gibbons et al 1984; Iacono et al 1992; Ross et al in press; Sweeney et al 1993). One component overlaps the scoring distribution of normal subjects fairly closely, whereas the other component captures subjects with poor eye tracking performance. The pattern of results suggests that ocular motor measures are capturing differences in kind among schizophrenia subjects, and that these differences are indexing biologically distinct subgroups of patients in whose families ocular motor abnormalities are an indicator of vulnerability for some form of this illness. This conclusion is consistent with a report suggesting linkage between eye tracking abnormalities and markers on chromosome 6 (Arolt et al 1995; see also Wang et al 1995). The above results, which rely primarily on "global" measures of eye tracking performance, should be considered in light of current trends to use "specific" measures, such as saccadic performance and the calculation of smooth pursuit gain, to