A model of smooth pursuit performance illustrates the relationship between gain, catch-up saccade rate, and catch-up saccade amplitude in normal controls and patients with schizophrenia

Abstract

Smooth pursuit eye-tracking performance of 37 unmedicated patients with schizophrenia and 45 normal controls was recorded with infrared oculography, and digitized for offline analysis of gain and other measures. Target velocity was 5°/sec. Patients with schizophrenia had significantly lower gain than normal controls, confirming several earlier reports. During low gain smooth pursuit tracking, when eye velocity is slower than target velocity, the saccadic system corrects for the accumulating position error with catch-up saccades (CUS). The rate of CUS and the median CUS amplitude were compared between groups. Patients had significantly more CUS than controls, but the median CUS amplitudes were not significantly different. A nonlinear mathematical model of the relationship between gain, CUS amplitude, and CUS rate during steady-state constant velocity tracking was developed to integrate these findings. According to this model, for a given gain, correction for position error can result from either a few large CUS or many small CUS. The fit of the model to empirical data, as assessed graphically and with linear and nonlinear regression techniques, was excellent. The model fit the data for both patients and controls. The psychiatric eye-tracking literature is discussed from the perspective suggested by the modeled relationship.