Metrics of two-dimensional smooth pursuit are diverse across participants and stable across days

Smooth pursuit eye movements are used to volitionally track moving objects, keeping their image near the fovea. Pursuit gain, the ratio of eye to stimulus speed, is used to quantify tracking accuracy and is usually close to 1 for healthy observers. Although previous studies have shown directional asymmetries such as horizontal gain exceeding vertical gain, the temporal stability of these biases and the correlation between oculomotor metrics for tracking in different directions and speeds have not been investigated. Here, in testing sessions 4 to 10 days apart, 45 human observers tracked targets moving along two-dimensional trajectories. Horizontal, vertical, and radial pursuit gain had high test-retest reliability (mean intraclass correlation 0.84). The frequency of all saccades and anticipatory saccades during pursuit also had high test-retest reliability (intraclass correlation coefficients = 0.66 and 0.73, respectively). In addition, gain metrics showed strong intermetric correlation, and saccade metrics separately showed strong intercorrelation; however, gain and saccade metrics showed only weak intercorrelation. These correlations are likely to originate from a mixture of sensory, motor, and integrative mechanisms. The test-retest reliability of multiple distinct pursuit metrics represents a "pursuit identity" for individuals, but we argue against this ultimately contributing to an oculomotor biomarker.