It has been reported that concurrent manual tracking enhances smooth pursuit of a moving target only when the target movement is predictable. Here we report a study in which such facilitation was found when the target movement was unpredictable. The experiment consisted of two conditions. In the eye-hand condition, participants were asked to use pursuit eye movements to track the movement of a cyan Gaussian target (??=0.6?°) on a computer screen (40?°Hx30?°V) as its horizontal position was perturbed by the sum of seven harmonically-unrelated sinusoids (0.1-2.19 Hz). In the meantime, they used a high precision mouse to control the horizontal position of a red Gaussian cursor (8?° below) to vertically align it with the target. In the eye alone condition, the target and cursor motion recorded in the eye-hand condition were replayed, and participants were asked to only track the target with pursuit eye movements. Across 13 participants, the overall eye tracking performance indicated by the RMS eye position error over the course of a 90-s trial did not differ between the two conditions. However, the frequency-response analysis on smooth pursuit eye movement data with saccades removed showed that the average smooth pursuit gain was significantly higher for the eye-hand than the eye alone condition (Cohena??s d=0.61), and the pursuit phase lag decreased significantly for four out of seven input perturbation frequencies. Furthermore, while the average saccade amplitudes and durations did not differ between the two conditions, the number of saccades made per trial in the eye-hand condition (mean?±SEs: 215?±6) was significantly less than that in the eye alone condition (250?±6). We conclude that concurrent manual tracking increases the gain and decreases the phase delay of smooth pursuit, and that this pursuit enhancement by concurrent manual tracking is not specific to predictable target movement but is a fundamental property of eye-hand coordination.
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