Discrimination of curvature from motion during smooth pursuit and fixation

Smooth pursuit and motion perception have mainly been investigated with stimuli moving along linear trajectories. Here we studied the quality of pursuit movements to curved motion trajectories in human observers and examined whether the pursuit responses would be sensitive enough to discriminate various degrees of curvature. In a two interval forced choice task subjects pursued a Gaussian blob moving along a curved trajectory and then they indicated in which interval the curve was flatter. We also measured discrimination thresholds for the same curvatures during a fixation. Motion curvature had some specific effects on smooth pursuit properties: trajectories with larger amounts of curvature elicited lower open-loop acceleration, lower pursuit gain and larger catch-up saccades compared to less curved trajectories. Initially, target motion curvatures were underestimated, however, around 300 ms after pursuit onset, pursuit responses closely matched the actual curved trajectory. We calculated perceptual thresholds for curvature discrimination, which were on the order of 1.5 degrees of visual angle (deg) for a 7.9 deg curvature standard. Oculometric sensitivity to curvature discrimination based on the whole pursuit trajectory was quite similar to perceptual performance. Oculometric thresholds based on smaller time windows were higher. Thus, smooth pursuit can quite accurately follow moving targets with curved trajectories, but temporal integration over longer periods is necessary to reach perceptual thresholds for curvature discrimination.

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