Temporal integration of focus position signal during compensation for pursuit in optic flow.

Observer translation results in optic flow that specifies heading. Concurrent smooth pursuit causes distortion of the retinal flow pattern for which the visual system compensates. The distortion and its perceptual compensation are usually modeled in terms of instantaneous velocities. However, apart from adding a velocity to the flow field, pursuit also incrementally changes the direction of gaze. The effect of gaze displacement on optic flow perception has received little attention. Here we separated the effects of velocity and gaze displacement by measuring the perceived two-dimensional focus position of rotating flow patterns during pursuit. Such stimuli are useful in the current context because the two effects work in orthogonal directions. As expected, the instantaneous pursuit velocity shifted the perceived focus orthogonally to the pursuit direction. Additionally, the focus was mislocalized in the direction of the pursuit. Experiments that manipulated the presentation duration, flow speed, and uncertainty of the focus location supported the idea that the latter component of mislocalization resulted from temporal integration of the retinal trajectory of the focus. Finally, a comparison of the shift magnitudes obtained in conditions with and without pursuit (but with similar retinal stimulation) suggested that the compensation for both effects uses extraretinal information.