Inability to pursue non-rigid motion produces instability of spatial perception

Many types of environmental motion are non-rigid, for example the movements of water, fire or smoke. Here we report a type of non-rigid motion that is perceived as moving but that cannot be pursued and that leads to a breakdown of spatial stability when one attempts to pursue it. Each frame of our non-rigid motion stimulus consisted of a random dot distribution devoid of structural information. Across frames, dots within a circular area were rotated around the area's center, forming a vortex motion pattern. Independent of the first-order motion within it, the vortex then moved across the screen. Participants were asked to pursue the vortex. Smooth pursuit gain was very low and compensated by frequent catch-up saccades. Interestingly, participants reported the vortex as jumping. In contrast, the vortex was perceived as moving smoothly if the vortex jumped backwards during the saccades to compensate its movement during saccade planning and execution. Saccades from fixation also caused the jump percept and the jump direction was always in the motion direction of the vortex, independent of saccade direction. We conclude that the movement of the vortex during saccades is not correctly predicted for perception, impairing spatial stability. Despite this, the saccades were accurate on the targeted vortex, so its motion is accurately predicted for saccade planning. In an additional experiment, we embedded the vortex within a ring of second-order motion, making the vortex pursuable. When asked to identify the formerly observed vortex pattern in a discrimination task, participants more often chose the pattern with additional first-order motion congruent with the vortex movement across the screen than the correct pattern. We propose that pursuit velocity and the prediction of position across saccades for perception is based on first-order motion, whereas saccade planning and non-rigid motion perception is based on the movement of an entity.