Quantifying stereokinetic depth: Divergence across methods despite robust within-subject precision

In the stereokinetic effect, two non-concentric circles rotating in the image plane are perceived as a tilted, deforming 3D cylinder extending in depth. The cylinder's finite length poses a theoretical puzzle: minimal deformation favors infinite depth by eliminating relative motion, whereas a slow 3D motion constraint would limit depth by reducing the velocity of the back circle. If both constraints operate, they should yield a finite length. We tested their contributions by measuring perceived cylinder length with four methods: linear perspective adjustment, wireframe sphere fitting, binocular disparity matching, and motor reaching. Across participants, all methods yielded precise within-subject measurements but revealed systematic quantitative differences between methods, with only linear perspective and reaching strongly correlated. Variations in rotational speed had no effect, providing no support for a slow 3D motion constraint. Instead, perceived length increased with inter-circle distance, supporting minimal deformation as a key constraint. We propose that the finite length may additionally reflect a generic viewpoint constraint disfavoring cylinders aligned with the line of sight, a hypothesis for future investigation. Quantitative estimates depend critically on measurement method, suggesting distinct computational mechanisms for stereo versus monocular cues and for pictorial versus action-oriented depth tasks.