Abstract
Introduction: Optic flow and image-based vision are treated by the Two Visual Systems hypothesis (Milner and Goodale (1996)) as anatomically separate systems. We advocate conversely that optic flow and image structure are functional components of a unitary perceptual system. Optic flow provides powerful but temporary depth information; image structure is persistent but weak in specifying depth. When combined, optic flow informs image structure that provides embodied memory. Method: Two random-textured planes–a large rear plane containing targets seen through holes in a smaller front plane (holes without targets were distracters)–rotated rigidly to reveal depth structure; then the rear plane translated in one of 8 diagonal directions and stopped with targets occluded. Participants marked locations of hidden targets after some delay, during which they saw either the static image or a blank screen. In Experiment 1, delays were 5s, 10s or 15s with 2 to 15 targets and distracters, respectively, in three conditions: image structure only–holes were outlined but translation of plane was discontinuous; optic flow only–holes were not outlined but translation was continuous; and optic flow plus image structure. In Experiment 2, delays were 5s or 25s and numbers of targets and distracters were 9, 12, 15 or 18, respectively. Results: In Experiment 1, participants could not locate targets with only image structure and no optic flow. With only optic flow, participants correctly located up to 3 targets. With both, participants correctly located more than 60% of the 15 targets with 15s delay. In Experiment 2, mean numbers of targets correctly located were 8.0 without blank regardless of delay lengths; 7.7 with blank and 5s delay; and 7.0 with blank and 25s delay. Conclusion: Optic flow and image structure contribute functionally distinct properties to a single visual system. Optic flow yields layout and image structure preserves it.