This was my first paper in the field of neuroscience. It represents work I did as an M.Sc. project with Richard Eagle, in the psychology department of Oxford University. Richard died very suddenly, aged 32, while we were working on this project, a personal tragedy for his friends and family and a great loss to vision science. I am very grateful to Simon Prince and Bruce Cumming for their kindness in helping me to write this up, and to Randy Blake, who as editor of Vision Research understood that it was difficult for me to submit my first paper in the field after Richard had died, and wrote me an encouraging letter to accompany the referees' reports.
A stereogram and a two-frame kinematogram presents analogous correspondence problems, in that both require matching features in one image with partners in a second. So one might expect there to be similarities in the way the visual system solves the correspondence problem in each case. On the other hand, there are also obvious differences which one would expect the visual system to exploit: most notably, although we used only horizontal motion in our experiments, motion can in principle occur in any direcion, whereas stereoscopic disparities are overwhelmingly horizontal (a point I later investigated in Read & Cumming 2004b). Richard was interested in examining further a result in the literature which suggested quite a fundamental difference between the two systems. This concerned anti-correlated stimuli, in which one image is replaced with its photographic negative. Anti-correlated random-dot kinematograms produce a reversed perception of depth, whereas anti-correlated random-dot stereograms produce no depth percept at all. Yet, for sinusoidal gratings, both systems must produce a reversed percept, since an anti-correlated sine-grating stereogram with near disparity is exactly the same stimulus as a correlated stereogram with far disparity. So for sufficiently narrow-band stimuli, the two systems must produce the same result, whereas at broader bandwidths, the literature showed they produced different results. With Richard, I examined human perception for filtered 1/f noise at a range of spatial frequency and orientation bandwidths. We found that for one-dimensional stimuli, containing only vertical orientations, both motion and stereo produced rather similar results, with anti-correlated stereograms causing weak reversed depth. However, for two-dimensional stimuli, containing all orientations, anti-correlated stereograms caused no depth percept, in agreement with previous studies, whereas anti-correlated kinematograms caused reversed motion. We suggested that this might be related to the anisotropy in stereo -- that disparities are overwhelmingly horizontal -- and that therefore conflict between different channels had a more devastating impact on depth perception.
A stereogram and a two-frame kinematogram presents analogous correspondence problems, in that both require matching features in one image with partners in a second. So one might expect there to be similarities in the way the visual system solves the correspondence problem in each case. On the other hand, there are also obvious differences which one would expect the visual system to exploit: most notably, although we used only horizontal motion in our experiments, motion can in principle occur in any direcion, whereas stereoscopic disparities are overwhelmingly horizontal (a point I later investigated in Read & Cumming 2004b). Richard was interested in examining further a result in the literature which suggested quite a fundamental difference between the two systems. This concerned anti-correlated stimuli, in which one image is replaced with its photographic negative. Anti-correlated random-dot kinematograms produce a reversed perception of depth, whereas anti-correlated random-dot stereograms produce no depth percept at all. Yet, for sinusoidal gratings, both systems must produce a reversed percept, since an anti-correlated sine-grating stereogram with near disparity is exactly the same stimulus as a correlated stereogram with far disparity. So for sufficiently narrow-band stimuli, the two systems must produce the same result, whereas at broader bandwidths, the literature showed they produced different results. With Richard, I examined human perception for filtered 1/f noise at a range of spatial frequency and orientation bandwidths. We found that for one-dimensional stimuli, containing only vertical orientations, both motion and stereo produced rather similar results, with anti-correlated stereograms causing weak reversed depth. However, for two-dimensional stimuli, containing all orientations, anti-correlated stereograms caused no depth percept, in agreement with previous studies, whereas anti-correlated kinematograms caused reversed motion. We suggested that this might be related to the anisotropy in stereo -- that disparities are overwhelmingly horizontal -- and that therefore conflict between different channels had a more devastating impact on depth perception.