Abstract
The Fraser illusion occurs when a line formed from a number of tilted segments itself appears tilted. Two explanations of this illusion were examined: (1) that the illusion results from lateral facilitation between orientation selective cells, and (2) that it is due to orientation being processed only at a local level. In Experiment 1, variations in the strength of the Fraser illusion and a comparable Zöllner illusion, with change in the angle of inducing elements, were examined. There were no differences between the angular functions, but it was shown that in both cases assimilation occurred at angles as great as 18° and that solid figures showed weaker contrast illusions at larger angles. Experiment 2 indicated that this might be due to the different Fourier composition of the solid figures. Experiment 3 measured the local effects of lateral inhibition/facilitation and confirmed that facilitation did not take place. That indicates that the illusion results from the direct contribution of local information provided by the tilted segments to the global perception of the line. Finally, Experiment 4 showed that when the amount of background field used in the Zöllner illusion was increased, it became negative at smaller angles, due to the increased contribution of lateral inhibition.
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Stuart, G.W., Day, R.H. The Fraser illusion: Simple figures. Perception & Psychophysics 44, 409–420 (1988). https://doi.org/10.3758/BF03210425
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DOI: https://doi.org/10.3758/BF03210425