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Abstract

Fourier phase plays an important role in determining image structure. For example,
when the phase spectrum of an image showing a
ower is swapped with the phase
spectrum of an image showing a tank, then we will usually perceive a tank in the
resulting image, even though the amplitude spectrum is still that of the
ower. Also,
when the phases of an image are randomly swapped across frequencies, the resulting
image becomes impossible to recognize. Our goal was to evaluate the eect of phase
manipulations in a more quantitative manner. On each trial subjects viewed two images
of natural scenes. The subject had to indicate which one of the two images contained
an animal. The spectra of the images were manipulated by adding random phase noise
at each frequency. The phase noise was uniformly distributed in the interval [;+],
where was varied between 0 degree and 180 degrees. Image pairs were displayed for
100 msec. Subjects were remarkably resistant to the addition of phase noise. Even with
[120; 120] degree noise, subjects still were at a level of 75 correct. The introduction
of phase noise leads to a reduction of image contrast. Subjects were slightly better
than a simple prediction based on this contrast reduction. However, when contrast
response functions were measured in the same experimental paradigm, we found that
performance in the phase noise experiment was signicantly lower than that predicted
by the corresponding contrast reduction.