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A.Y. Leonova, J.S. McLellan, S.A. Burns; Effect of Phase Reversals on Perception of Complex Visual Stimuli . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3616.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: It is known that high frequency phase reversals produced by the optics of the eye can have a devastating effect on letter identification in visual acuity tests. Here we investigate the perceptual effect of phase reversals when viewing more complex visual stimuli such as human faces. It is generally believed that humans are extremely good at detecting subtle changes in face expressions often due to high spatial frequencies. This suggests that face perception may be affected by phase reversals. We also looked at the effect of phase reversals on perception of large scale stimuli like natural scenes. Methods: We used black and white images of human faces and natural scenes convolved with 1) a defocused point spread function (PSF), and 2) a Gaussian filter approximating a shape of the point spread function. The important difference between the conditions is the presence of the phase reversals in the optically blurred image and their absence in the image convolved with a Gaussian matched filter. The amount of blur in the image was varied depending on the value of a Zernike defocus term (0<c4<2.5). Subjects were asked to discriminate between two face expressions of the same face using a ‘yes–no’ paradigm. Twenty faces, with two face expressions each were used in the experiment. Results: Our results showed that for both, faces and natural scenes, there was no marked difference between images convolved with a real PSF and the ones convolved with a Gaussian filter. Conclusions: Phase reversals did not affect overall perception of human faces and natural scenes. Because natural scenes have complex spectra, approximating a 1/f frequency falloff, and also a complex phase spectra, there are multiple clues over a range of spatial frequencies to most features. Thus, while phase reversals mainly occurred at higher spatial frequencies, these are not detrimental for general perception of natural images and human faces. Many of the current studies in blur perception have used Gaussian filtering to produce blur. This is the type of blur produced by scattering, but is dissimilar to the blur produced by defocus. Our results suggest that this difference is not critical for most vision, though it has previously been shown to be quite important for letter identification.
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