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Sarah L. Elliott, Michael A. Webster, Mark A. Georgeson; Adaptation to blur: normalization or repulsion?. Journal of Vision 2005;5(8):246. doi: 10.1167/5.8.246.
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© ARVO (1962-2015); The Authors (2016-present)
Adapting to a blurred or sharpened image alters the perceived blur of a focused image, shifting the subjective best-focus toward the level of the adaptation (Webster et al. Nature Neuro. 2002). This aftereffect could reflect repulsion in the appearance of stimuli away from the adapt level (similar to spatial frequency adaptation) or normalization in the appearance of the adapt stimulus so that it appears better focused (similar to chromatic adaptation). The two alternatives differ in whether or not the response to focused stimuli is special, and thus in the functional implications of blur adaptation. We used matching between pairs of images to examine the form of the perceptual change. Stimuli were 4-deg noise images with amplitude spectra filtered over log-log slopes from −2 (strongly blurred) to 0 (white noise). Subjects adapted for 120 s to a sequence of blurred or sharpened images to the left of fixation, along with ‘focused’ noise (slope = −1) on the right, and then used a 2AFC task to match the slope of intermittent test images. Normalization in the perceived focus of the adapting images was confirmed with a blur-difference task, in which an adapt pair (shown above fixation) was compared to the same unadapted image pair shown below. The 1/f images remained similar while the lower blurred (or sharpened) image had to be sharpened (or blurred) to match the adapted pair. Changes across different test blur levels were probed for images with slopes ranging from −1.75 to −.25. Adapting to a blurred (−1.5) or sharpened (−0.5) sequence produced similar shifts at test levels near the adaptation, but these diminished and in some cases changed sign for tests far removed from the adapt. These effects are inconsistent with a simple normalization and could reflect repulsions relative to each adapt level. Thus both forms of perceptual change may occur. The partial renormalization suggests that under some conditions the focus point does have a special status in the encoding of blur.
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