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M. Webster, S. Webster; Neural adjustments to image blur. Journal of Vision 2001;1(3):441. doi: 10.1167/1.3.441.
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© ARVO (1962-2015); The Authors (2016-present)
The degree of blur in retinal images can vary widely in natural viewing (e.g. because of optical errors). We examined whether the visual system adjusts to changes in the magnitude of blur through spatial contrast adaptation. Stimuli were images of natural scenes, filtered noise or edges presented in a 4-deg field on a monitor. Subjects viewed a blurred or sharpened version of an image for 5 min, and then judged the apparent focus of a series of 0.5-sec test images interleaved with 6-sec of readaptation. A 2AFC staircase procedure was used to vary the amplitude spectrum of successive tests to find the image that appeared in focus. Adapting to a blurred image causes a physically focused image to appear too sharp (so that the image that appears in focus is physically blurred). Opposite after-effects occur for sharpened adapting images. Large biases in perceived focus were induced over a wide range of adapting blur magnitudes, and were similar for different types of blur. After-effects were also similar in magnitude for the range of images examined, but tended to be weaker when the adapting and test stimuli were drawn from different images. Thus the adaptation is not adjusting simply to blur per se. In further experiments, we examined a spatial analog of these successive after-effects, by surrounding test images with 8 adjacent background images. The test and surround were presented concurrently for 0.5 sec, and perceived focus of the test was adjusted as before with the staircase. Blurred surrounds again caused the central test to appear sharper, while sharpened surrounds induced blur. This “simultaneous blur contrast” was generally weaker than the adaptation for centrally fixated targets, but was pronounced for images viewed in the periphery, and was again selective for the specific images in the center and surround. These adaptive adjustments may play an important role in calibrating the perception of blur both in normal vision and with refractive errors.
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