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Ljiljana Velisavljevic, James H. Elder; Eccentricity effects in the rapid visual encoding of natural images. Journal of Vision 2003;3(9):647. doi: 10.1167/3.9.647.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: In studies of natural image perception, eccentricity effects may arise from (1) a decline in visual performance as a function of retinal eccentricity (observer effect), and/or (2) a decline in the detail or salience of the image content as a function of image eccentricity (framing effect). Here we assess the role of both factors in the rapid visual coding of natural images. Method: A local recognition task was employed. Each trial sequence consisted of a fixation, test, mask and probe stimulus. The test and mask stimuli were randomly-selected 31×31 deg natural images, displayed for 59 and 506 ms, respectively. A black grid divided each of the images into 64 3.8 deg square blocks. The probe stimulus consisted of two blocks presented on either side of fixation, one drawn randomly from the test image, the other from a random image. The task was to identify which of the blocks was drawn from the test image. In order to independently assess the influence of observer and framing effects, the test stimuli were cropped from both central and non-central locations of larger natural images. Results: Recognition performance for coherent natural images was found to decline significantly with eccentricity. However, this effect disappeared completely when the 64 blocks of the test image were scrambled. Multiple regression analysis revealed that both retinal and image eccentricity were significant factors, although the magnitude of the observer effect was roughly twice that of the framing effect. Discussion: Both spatial acuity and chromatic sensitivity decline with eccentricity. However, the absence of an eccentricity effect for scrambled images argues against these being the primary factors. Rather, the observer effect appears to be due to a higher-level facilitation in foveal processing triggered by the coherent structure of the image.
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