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Abstract

We propose that perceived similarity between two grey levels is related to the way they have been previously associated through eye saccades, actually reflecting their probability of co-occurrence in space and time. In the present study, we modified relative perceptual distances between grey levels by artificially associating certain pairs of grey levels with small eye saccades and other pairs with large eye saccades. In the test stage, we varied the luminance of a test patch (5 levels from 15 to 21 cd m-2) to determine the luminance perceived as 'right-in-between' that of two reference stimuli, dark and light (10 and 26 cd m-2). Each trial involved simultaneous presentation of one test grey and the two reference greys. The subject's task was to indicate whether the test patch looked more similar to the light or dark reference grey. Adaptation trials involved successive presentation of one of each test greys centred on the screen followed by the dark stimulus with a 10° eccentricity or the light stimulus with a 20° eccentricity, with equal probability. A control condition was employed without eccentricity difference. Only in the main condition does the post-test show an increased relative similarity between the test greys and the dark patch.