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Abstract

Human perception of probability is systematically distorted, with low probabilities typically overestimated and high probabilities underestimated. The distortion is thought to arise from constraints in the internal representations of probabilities (Zhang, Ren & Maloney, 2019, bioRxiv). Here we present data suggesting that for visual stimuli, saliency may provide an additional contribution. In a visual scene, infrequent objects among frequent background objects tend to be more salient to attract attention, which in turn weights them more heavily in probability estimations. For example, if a gray visual field contains a few black dots among many more white dots of equal size and contrast, the black dots are more salient to attract attention. Thus, the proportion of dots that are black may appear larger than it actually is. Analogously, when the black dots are more numerous than the white dots, they become less salient than the white dots and their proportion can thereby appear smaller than it actually is. Since saliency effects are stronger peripherally (Zhaoping 2014, Oxford University Press), we predict that the probability distortion should be larger when objects are distributed in a larger field. This is because in a large field, more dots fall in the peripheral field for any given fixation location, making the probability estimation more reliant on the peripheral estimations of the summary statistics of the field. To test this, we implemented a variant of the task first reported by Zhang and Maloney (2012, Frontiers in Neuroscience). Four participants (three naïve) viewed a square field containing a mixture of black and white dots (diameter=0.26°, density=2/deg²) subtending either 10° or 20°. After freely viewing for 1.5 seconds, participants had to estimate the fraction of dots that were black. For all participants, probability distortions were greater for the large field.