A Bayesian account of context-induced orientation illusions

1. ECVP 2012, 2-6 September, Alghero, Italy A Bayesian account of context-induced orientation illusions Endel PõderInstitute of Psychology, University of Tartu, Estonia E-mail: endel.poder@ut.ee Background In classical tilt illusion (Gibson, 1937, Goddard et al., 2008), perceived orientation of a target object is repulsed from the orientation of surrounding context. My earlier study (Põder 2009, 2012) with crowded orientation stimuli showed strong assimilation bias at about the same target-surround differences, opposite to the classical tilt illusion. Model I use a Bayesian model with conditional priors (e.g. Stocker & Simoncelli, 2008). I suppose that observer automatically categorizes stimulus patterns as either homogeneous or target pop-out. Probability of pop-out increases with increasing target-flanker difference. The target likelihood function is combined with the different prior distributions for the pop-out and homogeneous trials. Responses are calculated as the means of posterior distributions. Total distribution of responses is a mixture of the two distributions with opposite biases. Perceived as homogeneous Target pop-out Purpose To understand the mechanisms ofcontextual effects in orientation perception. Experiment Contextual effects were measured with the same stimuli in fovea and periphery. In fovea, visibility was varied by using different contrasts. • Methods • Stimuli: A target Gabor (3.2 cpd) surrounded by 6 flanking Gabors, target-flanker distance 0.8 deg, target-flanker orientation difference varied from 0 to 64 deg. • Exposure duration: 24 ms. • Task: to reproduce the target orientation by rotating a probe Gabor. • Three conditions: • Central presentation, high contrast (0.92) • Central presentation, low contrast (0.17) • Peripheral presentation (eccentricity 5 deg, contrast 0.92). Results of experiment Results of simulation Conclusions The experimental results suggest that perceived homogeneity vs. target pop-out may determine the nature of illusion. A Bayesian model with context-dependent priors can produce qualitatively similar results. References Gibson, J. J. (1937). Adaptation, after-effect, and contrast in the perception of tilted lines II. Simultaneous contrast and the areal restriction of the after-effect. Journal of Experimental Psychology, 20, 553–569. Goddard, E., Clifford, C. W., & Solomon, S. G. (2008). Centre-surround effects on perceived orientation in complex images. Vision Research, 48, 1374–1382. Põder,E. (2009).Integration of crowded orientation signals.Perception,38,ECVP Supplement, 179. Põder, E. (2012). On the rules of integration of crowded orientation signals. i-Perception, 3, 440–454. Stocker, A. A., & Simoncelli, E. P. (2008). A Bayesian model of conditioned perception. In J. C. Platt, D. Koller, Y. Singer, & S. Roweis (Eds.), Advances in Neural Information Processing Systems. (pp. 1409–1416). Cambridge, MA: MIT Press. Reduced visibility (higher internal noise) of the target orientation increases the range of target-surround differences where assimilation bias is observed.