(a) Illustrated are visual stimuli used in psychophysical experiments. Visual discrimination threshold, quantified as the just-noticeable feature differences between two successively presented visual stimuli, was measured using a standard staircase procedure. Contextual illusion magnitude, quantified as the feature difference between two physically dissimilar stimuli that appeared perceptually equal because of the presence of the surrounding context, was measured using a standard method of constant stimuli. (b) Across participants, the orientation discrimination threshold is plotted against the orientation contextual illusion magnitude, illustrating a tradeoff between discrimination sensitivity and contextual modulation of orientation perception. For contrast and luminance perception, no such tradeoff was observed between the discrimination threshold and the contextual illusion magnitude. Each point represents a single participant (N=45) and the line is the best-fitting linear regression. Statistical values reflect Spearman’s ρ and its bootstrap confidence interval with FDR correction for multiple comparisons (α=0.025).