Cultural and learning differences in the Judd illusion.

In the present study, we examined whether individual differences in the perception of illusory Judd drawings point to variability in the pickup of informational variables. Two sources for these individual differences were addressed: culture and learning. East Asian (n = 24) and Western (n = 24) participants made perceptual judgments of the midpoint of the shaft of various Judd figures in a pretest-practice-posttest design. During practice, half of the participants received feedback about the actual midpoint after each trial, while the other half did not receive feedback. The results showed differences among perceivers of different cultures in judging the midpoints of the shafts of Judd figures, particularly with respect to their propensity to improve perceptual accuracy after repeated practice and feedback. For most participants, changes in illusory bias as a consequence of learning were shown to reflect either a change in what informational variable they exploited or a rescaling or calibration of the perception to the informational variable. However, the individual differences in illusory bias related to culture could not be unequivocally attributed to either of these perceptual-learning processes.