Decrement in the horizontal-vertical illusion: Are subjects aware of theirincreased accuracy?

47 subjects adjusted the extended vertical lines of 20 inverted-T figures t o make them appear equal to a horizontal line and raced the confidence in t heir accuracy after each tri;il. One group viewed figures of varying sizes, a second group viewed figures of standard size, and a third group viewed f igures of standard size but received feedback on their accuracy immediately after completing Trial 5. Except for a significant increase in accuracy on Trial 6 fur the Feedback Group, there mere no differences in performance a mong groups. Subjects made the vertical line significantly shorter than the horizontal line on initial trials but their accuracy improved over trials. In contrast, there was no consistent increase in confidence, and several a nalyses indicated that confidence ratings were unrelated to accuracy. These results suggest that the subjects were unaware of the decrement in illusio n that occurred over trials.