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

To examine whether individuals were aware of their improvement on a Horizontal-Vertical Illusion task, subjects adjusted vertical lines to be equal in length to horizontal lines in twenty inverted T figures and also rated confidence in their accuracy on each trial. One group of subjects viewed varying size figures, a second group viewed standard size figures, and a third group viewed standard size figures and also received feedback regarding their accuracy on Trial 5. In initial trials, subjects in each group adjusted the vertical line shorter than the horizontal line and their accuracy increased over trials. The feedback manipulation was effective as shown by the amount of increase in accuracy between Trials 5 and 6. However, confidence ratings were unrelated to the increase in accuracy within groups or within subjects in each group. In summary, subjects appear to be unaware of their improvement in accuracy over a series of Horizontal-Vertical Illusion trials.