The horizontal vertical illusion: evidence for strategic factors in feedback-induced illusion decrement

This study assessed whether feedback would improve performance on a horizontal vertical illusion task and whether such improvement would transfer to different forms of the illusion. Subjects shortened the extended vertical line of small, medium, and large inverted-T figures to be equal in length to the horizontal line before and after being shown a correctly adjusted medium-sized figure (visual feedback). Next, they were tested on three alternate forms of the illusion; a production task (drawing 1-in. lines in the horizontal and vertical planes), an adjustment task using L figures, and a task requiring them to choose which of several sailboat drawings had mast height equal to hull length. Prior to feedback, vertical lines were adjusted shorter than the horizontal lines on each size inverted-T figure. After feedback on the medium-size figure, experimental subjects were more accurate than the controls on each size inverted-T figure. The results suggest that transfer of illusion decrement was also obtained for the boat-selection and, to a lesser extent, the L-figure adjustment tasks but not to the line-production task. These findings are consistent with the notion that improvement after feedback is not due to structural changes in visual processing or to simple feedback-induced compensation in performance but involves some strategic or cognitive change in judging line length.