Visual inspection and decrement in the horizontal-vertical illusion

The present study examined decrement in the Horizontal-Vertical Illusion as a function of visual inspection of the illusion figure. Experimental subjects visually inspected either inverted-T or L figures for five minutes and then shortened the extended vertical lines of inverted-T figures in an attempt to make them equal to the horizontal lines. Subjects who inspected inverted-T figures for five minutes were more accurate on their first adjustment trial than controls (who did not inspect figures) or those who inspected L figures. All groups adjusted the vertical lines of the inverted-T figures to be shorter than the horizontal lines, but improved in accuracy over 20 trials. There was no difference among groups in accuracy of terminal performance. These results demonstrate that visual inspection of inverted-T figures can produce a decrement in the Horizontal-Vertical Illusion, and may account in part for the illusion decrement that occurs over a series of trials.