Visual Inspection Alone Produces a Decrement in the Horizontal-Vertical Illusion

Subjects inspected either inverted-T or L figures for 5 min. and then shortened the extended vertical lines of inverted-T figures in an attempt to make the vertical line equal in length to the horizontal line. Those who inspected inverted-T figures were more accurate on initial trials than controls (who did not inspect figures), those who inspected L figures, or those who inspected inverted-T figures and made adjustments on the inspected figures. The results indicate that visual inspection alone can produce a decrease in the Horizontal-Vertical illusion and may account, in part, for changes in strategic factors which have been hypothesized to underlie illusion decrement.