Horizontal-Vertical Illusion

In 1858 Wilhelm Wundt introduced the horizontal-vertical illusion. In this illusion, a vertical line appears up to 30% (according to Wundt) longer than a horizontal line of the same exact length:

There are several possible explanations for the horizontal-vertical illusion. However, none of them are entirely satisfactory. Misapplied constancy claims that we perceive the illusion as three dimensional and the vertical line appears to recede into the distance. The perceived size (length) of the vertical line depends on its actual size and perceived distance. Because the perceived distance is larger than the perceived distance of the horizontal line, the vertical line appears longer.

A second explanation is that line length depends on how much effort it take to move our eyes when looking at the line. Because us Westerners are used to reading from left to right (as opposed to top to bottom) it might take a tiny bit more effort to move our eyes up and down the vertical line than it does to move our eyes left to right for the horizontal line. That increased effort makes the vertical line look longer than the horizontal line of the same length.

A third explanation is that the illusion is due to a framing effect. Because our eyes are to the left and right of each other (as opposed to above and below each other), we have a wider field of view horizontally than vertically. Thus, horizontal lines take up proportionally less of our horizontal field of view than same sized vertical lines do of our vertical field of view. That is, our horizontal frame is less full than the vertical frame, so the vertical line looks longer than the same sized horizontal line. If that is true, then viewing the illusion monocularly (which restricts the horizontal frame but not the vertical frame) should reduce the magnitude of the illusion. Prinzmetal and Gettleman (1993) found exactly that.

The Activity

In this activity you will see a horizontal and a vertical line. Adjust the length of the vertical line by moving the vertical line length slider (below) left (to make the vertical line shorter) or right (to make the vertical line longer) until the vertical line seems to be the same length as the horizontal line. When you think the two line are the same length, click on the "Check" button (below). This will rotate the vertical line 90° to make it horizontal and will draw two additional vertical, dashed lines, one at each end of the horizontal line. Rotating the vertical line should not change its length (most modern computer monitors have pixels that are the same width as height. If you have a really old CRT monitor, it is possible that rotating the line will change its length. In such cases, you will need to get a ruler and physically measure the line lengths.) If you are unhappy with your answer, click on the "Hide solution" button, and adjust the length some more. You can also click on the "New Problem" button to try a new horizontal-vertical illusion.

Vertical line length:

Prinzmetal, W., & Gettleman, L. (1993). Vertical-horizontal illusion: One eye is better than two. Perceptual Psychophysics, 53, 81-88. http://dx.doi.org/10.3758/bf03211717