Above there are many small upright bars which are vertically displaced following a sine-wave. The question is: do all bars look alike in height? Many would agree they don’t. You can use the right slider “Compensate” on the right, for me a value of half way up is about right, no?
As you will already have guessed, they are all identical in height, but don’t look like it. If you use the left slider, the amplitude of the sine-wave can bereduced to zero, revealing the veridical height of the vertical lines.
After a Twitter discussion on this, I added the option to alternatively show horizontal stripes (uncheck Vertical).
The original authors (Day & Stecher, 1991) write in their abstract: “The illusion is explained in terms of a perceptual compromise between the vertical extent and the greater overall dimensions of the section at the turn of the sine-wave figure and is thereby held to be the same in principle as the Müller-Lyer illusion.” Hmm… When I have obtained the original article (which is non-trivial for me) I will comment more. Whatever, the illusion is rather strong for me.
The phenomenon has occurred in my laboratory a number of times, when we were looking at visual evoked responses in the EEG. Small structures that ride on a rising or falling edge are easily misjudged in size.
When switched to horizontal stripes, there is no size illusion, but a strong 3-D suggestion instead (but that’s another story…).
Day RH, Stecher EJ (1991) Sine of an illusion. Perception 20:49–55