Subjective contours triggered by border lines below the resolution limit

Abstract

A variant of the Muensterberg or Café Wall illusion, where rectangles appear wedge-shaped, has been investigated. In contrast to the classic design with parallel rows of alternating black and white tiles, all rectangles were patterned in the same way, each of them being shaded symmetrically from light to dark and back to light again and surrounded by a contour line. It was investigated how the intensity of the illusion depends on 1) on the hue of the contour line, 2) on its line width, and 3) on the distance of observation. As a result, it was found that the illusion gains its maximum at a hue of the contour line of 40% grey and a line width considerably less than 1 minute of arc (= 2.91·10^-^4 rad). This interpreted as due to diffraction. The intensity of the illusion is plotted versus the logarithm of the line width and a lognormal function is fitted to the results.