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Applet design: Al Seckel  Programing: Akos Feher  Twisted Cord©1997 IllusionWorks, L.L.C.

 Twisted Cord


     Do the vertical lines appear to bend back and forth? Use the control buttons to compare position one with position two. Clear the background to check that the vertical lines are actually straight!

What's Happening

     The vertical lines are actually straight and parallel, yet they are perceived as bent. The twisted cord illusion is one of the most powerful illusions known, of which there are several well-known varieties, each having its own striking effect.

     Twisted cord illusions include the Musterberg figure, the twisted cord, the cafe wall illusion, Fraser Spiral, and the hollow squares illusion. Several authors have also linked the Zollner illusion to twisted cords, but other than the fact that they are both illusions of direction, they are not associated.

What's Going On?

    The twisted cord illusion is most likely due to orientation-sensitive simple cells in the striate cortex, which interact to combine the closely-spaced tilted lines into a single tilted line, which occurs early on in the visual system during the encoding of the position of edges and brightness differences. This type of processing occurs prior to the cognitive processes of object recognition. Illusions that involve higher cognitive processes are not likely to be affected by stimulus changes that do not alter the informational content of the picture or object.

     No amount of testing or knowledge of their true nature can influence your perception of this illusion. This demonstrates that perception is not always influenced by what you know.

History of the Twisted Cord Illusion

     This distortion illusion was originally observed in the nineteenth century by mat weavers who noticed it in their patterns. In 1894, Münsterberg described the illusion in his popular collection of illusions, Pseudoptics. In 1897, Münsterberg again published an account of the illusion, under the name "Die verschobene Schachbrettfigur" ("shifted checkerboard figure"), and put forward one of the earliest theories to account for it. From then on it has become known as the Münsterberg figure.

     Münsterberg suggested that the illusory effect arose from irradiation. He argued that the white regions above or below the line appear to "invade" or "bore into" the black so that each short section of the edge appears to be slightly tilted rather than horizontal. The overall effect of these apparently tilted sections is an apparent slanting of the whole contour. This sort of illusory  effect from irradiation had been described earlier by Helmholtz in1856.


     Helmholtz used the principle of irradiation to explain the observation that two squares (seen above) do not appear to be identical: The white middle square appears larger than the black middle square. The term "irradiation" refers to the spreading of light areas into adjacent dark areas, where one would get an increase in the size of a bright area at the expense of an adjacent dark area. This gets apparent displacement of a black-white boundary so that the contour appears shifted in the direction of the dark area. Helmholtz was rather vague on how this could occur.

    Although Münsterberg first proposed that irradiation could have a role in this illusion,  he offered no suggestion on how it could actually produce an angular distortion.  In recent years,  however, many researchers have re-examined irradiation's role in Münsterberg figure.

     In 1980 Steven Taylor and J. Margaret Woodhouse constructed a series of series of patterns in an attempt to obtain effects similar to the Münsterberg figure but with irradiation effects removed or reduced.

     They found that if the outline of the squares was overlapped so that one side was common to the individual squares in adjacent lines, the lines appeared to be parallel. However, if the squares were made to touch, rather than overlap, the effect that was produced was similar to the Münsterberg figure, but with the direction of tilt reversed. This illusion is known as the Taylor-Woodhouse illusion or the hollow squares illusion.

     Taylor and Woodhouse found that the figure could be reduced to an even more basic form (Figure still to come). Here, with only two parallel lines,  the tilt remains even after removal of the vertical and peripheral lines. The illusion disappears with the removal of the overlapping lines. This illusion bears a strong resemblance to the twisted cord illusion developed by Fraser.

    In 1908, James Fraser, a British psychologist, published a paper entitled "A New Visual Illusion of Direction." He showed how one could adapt the Munsterberg figure into a new and powerful illusion of direction -- the twisted cord.

     Fraser called the basic unit of his figures, a line with a triangle at either end, a "directional unit." He maintained that this directional unit was very effective at misleading the visual system.

     Fraser used this unit on both straight and curved figures, including the famous Fraser Spiral.

    Fraser noticed that it was important for the individuality of the elements to be disguised, either by overprinting the discontinuous adjacent ends of the tilted lines or by placing the figure on a complex patterned background.

   The importance of this fact is clearly demonstrad by Fraser in his series of examples of the "LIFE" illusion.

     Here the letters are superimposed on a checkered background and the illusion is very strong.

     However, when you remove the background, the illusion is reduced.


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