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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. REFERENCES: Anstis, S. and C. Tyler, (1980) "Induced tilt from checkerboards: Edges vs. Fourier components," Investigative Ophthalmology and Visual Science, 19, 165 Bressan, P., (1985) "Revisitation of the family tie between Münsterberg and Taylor-Woodhouse illusions," Perception, 14, 579-585 Coren, S. and J. S. Girgus, (1978) Seeing is Deceiving: The Psychology of Visual Illusions, Hillsdale, New Jersey, Lawrence Erlbaum Associates Cowan, T. M. (1973) "Some variations of the twisted cord illusion and their analyses," Perception and Psychophysics, 14, 553-564 Day, R. H. (1978) "A note on the Münsterberg or café wall illusion," Perception, 7, 123-124 DeValois, K. K. and R. L. DeValois, et. al. (1979) "Responses of striate cortex cells to gratings and checkerboard patterns," London Journal of Physiology, 291, 483-505 Earle, D. and S. Maskell, (1993) "Fraser cords and the reversal of the café wall illusion," Perception, 22, 383-390 Fraser, J., (1908) "A new illusion of direction," British Journal of Psychology, 2, 307-320 Gregory, R. L. and P. Heard, (1979) "Border locking and the café wall illusion," Perception, 8, 365-380 Gregory, R. L. (1977) "Vision with isoluminant color contrast: 1. A projection technique and observations," Perception, 6, 113-119 Grossberg, S. and E. Mingolla, (1985) "Neural dynamics of form perception: Boundary completion, illusory figures, and neon color spreading," Psychological Review, 92, 173-211 Haig, N. (1989) "A new visual illusion, and its mechanism," Perception, 18, 333-345 Luckiesh, M., (1922) Visual Illusions: Their Causes, Characteristics and Applications, New York, Dover Publications, Inc., 1965 Marr, D. and E. C. Hildreth, (1980) "A theory of edge detection," Proceedings of the Royal Society of London, Section B, 207, 187-217 McCourt, M. E., (1983) "Brightness induction and the café wall illusion," Perception, 12, 131-142 Morgan, M. J. and B. Moulden, (1986) "The Münsterberg figure and twisted cords," Vision Research, 26, no. 11, 1793-1800 Moulden, B. and J. Renshaw, (1979) "The Münsterberg illusion and 'irradiation,'" Perception, 8, 275-301 Münsterberg, H., (1894) Pseudoptics, New York, Milton Bradley Münsterberg, H., (1897) "Die verschobene Schachbrettfigur," Zeitschrift für Psychologie, 5, 185-188 Robinson, J. O., (1972) The Psychology of Visual Illusion, London, Hutchinson University Library Stuart, G. W. and R. H. Day, (1991) "The Fraser illusion: Complex figures," Perception and Psychophysics, 49, No. 5, 465-468 Stuart, G. W. and R. H. Day, (1988) "The Fraser illusion: Simple figures," Perception and Psychophysics, 44, No. 5, 409-420 Taylor, S., and J. M. Woodhouse, (1980) "A new illusion and possible links with the Münsterberg and Fraser illusions of direction," Perception, 9, 479-481 Wade, N., 1982. The Art and Science of Visual Illusions, London, Rouledge & Kegan Paul Woodhouse, J. M. and S. Taylor, (1987) "Further Studies of the café wall and hollow squares illusions," Perception, 16, 467-471 Entire web site©1997 IllusionWorks, L.L.C. |
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