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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.
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553-564
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