In a previous post I looked at the Boogie-Woogie illusion which is quite impressive and tried to deconstruct it. The first diagram above produced a weak movement effect that occurred in the periphery of vision. Increasing the width of the circular line did not improve the movement effect. Instead an after effect was noted on looking at the circles and then looking away into white space.
There was something unique about the original illusion which was not being reproduced. Revisiting this I noted that the diamond shaped line in the original illusion was superimposed on a grey background and that a focus on the white circle was necessary to reproduce the illusion of movement. Taking the first design above a graded background was added and the circle was made to move across the screen against a white background. This can be seen in the video below.
I made a number of findings
1. The movement effect of the original illusion was not reproduced and therefore a grey background alone with a contrasting white and black circular line was not sufficient to produce the effect
2. The after effect on looking away at white space was reproduced weakly on the left side of the circle (with a predominantly white/white-grey background) but absent on the right (with a predominantly black/black-grey background) suggesting that the after-effect was a function both of discontinuous lines and also of a simple white-black contrast.
3. I identified a new illusion – this time involving movement. If you focus on the left side of the circle as it moves to the right it appears to become lighter at times. On the return journey (i.e right to left) there is a momentary ‘overspill’ of dark colour so that the left side becomes momentarily darker.
These preliminary findings suggested to me that illusions occur in the presence of normal physiological response but it is likely that the human visual apparatus is unable to process these stimuli in a reality-congruent manner. In evolutionary terms the illusion would represent an imperfect adaptation of the visual apparatus to the external world. In particular the visual stimuli that are sufficient to produce this response are likely to be artificial – not commonly occurring in the natural world. This is merely speculation and there is one clear exception – the waterfall illusion described by Robert Adaams after observing the Falls of Foyers. Nevertheless if illusions could be couched in terms of adaptation to the environment then we might also expect species to differ in their experience of illusions especially where the visual apparatus of the compared species are highly specialised for different environments.
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