Other Effects
The demonstration in this article shows a blurred, slightly green-grey disc in each gap. These can be seen by examining one frame of the image under high magnification with any adjoining lilac discs masked off. But these blurred discs are not required for the illusion. Michael Bach's demonstration contains no such discs, but yields the illusion.
It is not necessary to fixate the black cross for the effects to occur. As long as the eyes are held steadily on any point of the figure, even the centre of one of the discs, the illusion will occur.
If instead of fixating the black cross, one follows the moving gap with one's eyes, one will see only a moving gap and 12 lilac discs rather than a single green disc. This is because the green disc arises as an afterimage, requiring the eyes to be held steadily to occur, and the disappearance of the lilac discs is from Troxler's fading, also requiring the eyes to be held steadily to occur. Moving the eyes prevents the formation of afterimages and the occurrence of Troxler's fading.
If after looking at the effect for 30 seconds or so, one moves one's eyes elsewhere (e.g., to another point on the figure or to a blank sheet of white paper), one will see a stationary ring of 12 green discs that will fade after a short time. These green discs are the afterimages of the 12 lilac discs.
If one watches the illusion for long enough to see only the green disc and then moves away from the computer screen while keeping the eyes on the cross, one sees larger green spots outside a ring of lilac spots with a smaller green disc running around them. The smaller green disc may merge briefly with the outer green spots, making the spots appear to be radial blobs. The outer green spots soon fade. These outer green spots are afterimages that appear larger because of Emmert's law: the size of an afterimage becomes larger as its viewing distance is increased. They are outside because moving away from the computer screen has decreased the visual angle of the lilac spots. They fade because the lilac discs that constantly refresh the green afterimages are now projected onto a different part of the retina. If one moves towards the screen, the effects are opposite.
If one closes the right eye and moves close to the stimulus so that the nine-o'clock disc falls in the blind spot, one sees that the movement is no longer smooth. There is a noticeable pause when the disappearance of the disc occurs on the region of the retina having no rods or cones. This suggests there are limits to the filling-in that normally prevents us from noticing a black hole in our visual fields at the location of the blind spot.
Read more about this topic: Lilac Chaser
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