vc | Valentina Bernal Buitrago

linkOptical illusion

linkLilac Chaser illusion

The lilac chaser is a visual illusion, also known as the Pac-Man illusion. It consists of 12 lilac (or pink, rose, or magenta), blurred discs arranged in a circle (like the numbers on a clock), around a small black, central cross on a grey background. One of the discs disappears briefly, then the next, and the next, and so on, in a clockwise direction. When one stares at the cross for about 5 seconds or so, one sees three different things.

A gap running around the circle of lilac discs;

A green disc running around the circle of lilac discs in place of the gap;

The green disc running around on the grey background, with the lilac discs having disappeared in sequence.

The Lilac chaser illusion was described by Jeremy L. Hinton in 2005 and it was shown by Michael Bach on his website in the same year. Imagine a set of coloured dots forming a circle. A good colour to use is lilac, which is a pinkish-violet colour, but other colours can be used. The dots are placed at twelve fixed positions around the circle. Twelve positions works well but the number is not critical, they just have to make a nice circular ring. One of the twelve dots is always missing but this gap changes from one position to the next over time.

linkExplanation

The lilac chaser illusion combines three simple, well-known effects:

The phi phenomenon is the optical illusion of perceiving continuous motion between separate objects viewed rapidly in succession. The phenomenon was defined by Max Wertheimer in the Gestalt psychology in 1912 and along with persistence of vision formed a part of the base of the theory of cinema, applied by Hugo Münsterberg in 1916. The visual events in the lilac chaser initially are the disappearances of the lilac discs. The visual events then become the appearances of green afterimages (see next).

When a lilac stimulus that is presented to a particular region of the visual field for a long time (say 10 seconds or so) disappears, a green afterimage will appear. The afterimage lasts only a short time, and in this case is effaced by the reappearance of the lilac stimulus. The afterimage is a simple consequence of adaptation of the rods and cones of the retina. Colour and brightness are encoded by the ratios of activities in three types of cones (and also the rods under mesopic conditions). The cones stimulated by lilac get "tired". When the stimulus disappears, the tiredness of some of the cones means that the ratios evoked by the grey background are the same as if a green stimulus had been presented to these cones when they are fresh. Adaptation of rods and cones begins immediately when they are stimulated, so afterimages also start to grow. We normally do not notice them because we move our eyes about four to five times a second, so the image of a stimulus constantly falls on new, fresh, unadapted rods and cones. In the lilac chaser, we keep our eyes still, so the afterimages grow and are revealed when the stimulus disappears.

When a blurry and non blurry stimulus is presented to a region of the visual field, and we keep our eyes still, that stimulus will disappear even though it is still physically presented. This is called color adaptation.

linkVariations

There are 2 suggested variations on Michael's website. Both variations follow the same principles already described.

Variation by Sebastian Mathôt

Variation by Blelb

linkBibliography