Square 1: A pulsing shadow under the square is enough to create the illusion of movement. The expanding shadow only makes sense if the square stands out from the background. What experience makes plausible is not always what is happening in reality. The distortionof the moving square is heightened by adding a suitable synchronised sound since the different sensory perceptions have a common final stage in the cerebral cortex.

Square 2: Our visual system contains neural cells, which are effective as movement detectors. Our perceptions of movement, shape and colour are linked. The red square on the right seems to send out black lines and in the process swells a little. In the network ofneural cells in the cerebral cortex, excitation is always accompanied by a corresponding inhibitory process. Both work in a stationary state when an image is observed for a longish period of time. As long as thestimulation of movement is absent, only the inhibitory cells continue to work for a few seconds longer. The black lines together with the red squares seem to shrink until the initial conditioning disappears. The whole film is screened not in the retina but in the cortex. As proof of this, we can look with one eye only for thirty seconds until the end of the film and then observe the after effect with the other eye.What is interesting here is that there can be a break of 10 minutes or so with the hands over the eyes, between the end of the film and observation of the after effect. The rate of the gradual disappearance of the conditioning depends on active observation or at least on light falling on the eyes. When the black lines are absorbed by the red square instead of being emitted, the square shrinks during the movement phase and expands when the film stops together with all the lines for a brief time [2].

Square 3: In reality, the square is of the constant colour red. The surroundings, however, influence our perception of colour. This phenomenon has been known since the 18th century and is called simultaneous colour contrast. In the textile industry, these undesirable colour changes are a problem. A thread can undergo a marked change in colour and luminosity in the fabric. Wilhelm von Bezold studied the positive and negative consequences of this problem extensively [1]. This illusion has been ascribed to lateral inhibition between neighbouring neural cells of the optical system and seems to have had positive implications during evolution. For example, the colour of red berries shifts in the direction of the complementary colour of its surroundings. Luminosity increases when the surroundings are dark. This intensifying of contrast in colour and brightness would have helped hominids to gather food. The colour and brightness illusions can be observed well in the third red square. In addition, small changes in size seem to occur.

Square4: The pulsating surroundings of the fourth red square seem to bend the borders of the square. This illusion is intensified as the viewing distance increases. This illusion, which is called the Fraser effect, is illustrated in the Appendix to Spot 17.

Square 5: Our brain is provided with movement detectors, which register the speed of individual image elements? The elements on the edges of the red square have two speed components. One is parallel to the edge and the other is vertical to it. The first cannot be registered by the brain if the edges of the square are covered and the square has no structure at the edges. The second component contradicts that the fixed movement of the square and causes the illusion of growth and shrinking. [2]