Movement clockwise or counterclockwise?
Use your imagination to change the movement direction of both wheels, alternate between the same direction and opposing directions and make sure to observe the size illusion of the identical hubs.
The movie contains only two alternating single frames. Our brain is able to produce a continuous motion from single frames when the frequency is higher than sixteen frames per second.
Motion perception has been the main goal of both prey and potential attackers in the evolutionary process. Movements in nature are often interrupted by occlusions for example, the reconstruction of continuous movements corresponds to evolutionary principles. The word «illusion» might be considered confusing in this context. In 1912, Max Werthheimer, the founder of Gestalt psychology, named this special illusion the phi phenomenon.
Both picture frames of a single wheel allow the interpretation of two movement directions. Both are equally probable in this example as the missing connections between sequential frames have the same value. Thus, with a two-wheel system there are four possible equally valid motion interpretations.
In the old wild west films, we can observe that the wagon wheels rotate backwards even though the wagon is rolling forwards. This is because the connecting gaps between the shots of the wheels are shorter for backwards movement. Today, films are shot with a higher picture frequency so that undesirable illusions such as these are eliminated. The whole movie industry depends on motion illusion.
A circle appears smaller in the middle of larger circles and bigger when surrounded by smaller circles. Our sense of touch also works in the same way because our senses of perception are «programmed» in a similar way. If the circles and wheel hubs were spheres, someone with closed eyes would underestimate or overestimate the size of the central spheres. This feature of our perception follows the principles of the evolutionary process. Its aim is to bring order into the chaos of visual information. Differences in very similar picture elements are emphasized. Our cerebral cortex contains specific nerve cells which react to curvatures. Their differences are emphasised by a sort of lateral inhibition. In this way, our visual system extracts the various properties of the picture elements. In the same way, brightness and colour contrasts, as well as size and angle illusions, are produced in order to speed up the perception and identification of moving objects in space. Human faces with only tiny differences, for example nose shape, could not be recognised with the ease we are used to if it were not for the above-mentioned illusions. We always see caricatures and without even realizing it.
The following animation is composed of right-angled picture elements, which appear to be moving continuously when the picture frequency is high enough. This movie contains only four frames with only two different picture elements with changeable grey values.
Movement to the left or to the right?
Number of stripes
Graytones 1 bis 4
The time-space graph shows you the chosen grey value and the exact order of the picture frames. Each following frame is moved by a half element width. This distance must be connected by illusory movement. There are two possible ways that this bridging can occur: the following frame in the time sequence can be moved to the right or to the left. Firstly, our brains prefer to construct the shortest possible connections. In our applet, all the connecting bridges are of the same size as in the case of the two moving wheels discussed before. Secondly, our brains prefer to connect elements with similar brightness. In this case, the sum of all the brightness differences is crucial to how the brain decides. This bridging illusion in all kinds of stroboscopic movements is known as the correspondence problem . It is still unknown how the neural network functions.
The special case of Sigma motion: when the third and fourth scroll bars are set at the same level, the illusionary motion seems to stop – the picture just seems to flicker. The sum of the brightness differences is the same here, both for the motion to the right and for the motion to the left. If you move your eyes slowly left and right, in order to scan the stationary picture, the part of the picture that is being focused on seems to move in time with your eyes. This type of illusionary movement is called Sigma motion [2 and 3]. This eye movement mechanism can be explained by the so-called reafference principle.