by Marek Dobiecki 1994

        FIG 1

There are only a couple of ways of forming the illusion of movement. One is to present the eye with a series of rapidly changing images as in cinematography and video, the other is to step an image across a surface made of light bulbs in the fashion of neon advertising. Both methods require that visual cues undergo a physical change external to the eye. New images, discovered at 'Panetix' as far back as 1986, cause an illusory movement to occur from static visual cues alone. The author offers possible causes for this phenomenon.

The Field displayed in Fig.l is typical of the types studied and is the standard test field. To the observer it will appear to rotate in a clockwise direction and simultaneously outwards from the centre. If the image was to be a negative of itself so that the dark elements were white and the background black the illusory effect would be the exact opposite. The apparent rotation would be anti-clockwise and simultaneously toward the centre. The strength of the illusion is dependent on a number of factors concerning the external physics of the image and the internal physical state of the observer, for instance, the light or dark adaptation of the eye to the ambient light conditions. In the table of illusion depenencies below is a rough guide to the optimal viewing conditions however it must be remembered that because of the many variables associated with viewing any scene, there is a wide margin of error and what may seem optimal to one person in one set of environmental circumstances may not be so to someone else in other circumstances, however slightly they may differ.
During the course of this document I shall refer to the dark elements in Fig.l as 1-DEs.

l. The image should occupy a retinal region that is larger than the fovea by about three diameters. 
2. The observer should direct his/her attention away from the centre of visual focus. In other words attention should be paid to those parts of the image that are outside of the foveal region on the retina. 
3. The eyes should not remain stationary for longer than about a second. 
4.The best results are obtained when the size of each 1-DE occupies about 1:2500 of the visual field. This approximates to a 7cm long 
1-DE viewed from 2 metres away or about 3 degrees of arc. 
5.Illuminance values ranging from 0.015 to 20.0 lux provide good ambience for the effect. This corresponds roughly to a white object inside of a room on a dull day at the lowest value to the same object and room on a very bright sunny day. The illusion loses strength under direct lighting in either sunlight or artificial light. The best viewing is at about 15.0 lux. 
6. The contrast ratio of 1-DE to background should be at maximum ie. black to white. 
7.Heart rate somehow has a bearing and it would appear that higher heart rates enhance the illusion although I cannot conjecture as to the reason for this. 
8.The rate of change in orientation of each element relative the average rate of the total field of elements. 



A single 1-DE  has insignificant movement properties, however a whole array of polarised 1-DEs exhibits a synergy of movement (Fig.1) that is not predictable from a single element. Just what is it about these 1-DEs that incites the visual perceptive system to respond by giving the illusion of movement? The analysis seems simple from a purely descriptive point of view: It contrasts the background at one edge only and gradually fades into the background in all directions except at the contrasting edge. What could be simpler? The eye, however, does not treat this as a simple object. This is evident if we consider exchanging a field of 1-DEs for a negative of itself. The reason is that a spinning field possesses a chirality, a 'handedness'. If a negative field (Fig. 4) is transposed onto the positive there is a reversal of chirality. Clearly something is happening during processing which is probably an idiosyncrasy of structure and of functions specific to a variety of cells.


Last Revised: 30/12/99