Illusory Motion within Still Pictures: The L-Effect

Leonardo, Vol. 15, No. 3, pp. 222-223, 1982 Printed in Great Britain 0024-094X/82/030222-02$03.00/0 Pergamon Press Ltd. ILLUSORY MOTION WITHIN STILL PICTURES: THE L-EFFECT lsia Leviant” 1. Description and Characteristics of the L-Effect In 1980. while experimenting with pictorial configurations I found that. when a uniformly coloured band of an appropriate width, which I call a coloured avenue, crosses a black and white grating roughly at a right angle, one perceives within the avenue an intense illusory motion. As a result, still pictures becomelive: that is what led to the phenomenon becoming known as the LEffect . In addition, as the effect appears whenever one watches the pictures, the motion seemingly never stops: thus the name ‘Perpetuum Mobile’ pictures. I later learned that. in 1957, D. M. MacKay had reported that a ‘trickling motion’ could be observed when a black and white grating was simply interrupted on a certain width by a white space [I]. Further experimentation proved, however, that colour is indeed an essential factor of the L-Effect and that the intensity of the effect varies with the choice of colour. The L-Effect has the following characteristics: (a)The width of the avenue is critical and for each grating there is an optimum width producing the strongest effect. (b) When two or more avenues are superimposed on a grating with a certain separation between them. the illusion is accentuated. (c) For each picture there is an optimum illumination-neither too bright nor too dim-producing the most intense motion. (d) There is also an optimum distance between picture and viewer, which varies with thescaleofthegratingand the widthoftheavenues.(e)The L-Effect is much stronger with binocular than with monocular vision. ( f ) By introducing enough dark ‘figures’ (lines. spots etc. . .. ) within the uniformly coloured ‘ground’ formed by the avenues. the motion can be stopped. (8) A simple colour photograph of a Perpetuum Mobile picture also shows the motion in the coloured avenues. though it has to be observed at a closer distance. This allows to say. paradoxically. that whereas it takes a movie camera to photograph normal motion. the motion of the L-Effect can-seemingly-be photographed with a still camera. 2. Analysis of the L-Effect While discussing the L-Effect with several neuroscientists. it rapidly appeared that this phenomenon falls into a category of psycho-physiological occurrences for which our knowledge is still limited. Another phenomenon reported by J. Purkinje as early as 1823 [ 2 ] , later discussed by W. S. Hunter in 1915 [3] and extensively studied by D. M. MacKay [13 under the name ‘complementary after-image’, has remained somewhat of a mystery to this date. Purkinje stated that if one stares first at a grating of black and white lines and then at a uniformly white area. one will perceive not only the normal after-image but also a peculiar after-effect, that is, in addition to seeing the rapidly fading staticafter-image of the grating, one will perceive briefly a set of wavy lines, oriented approximately at right angles to the after-image of the *Painter,25 Rue Galilk, 751 16Paris, France. (Received 10December 1980). revised February 1982) grating. The wavy lines move orthogonally to the grating, the direction of the motion varying subjectively according to the viewer. Somewhat similar (illusory) orthogonal motions are perceived in other circumstances. as mentioned here below: When Dr. Bela Jules;.. Head of the Sensory and Perceptual Process Department at the Bell Laboratories, had the opportunity of seeing my Perpetuum Mobile pictures he made the following comments: ‘In our eye-brain system there is always visual noise present; however, our perceptual mechanism normally tends to eliminate it. Most probably the graphical configuration which you have created combined with the colors used in your colored avenues seem to amplify this visual noise. It is well known that dynamic noise such as “visual snow” on a television screen will appear to flow perpendicularly to a transparent grid placed over it. Similarly, when radial lines are placed over the dynamic noise concentric movement (perpendicular to the spokes) is observed. This so-called orthogonal after-effect is well known and it has been popularized...