Frisén’s Lazy Shadow

From Michael’s “Visual Phenomena & Optical Illusions”

What to do & see

This phenomenon is not well perceived under bright lighting conditions. Reduce light and follow the sequence below for an optimal demonstration.

1. Notice the rotating bright pentagon
2. Notice the larger dark blue “shadow pentagon”, also rotating, behind it
3. They both rotate with the same speed, but are they fully aligned? [In the pair of samples below, the left one is ‘aligned’.]
   
4. With the ‘lag’ and ‘lead’ buttons you can adjust the relative rotational alignment. On the very right the alignment angle is given, for me around –3° looks aligned. [After you have clicked once on the animation, you can also use the cursor keys ← and →.]
5. Once you are satisfied they match, press ‘stop’, or ‘backg. on’.
6. Do you see a misalignment now?

Apparent alignment depends on rotation speed. It also depends on the luminance difference, the darker the outer “shadow”, the stronger the effect.

You can try out various speeds (using the faster/slower buttons) and various shapes. If you see a counter-rotation when stopping the shapes: that’s the motion after-effect. It is also possible to choose a darker blue using the colour pickers at the left. With the RGB setting ‘#000033’ the effect is nearly double in size.

Comment

The main mechanism here is that processing speed within our visual system depends on luminance, “the brighter the faster”. The mechanism is located within the photoreceptors in the retina. One might think that the optional bright-background effect argues against this, because the dark shape remains dark – but there its edges are defined by the bright background.

The present demonstration could be seen as a rotatory version of the “Hess effect”, where the more intensely illuminated of a pair of laterally-moving targets appears displaced ahead of the other. However, possibly due to pursuit eye movements, this is more difficult to demonstrate. There is also a close relation to the Pulfrich phenomenon, which, however, can be seen in binocular vision only.

References

This display was suggested to me by Lars Frisén in August 2006 and I heartily thank him for his ideas and graciously allowing the result to be published here; he also invented the name “Lazy Shadow”. He has set up his own page on this topic now.

Alpern M (1954) The relation of visual latency to intensity. AMA Arch of Phth 51:369–374

Bach M, Waltenspiel S, Röver J (1984) Comparison of pattern-ERG and VEP latency depending on stimulus luminance. Perception 13 [A27]
{Bach M, Waltenspiel S, Bühler B, Röver J (1985) Sehbahndiagnostik mit simultaner Registrierung der retinalen und kortikalen Musterpotentiale. Fortschr Ophthalmol 82:398–401}

Hess CV (1904) Untersuchungen über den Erregungsvorgan im Sehorgan der Katze bei kurz- und bei länger dauernder Reizung. Pflügers Arch ges Physiolo 101:226–262

Mansfield RJW, Daugman JG (1978) Retinal mechanisms of visual latency. Vision Res 18:1247–1260

Wilson JA, Anstis SM (1969) Visual delay as a function of luminance. Amer J Psychol 83:350–358

Pulfrich effect home page