Kaleidoscopic motion and velocity illusions

Top 10 finalist in the 2007 Best Visual Illusion of the Year Contest

Illusory effects for the rotating wheel

The next figure gives a schematic overview of the motion and velocity illusions for the rotating wheel, with links to animations that illustrate these effects. The effects apply to the depth configuration in the figure above with a white or black background, and are given as a function of the background contrasts of the rotating wheel and the stationary shapes.

The animations (Java popup applets) may need a few moments to get going. This can be checked in the illusion-less Animation 4 in which, both physically and perceptually, the wheel should rotate smoothly. The illusions occur in the other animations which were made exactly the same way and in which, physically, the wheel rotates just as smoothly.

	Animation 1 Animation 2 Animation 5
Animation 7 Animation 8		Animation 6 Animation 9 Animation 10 Animation 3 Animation 4

In both Animation 5 and Animation 6, jolts backwards can be seen to alternate with jolts forward, but the difference between these two animations is clear when viewing them side by side; the same applies to Animations 7 and 8.

Animations 9 and 10 show, apart from jolts backwards and forwards, that the background patches between the shapes seem to grow much faster than they shrink though, physically, they grow just as fast as they shrink.

Start Animation 11 to see how the illusory effects turn from one into another when contrasts change gradually.

Notice that illusory effects occur also for the inner stationary shape; for instance, it wiggles in Animations 1 and 2.

Interactive exploration of the illusions

The class of kaleidoscopic motion and velocity illusions can be explored interactively in the following two ways:

1. Launch the Java application Kaleidoscope to open a stand-alone window outside the browser.
2. Start, inside the browser, the Java applet below.

In both cases, one may produce all sorts of illusions and illusion strengths by combining variations in:

• the colours of individual stimulus parts --- by way of colour choosers
• the rotation speed --- maximum speed depends on computer
• the size of the stimulus --- large or small
• the transparency of the wheel --- four transparency values
• the relative depth positions of the shapes --- four depth configurations

Toggle "Inverse" to stop the clockwise rotating wheel and to let the stationary shapes rotate counterclockwise; this motion inversion helps to assess the influence of relative motion on the illusory effects in the normal situation.

Toggle "Phase" to let the wheel align alternately with only one of the stationary shapes; this helps to dissociate local effects (requiring one stationary shape) from global effects (requiring both stationary shapes). This dissociation can also be achieved by removing a stationary shape via its depth and colour settings. If the inner stationary shape is removed, one may see, as a bonus illusion, that the hole in the wheel seems to rotate independently the moment the wheel pulsates.

Toggle "Start/Stop" now and again to see, as a further bonus, motion-after effects (illusory counterclockwise rotation).

Kaleidoscopics