illusions presented here occur in stimuli with the
starlike inner and outer shapes, a
starlike wheel rotates with constant velocity about its own center. It
rotates partly behind the inner shape and partly in front
of the outer shape, but at regular intervals, it precisely fills the
between the stationary shapes. At these moments, various
motion and velocity illusions occur, for both the
rotating wheel and the inner stationary shape.
For instance, depending on the contrasts in the image, the rotating
wheel may pulsate, jolt, or accelerate, and the inner stationary shape
may wiggle. These dynamic effects seem to be caused by colour
assimilation and ambiguous figure-ground segregation.
For an overview with Java animations --- see below
For an interactive Java exploration --- see further
Alternatively, try Michael
Bach's Flash version
The required free Java software can be downloaded here
van der Helm, P. A. (2007).
Kaleidoscopic motion and
Illusory effects for
The next figure gives a schematic overview of the motion and velocity
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
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.
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.
to see how the illusory effects turn from
another when contrasts change gradually.
Notice that illusory effects
occur also for the inner stationary shape; for instance, it
of the illusions
The class of kaleidoscopic motion and velocity illusions can be
explored interactively in the following two ways:
- Launch the Java application Kaleidoscope
to open a stand-alone window outside the browser.
- 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
- the rotation speed --- maximum speed depends on computer
- the size of the stimulus --- large or small
- the transparency of the wheel --- four
- the relative depth positions of the shapes --- four depth
Toggle "Inverse" to stop
the clockwise rotating wheel and to let the
shapes rotate counterclockwise; this motion inversion helps to assess
the influence of
relative motion on the illusory effects in the
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
Toggle "Start/Stop" now and again to see, as a further
effects (illusory counterclockwise rotation).