** A new anomalous motion illusion: the "central drift illusion"  **
** Akiyoshi Kitaoka (Ritsumeikan University) and Hiroshi Ashida (Kyoto
University)  **
Presented in the 2004's winter meeting of the Vision Society of Japan, Tokyo,
Japan (January 26 - 28, 2004)
_ Presented January 26, 2004, using this page  _
* * *
**Summary** . We found a new anomalous motion illusion, in which observers see
illusory motion in a stationary image. Although a variety of anomalous motion
illusions have recently been discovered, most of them require retinal slips of
images to generate the illusory motion. Among them, the only exception was the
peripheral drift illusion, in which observers see illusory motion in the
peripheral vision without any retinal slip (  Kitaoka and Ashida, 2003,
VISION  ). The present
new illusion appears without retinal slips of images and can be seen in the
central vision as well as in the peripheral vision. The critical factor to
cause this illusion seems to be the difference in contrast between the inner
and outer areas of elongated bars or ellipses.
* * *
We found a new anomalous motion illusion in the figures of  visual phantoms
 , in which phantoms
appear to expand in the horizontal direction when we observe them carefully.
But the effect is too faint for most observers.

There are two dark phantoms across the dark occluder, each of them appears to
expand horizontally.

There are three light phantoms across the light occluder, each of them appears
to expand horizontally.

The sinusoidal grating itself does not show any phantoms or any anomalous
motion illusion.
* * *
A special type of visual phantoms (pseudo second-order phantoms) strongly
induces this new illusory motion, in which phantoms bridging low-contrast
stripes appear to expand in the horizontal direction.

Actually, the low-contrast areas in the stripes appear to expand.
Subsequently, phantoms that are induced by them appear to expand.
* * *
One of Zavagno's figures also shows this type of anomalous motion illusion, in
which the outer "misty" darkness appears to move into the inner space. The
effect is rather small.

Some observers may see the reverse effect just after saccades or blinks,
probably due to the difference in visual latency depending on luminance.
* * *
When contrast of the Zavagno figure decreases, the anomalous motion illusion
increases. The outer "mist" appears to move into the inner space (left) . The
inner "mist" appears to expand (right) .

* * *
When the ellipses are rotated through 90 deg, the rings made up of the
ellipses appear to rotate slowly. The ring appears to rotate clockwise (left).
The ring appears to rotate counterclockwise (right).

* * *
The shape shown below (round heads and rectangular tails) generate stronger
illusion than the ellipse version. The ring appears to rotate clockwise
(left). The ring appears to rotate counterclockwise (right). Also see " [
Boats ](kabe-e.html#boats) ".

* * *
This illusion does not depend on the luminance gradient of inducers. Although
the luminance gradients are the same (white to gray) between the two figures
shown below, the direction of illusory motion is opposite to each other. The
upper two rows in the left figure appear to move leftward while those in the
right figure appear to slide rightward; the lower two rows in the left figure
appear to move rightward while those in the right figure appear to slide
leftward.

The only difference in stimuli is whether the background is gray or white. It
is therefore suggested that the critical factor to generate this illusion be
contrast. In this case, the direction of illusory motion can be described as "
**low-to-high** contrast".