Flash Version
This new illusion won second place at the recent international "Illusion of the Year" competition at the annual European Conference on Visual Perception, A Coruna, Spain.
The illusion contains two pattern frames depicting a moving image (hence two-stroke) which are displayed using a technique that creates an impression of continuous forward movement. Any two frames from a motion sequence can be used. The two images below show two views of a motorcycle as it advances along the road.
When the two images are presented in alternation, you will see back-and-forth motion. (Note: there are several QuickTime animations on this page. They may appear jerky if all are running at once; try stopping some to get the smoothest animation)
An extra trick is needed to create the impression of continuous forward motion. Several research papers have reported that when two frames in a motion sequence are separated by a brief, blank inter-stimulus interval (ISI), a large number of errors are obtained in direction discrimination tasks; the direction of apparent motion reverses. A blank ISI is inserted into the sequence shown above, after the second frame.
Notice that the apparent motion now appears unidirectional.
The switch from frame-1 to frame-2 creates forward apparent motion. The switch from frame-2 back to frame-1 would normally create backward apparent motion, but the blank ISI reverses this motion to create forward motion; the motion sequence appears unidirectional.
The dramatic effect of the blank ISI is due to the visual system's temporal response - how it responds to sudden changes in illumination. In bright conditions the response to a sudden change shows an initial positive phase followed by a brief negative phase. The latter can be viewed as creating a negative neural image of whatever preceded the change. So during the brief ISI, a negative neural image of frame-2 is created in the visual system. This negative response combines with the positive response created by frame-1 to evoke apparent motion. The combination of positive and negative images in motion sequences is already known to produce reversed apparent motion, as can be seen in four-stroke motion.
Four-stroke motion displays consist of four pattern frames containing repetitive forward and backward displacements. In frame transitions involving backward displacements, the second frame is contrast-reversed relative to the first, so that the resultant motion is also reversed. The result is forward apparent motion.
Two-stroke motion contains only two pattern frames instead of four. The contrast-reversed image needed to produce unidirectional apparent motion is created in the visual system by the ISI, rather than being a part of the stimulus sequence itself.
