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.
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.
The lefthand animation contains two frames. The righthand animation contains a blank frame after the second frame.