TWO-STROKE APPARENT MOTION
George Mather, Sussex University, UK.
Several studies of apparent motion in two-frame animation sequences have reported that when the two frames are separated by a brief, uniformly grey field (inter-stimulus interval, or ISI), the direction of apparent motion reverses (eg. Braddick, 1980; Shioiri and Cavanagh, 1990; Takeuchi and De Valois, 1997). The reversal of apparent direction has been attributed to the negative phase of a biphasic temporal impulse response in low-level motion detectors. This response produces a negative neural image of the first frame, which combines with the positive image of the second frame to produce reversed apparent motion (see Shioiri and Cavanagh, 1990; Strout et al.,
1994; it is well known that contrast-reversing one frame of a two-frame motion sequence produces reversed apparent motion; Anstis and Rogers, 1975).
On the basis of these reports, and the explanation in terms of low-level motion responses, it should be possible to create a new illusion of motion. If a two-frame animation sequence is presented repeatedly in a loop with no ISI, the observer should report oscillatory motion. The switch from frame one to frame two produces forward motion, and the switch from frame two back to frame one produces backwards motion. If a grey ISI is inserted between the offset of frame two and the reappearance of frame one, it should reverse the backward motion normally seen to create a second phase of forward motion. The two-frame display should appear unidirectional. I have called this kind of display ‘two-stroke motion’, since it contains just two pattern displacements.
Previous studies have reported a related stimulus called ‘four-stroke motion’, containing four frames of pattern displacement (Anstis and Rogers, 1986; Mather,
1991). The first two frames contain standard two-frame apparent motion; the second two contain contrast-reversed versions of the first two frames. This four-frame sequence also appears unidirectional because of the effect of contrast reversal mentioned earlier.
The new and distinctive features of two-stroke motion are that (i) it contains the bare minimum of information to create apparent motion - just two pattern displacements - yet appears unidirectional; and (ii) it relies on contrast-reversal created in the visual system, rather than in the stimulus. Prolonged viewing can give rise to a motion aftereffect.
The files submitted for this illusion are in Gif and Mov format. Other formats
(including HTML) or enlargements can be supplied on request. “TwoStrokeSmall” is the illusion, containing two frames plus ISI; “TwoStrokeSmallControl” contains two frames with no ISI.
References
Anstis S M, Rogers B J (1975) Vision Research, 15 , 957-961.
Anstis S M, Rogers B J (1986) Perception, 15 , 627-640.
Braddick O J (1980) Philosophical Transactions of the Royal Society of London B, 290 , 137-151.
Mather G (1991) Vision Research, 31 , 161-167.
Shioiri S, & Cavanagh P (1990) Vision Research, 30 , 757-768.
Strout J J, Pantle A, Mills S L (1994) Vision Research, 34 , 3223-3240.
Takeuchi T, & De Valois K K (1997) Vision Research, 37 , 745-755.