Spot B (right), moving in a
circular path, is peceived
veridically when spot A (left) is
not visible. When spot A is
visible, spot B appears to move
back and forth horizontally 
while both spots as a group 
appear to move vertically.

    Johansson has shown that 
this kind of perception can 
occur even when no visible 
structure serves as a frame of 
reference. As shown at left, one 
spot, A, moves up and down. 
Another spot, B, moves along 
the path of a circle. However, 
B’s motion is linked to A’s 
vertical motion so that B 
arrives at the top and bottom of 
its circular path when A arrives 
at the top and bottom of its 
path. If A is not visible, B 
appears veridically to move 
around a circular path. But 
when A is visible, B is not 
perceived to move in a circle. 
Rather, it appears to move back 
and forth horizontally, 
approaching and receding from 
A. In addition, both A and B 
appear to move up and down 
together.

    There is some disagreement 
over how to explain this effect. 
One explanation is that B’s 
motion is purely horizontal 
relative to A’s, and that such 
relative change is salient in 
our perceptual experience. 
Therefore, we perceive this 
change. But, in addition, both 
spots are going up and down 
together, and we perceive this 
fact secondarily. An alternative 
explanation evokes the 
principle of grouping by 
"common fate," discussed in 
Chapter 5: When objects move 
together in the same direction 
and at the same speed, we tend 
to perceive them as belonging 
together. The spots in our 
example are moving together in 
the vertical direction. Once we 
see both spots as a group, this 
structure becomes a frame of 
reference with respect to 
which the horizontal 
component of B’s motion is 
noted. In both explanations, 
however, the net result is that 
the circular motion of B is 
divided into two vectors of 
perceptual motion.