An illusion of direction of 
motion that is very different 
from the kinds thus far 
discussed is the barber-pole 
illusion. One tends to see 
stripes moving down (or up), 
although in fact any region on 
the colored helical stripe that 
curves around a barber pole is 
simply rotating around the pole 
and not displacing downward or 
upward. In order to see this 
motion veridically, however, 
we would have to distinguish 
some specific point on the 
helix. Then we would detect its 
rotation in a horizontal plane. 
Without such a distinct point, 
however, we have no good 
information as to how the 
contour of the helix is moving. 
The stimulus is thoroughly 
ambiguous. Under the 
circumstances, we tend to 
"assume" that visible parts of 
the contour at time 1 are the 
same as those at time 2--that is, 
that these regions have the 
same physical identity. In fact, 
they are different because the 
helix is rotating. Thus those 
visible at time 1 will be 
occluded by time 2. If the parts 
were the same, the stripe would 
have to be moving directly 
downward, and that is precisely 
what we perceive.