(Left) A spot moving along an
oblique path in a homogeneous
field is perceived veridically.
(Right)  When a horizontally
moving frame surrounding the
spot moves along with it, the 
spot appears to be moving
vertically.

    Before trying to understand 
these perceptions, consider the 
following example illustrated 
at left. If we view a spot of light 
traveling along an oblique path 
in an otherwise dark room, we 
will perceive its motion 
correctly. If we view the spot 
traveling the same path, but 
this time surrounded by a 
rectangular frame that is 
moving horizontally along with 
the spot such that both will 
reach their rightmost and 
leftmost positions at the same 
moment, the spot will no 
longer appear to move 
obliquely. Our dominant 
impression will be that it is 
moving up and down. However, 
we will also have the 
impression, although a less 
strong one, that the spot 
belongs to the rectangle and is 
moving horizontally along with 
it.