Now we can reconsider the 
spot on the rim of the rolling 
wheel. The spot’s true path 
through space is not perceived 
unless the rest of the wheel is 
invisible, as in the case of the 
bicycle seen in the dark. When 
just a few additional spots on 
the wheel are visible, the 
wheel becomes a frame of 
reference for the spot. The spot 
is now seen as revolving around 
its axis. Because it "belongs" to 
the wheel, it also is seen as 
partaking of the wheel’s 
horizontal, linear motion. 
Therefore, there are two 
components of motion that are 
perceived.

    These examples of directional 
illusion and those of induced 
motion suggest the following 
conclusions: The motion of one 
object relative to other objects 
is particularly important in the 
perception of movement. Under 
certain conditions, one object 
in the field will serve as frame 
of reference with respect to 
which other objects will be 
seen to move. The frame of 
reference is often "assumed" to 
be stationary so that any 
motion relative to it is 
attributed to other objects. 
When the frame of reference is 
seen to be in motion, objects 
seen with respect to it will 
appear to have different 
components of movement, one 
based directly on relative 
change and another based on 
their belonging to the frame 
and partaking of its motion. 
Finally, under the right 
conditions, observers may 
perceive themselves to be in 
motion when a nearby 
structure that is in motion is 
assumed to be stationary.