Relative change is clearly the 
determining factor in these 
cases. Unless we have clear 
information as to our own 
motion, such as when we are 
walking, information about the 
change in direction of the 
surrounding scene will be 
ambiguous. It could be the 
result of motion of the outer 
object or objects, or it could be 
the result of our own motion. If 
the moving object fills most of 
our visual field and no 
stationary objects surrounding 
the moving one are visible, the 
visual experience is essentially 
what it would be if we were 
moving in a stable 
environment.

    In the laboratory, induced 
self-motion is studied by 
seating an observer inside a 
rotating drum lined with 
stripes. Ideally, the stationary 
floor and ceiling are not 
visible. If we were in the 
observer’s place under these 
conditions, how could we tell 
whether it was the drum that 
was turning or we that were 
turning while the drum 
remained stationary? The 
visual input would be highly 
ambiguous.