Gibson emphasized that 
the texture of a plane that 
yields motion perspective 
provides richer, more 
naturalistic information than 
do laboratory environments 
that consist of such artificial 
elements as one or more spots 
floating in space. Wallach 
noted that in kinetic depth 
displays, in contrast to simple 
examples of motion parallax, 
not only do points within the 
object shift in direction as the 
object moves but the contours 
of the object simultaneously 
change in orientation and 
length. He believes it is this 
coupling of length change and 
orientation change that is 
crucial to the perception of 
depth generated by motion. 
 
    Other factors may 
influence the effectiveness of 
motion as a depth cue. For 
example, in the kinetic depth 
effect the stimulus is a 
connected object. Stanley 
Anton demonstrated in our 
laboratory that when only the 
ends of the rotating rod are 
visible, as separate spots, the 
depth effect does not occur. The 
amount of depth separation may 
also influence the 
effectiveness of motion as a 
depth cue. In the case of 
stereopsis, when the disparity 
is too great, one typically sees 
double images of the non-
fixated contour rather than 
one contour farther or nearer 
than the fixated one. If this 
rule holds for motion parallax, 
it may explain the impressive 
sense of depth achieved by 
motion of a band within a 
random dot pattern in the 
experiment Rogers and Graham 
conducted. The actual depth 
simulated in that experiment 
was far less than that employed 
in the experiment on luminous 
circles described earlier. Still 
another factor may be whether 
it is the display or the observer 
that is moving. For ideal 
conditions such as in the 
Rogers and Graham experiment 
or in a kinetic depth display, 
object movement suffices, but 
for poorer conditions such as in 
the luminous circles 
experiment, only movement of 
the observer may yield a depth 
effect. When the observer 
moves, information is available 
that the changing retinal 
image is not necessarily the 
result of object motion.