The optokinetic response in 
humans causes your eyes to 
follow these moving lines in 
the direction in which they are 
moving, in this case, left to 
right.  A similar reflex was 
shown in an experiment in 
which a fly was placed inside a 
rotating drum of vertical 
stripes.  Lacking the ability to 
turn its eyes, the fly turned its 
entire body, displaying the 
optomotor response.

    These findings tell us that 
the specific relations between 
a change of an object’s 
direction and a change in the 
observer’s position that yield 
an impression of motion or 
position constancy can be 
learned or relearned. They do 
not necessarily imply that the 
relations had to be learned in 
the first place, however. For 
some animals tested, position 
constancy is innately 
determined. In one 
experiment, the head of a fly 
was surgically rotated 180 
degrees and kept in that 
position. This had the effect of 
reversing the direction of the 
motion of the image of the 
stationary scene during the 
fly’s motion, much as did the 
lenses in Stratton’s 
experiment. What did the fly 
perceive when it moved? A 
simple but ingenious 
experiment, conducted by Horst 
Mittelstaedt of the Max Planck 
Institute, that made use of a 
known reflexlike effect 
supplied an answer.