How does the brain "know" 
whether or not the eyes are 
moving and, if they are, in 
what direction and at what 
speed? Given what is 
understood about how the brain 
gains information about 
movement of other parts of the 
body, we might suppose that 
such information derives from 
sensory feedback. For example, 
physiologists believe that, 
when the arm bends, receptor 
cells in the elbow joint signal 
the change. Such 
proprioceptive information has 
long been held to come from the 
activity of muscles as well as 
joints. Receptor cells in the eye 
muscles were thus assumed to 
be the source of similar 
information about eye 
movement.

    It is unlikely, however, that 
the information that tells us 
about eye movement derives 
from sensory feedback. 
Consider two countercases. 
First, there are circumstances 
in which the eyes remain 
perfectly still—–and thus there 
is no proprioceptive feedback 
that they are moving—–but 
nonetheless the eyes are 
interpreted as moving. If the 
eye muscles are paralyzed or are 
otherwise prevented from 
moving, the observer may still 
attempt to look at an object in 
the periphery. Each time this 
happens, the entire visual field 
appears to move rapidly in the 
direction of the intended eye 
movement. Helmholtz and 
subsequent investigators 
inferred from this result that 
the perceptual system treats 
the intention or command to 
move the eyes as equivalent to 
actual eye movement. 
Ordinarily the command would 
be followed immediately by eye 
movement. Thus the image 
displacement of a stationary 
thing would not be improperly 
interpreted as signifying that 
the object was in motion. But if 
the eyes cannot move, the 
command is still recorded, and 
the eyes are interpreted as 
moving. Consequently, the 
stationary image is incorrectly 
interpreted as signifying that 
the object is in motion.