of the brain. We have also
learned that the eyes are in a
constant state of very rapid
oscillation, or tremor, and that
such motion is crucial for
vision because it causes the
image to be continuously
shifted over the retina. Visual
cells respond more to the onset
and cessation of stimulation
than to steadily maintained
stimulation. The stabilization
of the image on the retina by
various methods will thus
eliminate visual perception of
contours. We know from the
recording of single cells in the
visual system that, in some
species, neurons exist that
discharge rapidly only when a
certain stimulus pattern is
present in the appropriate
region of the retina. These
neurons can be considered to
be detectors of such features as
spots, contours, or edges. We
also now know that light
stimulation of corresponding
regions of the two retinas
transmits signals to one region
in one hemisphere of the brain
and yields singleness of vision
and that stimulation of slightly
noncorresponding regions by a
similar pattern yields depth
perception.