where there are no rods and no cones—no photoreceptors at all. This
is the place where the axons from the neurons in the retina come to-
gether into a bundle called the optic nerve and exit the eyeball on the
way to the brain. The large number of nerve fibers (about one million)
occupy so much space that there is no room for any photoreceptor
cells in that small region of the retina. This region is called the “blind
spot,” because any light falling on that part of the retina will not be de-
tected. Although we are completely blind to the region of visual space
corresponding to the retinal blind spot, we are completely unaware of
this blindness, unless specific effort is made to draw attention to it.
To detect your visual blind spot, hold this page about 15 cm (6
inches) in front of your face.
The center dot should be directly in front of your nose, such that a
line connecting your nose with the center dot would be parallel to
the floor. Now, close one eye and look at the center dot. Then, keeping
your open eye firmly focused on the center dot, move the page gently
toward and away from your face until the dot on the same side as your
open eye disappears. That is, if your left eye is open, then the left-side
dot will disappear; if your right eye is open, the right-side dot will
disappear.
Clearly, there is a blind spot in each of our eyes. Yet, we are not aware
of this, unless we perform some trick to make the blindness visible.
How so?
Because the location in space of the blind spot is different for each
of the two eyes—if both eyes are open and functioning, the visual
system can use the information received in one eye to fill in the blind
spot for the other eye. However, even if we close one eye and walk