||Map Your Blind Spot
Find out how big your visual blind spot is and
how your brain fills the hole so you don't notice
Coating the back of each eye are
photoreceptors that catch light and convert it to nerve impulses to
send to the brain. This surface, the retina,
isn't evenly spread with
receptors—they're densest at the center and
sparse in peripheral vision [Hack #14]. There's
also a patch that is completely devoid of receptors; light that falls
here isn't converted into nerve signals at all,
leaving a blind spot in your field of view—or actually two
blind spots, one for each eye.
First, here's how to notice your blind spot (later
we'll draw a map to see how big it is). Close your
left eye and look straight at the cross in Figure 1. Now hold the book flat about 10 inches from
your face and slowly move it towards you. At about 6 inches, the
black circle on the right of the cross will disappear, and where it
was will just appear grey, the same color as the page around it.
Figure 1. A typical blind spot pattern
You may need to move the book back and forth a little. Try to notice
when the black circle reappears as you increase the distance, then
move the book closer again to hide the circle totally.
It's important you keep your right eye fixed on the
cross, as the blind spot is at a fixed position from the center of
vision and you need to keep it still to find it.
Now that you've found your
blind spot, use Jeffrey Oristaglio and Paul
Grobstein's Java applet at the web site Serendip
Java) to plot its size.
The applet shows a cross and circle, so, as before, close your left
eye, fix your gaze on the cross, and move your head so that the
circle disappears in your blind spot. Then click the Start button (at
the bottom of the applet) and move your cursor around within the
blind spot. While it's in there, you
won't be able to see it, but when you can (only
just), click, and a dot will appear. Do this a few times, moving the
cursor in different directions starting from the circle each time.
Again, be careful not to move your head, and keep focused on the
cross. You'll end up with a pattern like Figure 2. The area inside the ring of dots is your
Figure 2. Matt's blind spot mapped
Here's a fun way of playing with your blind spot. In
a room of people, close one eye and focus on your index finger. Pick
a victim and adjust where your finger is until your blind spot makes
his head disappear and the background takes its place. Not very
profitable, but fun, and not as obvious as making as if to crush his
head between your thumb and index finger.
How It Works
The blind spot for each eye corresponds to
a patch on the retina that is empty of photoreceptors. With no
photoreceptors, there's nothing to detect light and
turn it into information for use by the visual system, hence the
Each receptor cell is
connected to the brain via a series of cells that aggregate the
signal before reporting it to the brain by an information-carrying
fiber called an axon (see [Hack #9]). Bizarrely, the part of
the photoreceptor responsible for detecting light is
behind the fibers for carrying the information
into the brain. That's right—the
light-sensitive part is on the side furthest from the light. Not only
does this seem like bad design, but also it means that there has to
be a gap in surface of the retina where the fibers gather together to
exit the eyeball and run to the brain—and that exit point is
the blind spot.
At first sight, there doesn't appear to be any
particular reason for this structure other than accident. It
doesn't have to be this way. If the light-detecting
parts of the cells were toward the light, you
wouldn't need a blind spot; the fibers could exit
the eye without interrupting a continuous surface of photoreceptors
on the retina.
Can we be sure that this is a bug and not a
feature? One bit of evidence is that in the octopus eye it was done
differently. The eye evolved independently in octopuses, and when it
did, the retinal cells have the photoreceptors in front of the nerve
fibers, not behind, and hence no blind spot.
Conversely, there are benefits to the arrangement of the human
retina: it allows a good blood supply close to the retina to both
nourish the photoreceptors and help metabolize debris that
accumulates there. Both orientations of the retina have their
We don't normally notice
these two great big holes in our field of vision. Not only do our
eyes move around so that there's no one bit of
visual space we're ignoring, but the blind spots
from the two eyes don't overlap, so we can use
information from one eye to fill in the missing information from the
However, even in situations in which the other eye
isn't providing useful information and when your
blind spot is staying in the same place, the brain has evolved
mechanisms to fill in the hole.1 This
filling in is why, in the demostration above, you see a continuous
grey background rather than a black hole.
Hacking the Hack
Cheshire Cat experiment (http://www.exploratorium.edu/snacks/cheshire_cat.html;
full instructions) shows a really good interaction of the blind spot,
the filling-in mechanisms and our innate disposition to notice
movement competing against our innate disposition to pay attention to
faces. With a blank wall, a mirror, and a friend, you can use your
blind spot to give yourself the illusion that you can slowly erase
your friend's head until just her smile
"Seeing More Than Your Eye Does"
is a fun tour through the capabilities of your blind spot (the link
at the bottom of each page's article will lead you
to the next page). It demonstrates how your brain uses colors and
patterns in the area surrounding the blind spot to make a good guess
of what should be in the blind spot itself and will report that to
your conscious mind.
Ramachandran, V. S. "Blind Spots."
Scientific American, May 1992, 86-91.
Ramachandran, V. S., & Gregory, R. L. (1991). Perceptual filling
in of artificially induced scotomas in human vision.
Nature, 350, 699-702.
There is an interesting discussion of the blind spot, filling in, and
what that implies for the nature of experience in Daniel
Explained, 344-366. Boston: Little, Brown and Co., 1991.
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