Mind Hacks™: Tips & Tools for Using Your Brain (10 page)

Find out how big your visual blind spot is and how your brain fills the hole
so you don’t notice it.

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 2-6
. 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 2-6. 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 (
http://serendip.brynmawr.edu/bb/blindspot
; 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 2-7
. The area inside
the ring of dots is your blind spot.

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 blind spot.

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
The Neuron
). 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.

Figure 2-7. Matt’s blind spot mapped

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.

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 other.

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.

The Cheshire Cat experiment (
http://www.exploratorium.edu/snacks/cheshire_cat/
; 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 remains.

Our eyes constantly dart around in extremely quick movements called saccades. During
each movement, vision cuts out.

Put your face about 6 inches from a mirror and look from eye to eye. You’ll notice
that while you’re obviously switching your gaze from eye to eye, you can’t see your own
eyes actually moving — only the end result when they come to rest on the new point of focus.
Now get someone else to watch you doing so in the mirror. They can clearly see your eyes
shifting, while to you it’s quite invisible.

With longer saccades, you can consciously perceive the effect, but only just.

Hold your arms out straight so your two index fingers are at opposite edges of your
vision. Flick your eyes between them while keeping your head still. You can just about
notice the momentary blackness as all visual input from the eyes is cut off. Saccades of
this length take around 200 ms (a fifth of a second), which lies just on the threshold of
conscious perception.

What if something happens during a saccade? Well, unless it’s really bright, you’ll
simply not see it. That’s what’s so odd about saccades. We’re doing it
constantly, but it doesn’t look as if the universe is being blanked out a
hundred thousand times a day for around a tenth of a second every time.

Saccadic suppression exists to stop the visual system being confused by blurred images
that the eye gets while it is moving rapidly in a saccade. The cutout begins just before
the muscles twitch to make the eyes move. Since that’s before any blur would be seen on
the retina, we know the mechanism isn’t just blurred images being edited out at processing
time. Instead, whatever bit of the brain prepares the eyes to saccade must also be sending
a signal that suppresses vision. Where exactly does that signal come from? That’s not
certain yet.

One recent experiment proves that suppression definitely occurs before any visual
information gets to the cortex. This isn’t the kind of experiment that can be done at
home, unfortunately, as it requires
transcranial magnetic stimulation
(TMS). TMS
[
Transcranial Magnetic Stimulation: Turn On and Off Bits of the Brain
]
essentially lets you
turn on, or turn off, parts of the brain that are close enough to the surface to be
affected by a magnet. The device uses rapid electromagnetic pulses to affect the cells
carrying signals in the brain. Depending on the frequency of the pulses, you can enhance
or suppress neuronal activity.

Kai Thilo and a team from Oxford University
¹
used TMS to give volunteers small illusionary spots, called phosphenes, in
their vision.

When phosphenes were made at the retina, by applying TMS to the eye, saccadic
suppression worked as normal. During a saccade, the phosphenes disappeared, as would be
expected. The phosphenes were being treated like normal images on the retina. But when the
spots were induced later in visual processing, at the cortex, saccades didn’t affect them.
They appeared regardless of eye movements.

So, suppression acts between the retina and the cortex, stopping visual information
before the point where it would start entering conscious experience. Not being able to see
during a saccade isn’t the same kind of obstruction as when you don’t see because your
attention is elsewhere. That is what
happens during change blindness
[
Blind to Change
]
— you don’t notice changes because your
attention is engaged by other things, but the changes are still potentially
visible.

Instead, saccadic suppression is a more serious limitation. What happens during a
saccade makes it nowhere near awareness. It’s not just that you don’t see it, it’s that
you can’t.