Despite the fact that the eye has a
blind spot, an uneven distribution of color perception, and can make
out maximal detail in only a tiny area at the center of vision, we
still manage to see the world as an uninterrupted panorama. The eye
jumps about from point to point, snapshotting high-resolution views,
and the brain assembles them into a stunningly stable and remarkably
These rapid jumps with the eyes are called
saccades, and we make up to five every second.
The problem is that while the eyes move in saccade all visual input
is blurred. It's difficult enough for the brain to
process stable visual images without having to deal with motion blur
from the eye moving too. So, during saccades, it just
doesn't bother. Essentially, while your eyes move,
you can't see.
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
With longer saccades, you can consciously perceive the effect, but
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
Saccadic suppression may even be one of
the ways some magic tricks work. We know that sudden movements grab
attention [Hack #37]. The
magician's flourish with one hand grabs your
attention, and as your eyes are moving, you aren't
able to see what he does with the other hand to pull off the trick.
How It Works
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 [Hack #5]
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
University1 used TMS to give volunteers
small illusionary spots, called phosphenes, in their vision.
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
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 [Hack #40] —you
don't notice changes because your attention is
engaged by other things, but the changes are still potentially
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.
Thilo, K. V., Santoro, L., Walsh, V., & Blakemore, C. (2004). The
site of saccadic suppression. Nature Neuroscience,