Mind Hacks™: Tips & Tools for Using Your Brain (9 page)

The high-resolution portion of your vision is only the size of your thumbnail
at arm’s length. The rest of your visual input is low res and mostly colorless, although
you seldom realize it.

Figure 2-1
is a variant of the usual
eye chart you will have encountered at the optometrist, constructed by Stuart Anstis. Hold
it in front of you, and rest your gaze on the central dot. The letters in the chart are
smallest in the middle and largest at the outer edge; they scale up at a rate to exactly
compensate for your eyes’ decrease in resolution from the central fovea to the
periphery.

That means that, holding your gaze on the center of the chart, it should be as easy
for you to read one of the letters near the middle as one of the bigger ones at the
edge.

Figure 2-1. When you fixate on the center of this chart, all the letters are scaled to have the
same resolution
¹

What this eye chart doesn’t show is our relative decrease in color-sensing
ability as we edge toward peripheral vision. Have a friend hold pieces of colored card up
to the side of your face while you keep your head, and eyes, looking forward. Notice that,
while you can see that she’s moving the card off in the corner of your eye, you can’t tell
what color the card is.

Because peripheral vision is still good at brightness, you’ll need to use pieces of
card that won’t give you any clues simply from how bright the card looks. A dull yellow
and a bright blue will do. If you’d like to perform a more rigorous experiment, the
Exploratorium museum provides instructions on how to
make yourself a collar to measure the angles at which your color vision
becomes useful (
http://www.exploratorium.edu/snacks/peripheral_vision/
).

When you’re looking at Anstis’ eye chart,
Figure 2-1
, all the letters are equally
legible because the light from each is falling on the same number of photoreceptors at the
back of the eye. The central letters fall in the center of your retina, where the
photoreceptors are densest; the outer letters fall in the periphery where the cells are
spread thinner, but the letters are larger so the same number of cells are covered.

The distribution of light-sensitive cells across the retina is shown in
Figure 2-2
. There are two different curves,
one for
rods
and one for
cones
, corresponding to
the two types of photoreceptor cells we possess, so named because of their shapes. You can
see how they’re both densest toward the center of the eye and drop away toward the
periphery, although at different rates. Assuming you’re reading this book in anything
above dim light, you’ll have been using your cones to look at the eye chart — they’re the
ones that drop away fastest, and that rate determines the resolution of vision.

That’s why our color vision suffers outside the fovea. Cones work best in normal,
daytime light levels, and they also respond to color. But the rods are relatively more
numerous outside the fovea, and they don’t respond to color. They’re also extremely
sensitive to light, so during the day they’re not too much help at all, but you can still
see how they’re useful when cones are sparse. They’re why you could see your friend moving
the colored card in the earlier experiment, but they couldn’t help you figure out whether
the card was yellow, blue, or whatever.

Rods, because of their sensitivity to light, are also handy when light is very poor.
In dim conditions, our cones shut down (over a period of about 5 minutes) and we use our
rods to see (the rods reach maximum sensitivity after about half an hour). But notice that
rods are actually densest just outside the
fovea, which means the best way to spot really faint light is to look at it
slightly off-center. You can use this to look for faint stars on a dark night, and you’ll
see slightly more stars slightly outside the exact center of your vision.

Figure 2-2. The distribution of different photoreceptors on the retina
²

Curiously, aside from experiments like the colored card one, you don’t normally notice
that not all of your visual world is high resolution. This is because you move your eyes
to what you want to look at, and as you move your eyes, the area of high resolution
follows. This process of active vision
[
To See, Act
]
is much more efficient than having high
resolution everywhere.

Of course, before you move your eyes to something, your visual system has to
preconsciously spot it using your peripheral vision and move your attention there. The
events best noticed by peripheral vision are described in
Grab Attention
and are mainly sudden changes of movement and light. These are events that signify that
something needing an urgent response could be happening — it’s not surprising we are
designed to notice them even outside the high-resolution center of the eye.

Think of perception as a behavior, as something active, rather than as something
passive. Perception exists to guide action, and being able to act is key to the
construction of the high-resolution illusion of the world we experience.

One example of active vision that always happens, but that we don’t normally notice,
is moving our eyes. We don’t normally notice our blind spots
[
Map Your Blind Spot
]
or our poor peripheral vision
[
See the Limits of Your Vision
]
,
because our gaze constantly flits from place to place. We sample constantly from the
visual world using the high-resolution center of the eye — the
fovea —
and our brain constructs a constant, continuous, consistent,
high-resolution illusion for us.

Constant sampling means constant eye movement: automatic, rapid shifts of gaze called
saccades
. We saccade up to five times a second, usually without
noticing, even though each saccade creates a momentary gap in the flow of visual
information into our brains
[
Glimpse the Gaps in Your Vision
]
. Although the target
destination of a saccade can be chosen consciously, the movement of the eyes isn’t itself
consciously controlled. A saccade can also be triggered by an event we’re not even
consciously aware of — at least not until we shift our gaze, placing it at the center of our
attention. In this case, our attention’s been captured involuntarily, and we had no choice
but to saccade to that point
[
Grab Attention
]
.

Each pause in the chain of saccades is called a
fixation
.
Fixations happen so quickly and so automatically that it’s hard to believe that we don’t
actually hold our gaze on anything. Instead, we look at small parts of a scene for just
fractions of a second and use the samples to construct an image.

Using eye tracking devices, it is possible to construct images of where people fixate
when looking at different kinds of objects — a news web site, for instance. The Poynter
Institute’s Eyetrack III project (
http://www.poynterextra.org/eyetrack2004/
) investigates how Internet news readers go about perusing news online (
Figure 2-3
) and shows the results of their
study as a pattern of where eye gaze lingers while looking over a news web site.

Part of developing speed-reading skills is learning to make fewer fixations on each
line of text and take in more words at each fixation. If you’re good — and the lines are
short enough — you can get to the point of one fixation per line, scanning the page from top
to bottom rather than side to side.
Figure 2-4
shows typical fixation patterns
while reading.

Figure 2-3. The pattern of eye fixations looking over a news web site; the brighter patches
show where eyes tend to fixate
²

Figure 2-4. A typical pattern of eye fixations when reading
³

Figure 2-5
shows a typical
pattern of what happens when you look at a face. You fixate enough to get a good idea of
the shape of the whole face with your peripheral vision, fixating most on those details
that carry the most information: the eyes.

Figure 2-5. A pattern of fixations over 8 seconds when looking at a face (Matt’s, in this case)
⁴