Mind Hacks™: Tips & Tools for Using Your Brain (11 page)

Our sense of time lends a seamless coherence to our conscious experience of the world.
We are able to effortlessly distinguish between the past, present, and future. Yet, subtle
illusions show that our mental clock can make mistakes.

Sometimes you’ll glance at a clock and the second hand appears to hang, remaining
stationary for longer than it ought to. For what seems like a very long moment, you think
the clock may have stopped. Normally you keep looking to check and see that shortly
afterward the second hand starts to move again as normal — unless, that is, it truly has
stopped.

This phenomenon has been dubbed the
stopped clock
illusion
. You can demonstrate it to yourself by getting a silently moving
clock and placing it off to one side. It doesn’t need to be an analog clock with a
traditional second hand; it can be a digital clock or watch, just so long as it shows
seconds. Position the clock so that you aren’t looking at it at first but can bring the
second hand or digits into view just by moving your eyes. Now, flick your eyes over to the
clock (i.e., make a saccade
[
To See, Act
]
). The movement needs to be as quick as possible, much as might happen if
your attention had been grabbed by a sudden sound or thought
[
Grab Attention
]
; a slow, deliberate movement won’t cut
it. Try it a few times and you should experience the “stopped clock” effect on some
attempts at least.

When our gaze falls on an object, it seems our brain makes certain assumptions about
how long that object has been where it is. It probably does this to compensate for the
suppression of our vision that occurs when we move our eyes
[
Glimpse the Gaps in Your Vision
]
. This suppression means vision
can avoid the difficult job of deciphering the inevitable and persistent motion blur that
accompanies each of the hundred thousand rapid saccadic eye movements that we make daily.
So when our gaze falls on an object, the brain assumes that object has been where it is
for at least as long as it took us to lay eyes on it. Our brain
antedates
the time the object has been where it is. When we glance
at stationary objects like a lamp or table, we don’t notice this antedating process. But
when we look at a clock’s second hand or digits, knowing as we do that they ought
not
be in one place for long, this discord triggers the
illusion.

This explanation was supported and quantified in an experiment by Keilan Yarrow and
colleagues at University College, London and Oxford University.
¹
They asked people to glance at a number counter. The participants’ eye
movements triggered the counter, which then began counting upward from 1 to 4. Each of the
numerals 2, 3, and 4 was displayed for 1 second, but the initial numeral 1 was displayed
for a range of different intervals, from 400 ms to 1600 ms, starting the moment subjects
moved their eyes toward the counter. The participants were asked to state whether the time
they saw the numeral 1 was longer or shorter than the time they saw the
subsequent numerals. Consistent with the stopped clock illusion, the
participants consistently overestimated how long they thought they had seen the number 1.
And crucially, the larger the initial eye movement made to the counter, the more
participants tended to overestimate the duration for which the initial number 1 was
visible. This supports the saccadic suppression hypothesis, because larger saccades are
inevitably associated with a longer period of visual suppression. And if it is true that
the brain assumes a newly focused–on target has been where it is for at least as long as
it took to make the orienting saccade, then it makes sense that longer saccades led to
greater overestimation. Moreover, the stopped clock illusion was found to occur only when
people made eye movements to the counter, not when the counter jumped into a position
before their eyes — again consistent with the saccadic suppression explanation.

You’ll experience an effect similar to the stopped clock illusion when you first pick
up a telephone handset and get an intermittent tone (pause, beeeep, pause, beeeep,
repeat). You might find that the initial silence appears to hang for longer than it ought
to. The phone can appear dead and, consequently, the illusion has been dubbed the
dead phone illusion
.

The clock explanation, however, cannot account for the dead phone illusion since it
doesn’t depend on saccadic eye movement.
²
And it can’t account, either, for another recent observation that people
tend to overestimate how long they have been holding a newly grasped object,
³
which seems like a similar effect: the initial encounter appears to last
longer.

One suggestion for the dead phone illusion is that shifting our attention to a new
auditory focus creates an increase in arousal, or
mental interest
.
Because previous research has shown that increased arousal — when we’re stressed, for
instance — speeds up our sense of time, this could lead us to overestimate the duration of a
newly attended–to sound. Of course, this doesn’t fit with the observation mentioned
before, that the stopped clock illusion fails to occur when the clock or counter moves in
front of our eyes — surely that would lead to increased arousal just as much as glancing at
a clock or picking up a telephone.

So, a unifying explanation for “when time stands still” remains elusive. What
is
clear is that most of the time our brain is extraordinarily
successful at providing us with a coherent sense of what happened when.

— Christian Jarrett

It takes longer to shift your attention to a new object if the old object is still
there.

The gap effect can be spotted if you’re asked to stare at some shape on a screen, then
switch your gaze to a new shape that will appear somewhere else on the screen. Usually,
switching to the new shape takes about a fifth of a second. But if the old shape vanishes
shortly before the new shape flashes up, moving your gaze takes less time, about 20%
less.

It has to be said: the effect — on the order of just hundredths of a second — is tiny in
the grand scheme of things. You’re not going to notice it easily around the home. It’s a
feature of our low-level cognitive control: voluntarily switching attention takes a little
longer under certain circumstances. In other words, voluntary behavior isn’t as voluntary
as we’d like to think.

We take in the world piecemeal, focusing on a tiny part of it with the high-resolution
center of our vision for a fraction of a second, then our eyes move on to focus on another
part. Each of these mostly automatic moves is called a saccade
[
To See, Act
]
.

We make saccades continuously — up to about five every second — but that’s not to say
they’re fluid or all the same. While you’re taking in a scene, your eyes are locked in.
They’re resistant to moving away, just for a short time. So what happens when another
object comes along and you want to move your eyes toward it? You have to overcome that
inhibition, and that takes a short amount of time.

Having to overcome resistance to saccades is one way of looking at why focusing on a
new shape takes longer if the old one is still there. Another way to look at it is to
consider what happens when the old shape disappears. Then we can see that the eyes are
automatically released from their fixation, and no longer so resistant to making a
saccade — which is why, when the old shape disappears before the new shape flashes up, it’s
faster to gaze-shift. In addition, the disappearing shape acts as a warning signal to the
early visual system (“There’s something going on, get ready!”), which serves to speed up
the eyes’ subsequent reaction times. It’s a combination of both of these factors — the
warning and the eyes no longer being held back from moving — that results in the
speedup.

Just for completeness, it’s worth knowing that the old point of fixation should
disappear 200 milliseconds (again, a fifth of a second) before the new object appears, to
get maximum speedup. This time is used for the brain to notice the old object has vanished
and get the eyes ready to move again. Now, in the real world, objects rarely just vanish
like this, but it happens a lot on computer screens. So it’s worth knowing that if you
want someone to shift his attention from one item to another, you can make it an easier
transition by having the first item disappear shortly before the second appears (actually
vanish, not just disappear behind something, because we keep paying attention to objects
even when they’re temporarily invisible
[
Feel the Presence and Loss of Attention
]
). This will facilitate
your user’s disengagement from the original item, which might be a dialog box or some
other preparatory display and put her into a state ready for whatever’s going to need her
attention next.

How do you figure out the three-dimensional shape of objects, just by looking?
At first glance, it’s using shadows.

Have a look at the shaded circles in
Figure 2-8
, following a similar illustration
in Kleffner and Ramachandran’s “On the Perception of Shape from Shading.”
¹

I put together this particular diagram myself, and there’s nothing to it: just a
collection of circles on a medium gray background. All the circles are gradient-filled
black and white, some with white at the top and some with white at the bottom. Despite the
simplicity of the image, there’s already a sense of depth.

The shading seems to make the circles with white at the top bend out of the page, as
though they’re bumps. The circles with white at the bottom look more like depressions or
even holes.

To see just how strong the sense of depth is, compare the shaded circles to the much
simpler diagram in
Figure 2-9
, also
following Kleffner and Ramachandran’s paper.

The only difference is that, instead of being shaded, the circles are divided into
solid black and white halves. Yet the depth completely disappears.

Shadows are identified early in visual processing in order to get a quick first
impression of the shape of a scene. We can tell it’s early because the mechanism it uses
to resolve light source ambiguities is rather hackish.

Ambiguities occur all the time. For instance, take one of the white-at-top circles
from
Figure 2-8
. Looking at it, you could
be seeing one of two shapes depending on whether you imagine the shape was lit from the
top or the bottom of the page. If light’s coming from above, you can deduce it’s a bump
because it’s black underneath where the shadows are. On the other hand, if the light’s
coming from the bottom of the page, only a dent produces the same shading pattern. Bump or
dent: two different shapes can make the same shadow pattern lit from opposite
angles.

Figure 2-8. Shaded figures give the illusion of three-dimensionality

There’s no light source in the diagram, though, and the flat gray background
gives no clues as to where the light might be coming from. That white-at-top circle
should, by rights, be ambiguous. You should sometimes see a bump and sometimes see a
dent.

What’s remarkable is that people see the white-at-top circles as bumps, not dents,
despite the two possibilities. Instead of leaving us in a state of confusion, the brain
has made a choice: light comes from above.
²

Assuming scenes are lit from above makes a lot of sense: if it’s light, it’s usually
because the sun is overhead. So why describe this as a hackish mechanism?

Although the light source assumption seems like a good one, it’s actually not very
robust. Try looking at
Figure 2-8
again.
This time, prop the book against a wall and turn your head upside-down. The bumps turn
into dents and the dents turn into bumps. Instead of assuming the light comes from high up
in the sky, your brain assumes it comes from the top of your visual field.

Figure 2-9. Binary black-and-white “shading” doesn’t provide a sense of depth

Rather than spend time figuring out which way up your head is and then deducing where
the sun is likely to be, your brain has opted for the “good enough” solution. This
solution works most, not all, of the time (not if you’re upside-down), but it also means
the light source can be hardcoded into shape perception routines, allowing rapid
processing of the scene.

It’s this rapidity that allows the deduction of shape from shadows to occur so early
in processing. That’s important for building a three-dimensional mental scene rather than
a flat image like a photograph. But the shaded circles have been falsely tagged as
three-dimensional, which gives them a compelling sense of depth.

What’s happened to the shaded circles is called “pop-out.” Pop-out means that
the circles jump out from the background at you — they’re easier to notice or give attention
to than similar flat objects. Kleffner and Ramachandran, in the same paper as before,
illustrate this special property by timing how long it takes to spot a single bump-like
circle in a whole page of dents. It turns out to not matter how many dents are on the page
hiding the bump. Due to pop-out, the bump is immediately seen.

If the page of bumps and one dent is turned on its side, however, spotting the dent
takes much longer. Look one more time at
Figure 2-8
, this time holding the book on its
side. The sense of depth is much reduced and, because the light-from-above assumption
favors neither type of circle, it’s pretty much random which type appears indented and
which appears bent out of the page. In fact, timings show that spotting the one different
circle is no longer immediate. It takes longer, the more circles there are on the
page.

The speed advantage for pop-out is so significant that some animals change their
coloring to avoid popping out in the eyes of their predators. Standing under a bright sun,
an antelope would be just like one of the shaded circles with a lit-up back and shadows
underneath. But the antelope is dark on top and has a white belly. Called
“countershading,” this pattern opposes the shadows and turns the animal an even shade,
weakening the pop-out effect and letting it fade into the background.

Given pop-out is so strong, it’s not surprising we often use the shading trick to
produce it in everyday life.

The 3D beveled button on the computer desktop is one such way. I’ve not seen any
experiments about this specifically, but I’d speculate that Susan Kare’s development of
the beveled button in Windows 3.0 (
http://www.kare.com/MakePortfolioPage.cgi?page=6
) is more significant than we’d otherwise assume for making more obvious what
to click.

My favorite examples of shade from shading are in Stuart Anstis’ lecture on the use of
this effect in the world of fashion (
http://psy.ucsd.edu/~sanstis/SAStocking.htm
). Anstis points out that jeans faded white along the front of the legs are
effectively artificially shadowing the sides of the legs, making them look rounder and
shapelier (
Figure 2-10
). The same is true of
stockings, which are darker on the sides whichever angle you see them from.

Among many examples, the high point of his presentation is how the apparent shape of
the face is changed with makeup — or in his words, “painted-on shadows.” The with and
without photographs (
Figure 2-11
)
demonstrate with well-defined cheekbones and a sculpted face just how compelling shape for
shading really is.

Figure 2-10. Shaded jeans add shape to legs