Mind Hacks™: Tips & Tools for Using Your Brain (33 page)

When it comes to visual processing in the brain, it’s all about job delegation. We’ve
got one pathway for consciously perceiving the world — recognizing what’s what — and another
for getting involved — using our bodies to interact with the world out there.

In the mid-’90s, Salvatore Aglioti
¹
and colleagues showed that when people are presented with the Ebbinghaus
illusion (see
Figure 6-6
) they find the
disk surrounded by smaller circles seems larger than an identically sized disk surrounded
by larger circles, and yet, when they reach for the central disks, they use the same,
appropriate, finger-thumb grip shape for both disks. The brain’s conscious perceptual
system (the ventral pathway) appears to have been tricked by the visual illusion, whereas
the brain’s visuomotor (handeye) system (the dorsal pathway) appears immune.

Figure 6-6. The Ebbinghaus Illusion. Both central circles are the same size; although they
don’t look it to your perceptual system, your visuomotor system isn’t fooled

There are many examples of situations in which our perception seems to be tricked
while our brain’s visuomotor system remains immune. Here’s one you can try. You’ll need a
friend and a tape measure. Find a sandy beach so you can draw in the sand or a tarmac area
where you can draw on the ground with chalk. Tell your friend to look away while you
prepare things.

Part 1

Draw a line in the sand, between 2 and 3 meters long. Now draw a disk at the end,
about 70 cm in diameter, as in
Figure 6-7
. Ask
your friend to stand so her toes are at the start of the line, with the disk at far end,
and get her to estimate how long the line is, using whichever units she’s happy with.
Then blindfold her, turn her 90°, and get her to pace out how long she thinks the line
is. Measure her “walked” estimate with your tape measure.

Figure 6-7. A draw-it-yourself visual illusion

How it works depends upon whom you ask. Advocates of the dual-stream theory of visual
processing argue that these demonstrations, of the immunity of our actions to visual
illusions, are evidence for the separateness of the dorsal (action) and ventral
(perception) streams. The ventral stream is susceptible, they argue, because it processes
objects relative to their surroundings, assessing the current context in order that we
might recognize things.
The dorsal stream, by contrast, is invulnerable to such illusions because it
processes objects of interest in egocentric coordinates, relative to the observer, so that
we might accurately interact with them.

Doubters of the dual-stream theory take a different view. One reason we are sometimes
duped by illusions, and sometimes not, they argue, is all to do with the type of task, far
less to do with there being separate processing pathways in our brain. For instance, when
we view the Ebbinghaus illusion (
Figure 6-6
), we are typically asked to compare the two central disks. Yet, when we reach for one
of the disks, we are focused on only one disk at a time. Perceptual tasks tend to involve
taking context and nearby objects into account, whereas motor tasks tend to involve
focusing on one object at a time and, by necessity, using egocentric coordinates to
interact accurately. When changing the task conditions reverses these tendencies, the
visuomotor system can be found to be susceptible to illusion or the perceptual system
invulnerable.

Which argument is right? Well, there’s evidence both ways and the debate will probably
roll on for some time yet.
^2
,
3
What is clear, is that this phenomenon
provides yet another example
[
The Broken Escalator Phenomenon: When Autopilot Takes Over
]
of how our illusory
sense of a unified self keeps all these conflicting processes conveniently out of
mind.

Does the world really appear as you’re seeing it? Who cares? Just sit back and enjoy
the view, accurate or not, while your neurons fight things out.

— Christian Jarrett

When we see objects, they automatically trigger the movements we’d make to use
them.

You may not believe me yet, but I’m sure you can think of a time when your movements
appeared to be automatically captured by something in your environment. Have you ever seen
a door handle with a “Push” sign clearly displayed above it, yet found yourself
automatically pulling the door toward you? The shape of the pullable handle suggests that
you should pull it, despite the contradictory instruction to push it. I go through such a
door several times a week and still find myself making that same mistake!

Try finding such a door near where you live or work. Sit down and watch how people
interact with it. What happens if you cover up the “Push” sign with a blank piece of
paper? Or cover it with a piece of paper labeled “Pull”; does this appear to affect how
often people pull rather than push, or is the shape of the handle all they’re really
paying attention to?

Perhaps you’ve found yourself picking up a cup or glass from the table in front of
you, even though you didn’t mean to (or even knowing that it belonged to someone
else)?

Effects of object affordances have been found in experiments: Tucker and Ellis
¹
asked subjects to press a button with their left or right hand, to indicate
whether a picture of an object was the right way up or inverted. Even though subjects were
not thinking about the action they would use for that object, it had an effect. If they
saw a cup with a handle pointing toward the right — evoking a right-hand grasp — they were
faster to react if their response also happened to require a right-hand response. That is,
the reaction time improved if the hand used for the button press coincided with the hand
that
would
be used for interacting with the object. This is called a
compatibility effect
. (The Simon Effect
[
Don’t Go There
]
shows that reaction times
improve when stimuli and response match in the more general case. What’s happening here is
that the stimulus includes not just what you perceive directly, but what affordances you
can perceive too.)

The graspability of objects can affect judgments, even when people are not
making any kind of movement. de’Sperati and Stucchi
²
asked people to judge which way a moving screwdriver was rotating on a
computer screen. People were slower to make a judgment if the handle were in a position
that would involve an awkward grasping movement with their dominant hand. That is,
although they had no intention to move, their own movement system was affecting their
perceptual judgment.

Brain imaging has helped us to understand what is happening when we see
action-relevant objects. Grèzes and Decety
³
looked at which brain areas are active when people do the Tucker and Ellis
judgment task. Bits of their brain become active, like the supplementary motor area and
the cerebellum, which are also involved in making real movements. In related research in
monkeys, cells have also been discovered that respond both when the monkey sees a
particular object and also when it observes the type of action that object would
require.

People with damage to their frontal lobes sometimes have problems suppressing the
tendency to act upon objects. They might automatically pick up a cup or a pair of glasses,
without actually wishing to do so (or even when they’re told not to). It is thought that
we all share these same tendencies, but with our intact frontal lobes, we are better at
stopping ourselves from acting them out. (Frontal patients can also have trouble
suppressing other impulses; for instance, some become compulsive gamblers.)

So, objects can produce movements within our mind, but just how do they do so? We
don’t know the answer to this yet. One possibility is that these effects happen
automatically, as Gibson suggested. Our system for visual perception has two routes
[
Trick Half Your Mind
]
: the ventral
(or “what?”) route, concerned with the identity of the object and the dorsal (“where?” or
“how?”) route, concerned with location and action. Affordances may act directly on the
dorsal stream, without relying on any higher processing; information about the type of
movement might be extracted directly from the shape or location of the object.

However, our knowledge about objects must play a role. We certainly couldn’t have
evolved to respond to everyday objects of today — prehistoric man didn’t live in a world
filled with door handles and coffee mugs! These automatic responses must be learned
through experience. Recently, Tucker and Ellis
⁴
found that merely seeing an object’s name was enough to speed reaction
times to produce the relevant size of grasp. Thus, our previous experience and knowledge
about acting upon objects become bound up
with the way that we represent each object in our brains. So, whenever you see
(or simply consider) an object, the possibility of what you might do with it is
automatically triggered in your mind.

— Ellen Poliakoff