Mind Hacks™: Tips & Tools for Using Your Brain (32 page)
Read Mind Hacks™: Tips & Tools for Using Your Brain Online
Authors: Tom Stafford,Matt Webb
Tags: #COMPUTERS / Social Aspects / Human-Computer Interaction
Your body image is mutable within only a few minutes of judicious — and
misleading — visual feedback.
Our brains are constantly updated with information about the position of our bodies.
Rather than relying entirely on one form of sensory feedback, our bodies use both visual and
tactile feedback in concert to allow us to work out where our limbs are likely to be at any
one moment.
Proprioception —
generated by sensory receptors located in
our joints and muscles that feed back information on muscle stretch and joint position — is
another sense that is specifically concerned with body position.
The brain combines all this information to provide a unified impression of body position
and shape known as the
body schema
. Nevertheless, by supplying
conflicting sensory feedback during movement, we can confuse our body schema and break apart
the unified impression.
Find a mirror big enough so you can stand it on its edge, perpendicular to your body,
with the mirrored side facing left. Put your arms at your sides (you’ll probably need a
friend to hold the mirror). This whole setup is shown in
Figure 6-2
. Look sideways into the mirror so
you can see both your left hand and its reflection in the mirror, so that it appears at
first blush to be your hidden right hand. While keeping your wrists still and looking into
the mirror, waggle your fingers and move both your hands in synchrony for about 30
seconds. After 30 seconds, keep your left hand moving but stop your right. You should
sense a momentary feeling of “strangeness,” as if disconnected from your right hand. It
looks as if it is moving yet feels as if it has stopped.
of a haircut isn’t essential for the experiment)
One easy way of moving your hands together is to run a curtain rail under the
mirror, if you have one handy, and place each hand on a curtain ring (this is what I’m
doing in
Figure 6-2
). Move your hands
toward and away from the mirror for 30 seconds, until your brain has confused your right
hand and your reflected left hand in the mirror — then release the curtain ring from your
right hand. You can feel the ring has gone, but in the mirror it looks as though you’re
still holding it. To me, the disconnect felt like pins and needles, all through my right
hand.
Alternatively, you can manipulate your body schema into incorporating a table as part
of yourself.
1
Sit at a table with a friend at your side. Put one hand on your knee, out
of sight under the table. Your friend’s job is to tap, touch, and stroke your hidden hand
and — with identical movements using her other hand — to tap the top of the table directly
above. Do this for a couple of minutes. It helps if you concentrate on the table where
your friend is touching, and it’s important you don’t get hints of how your friend is
touching your hidden hand. The more irregular the pattern and the better synchronized the
movements on your hand and on the table, the greater the
chance this will work for you. About 50% of people begin to feel as if the
tapping sensation is arising from the table, where they can see the tapping happening
before their very eyes. If you’re lucky, the simultaneous touching and visual input have
led the table to be incorporated into your body image.
These techniques provide conflicting touch and visual feedback, making it difficult to
maintain a consistent impression of exactly where body parts are located in space. They’re
similar to the crossed hands illusion
[
Keep Hold of Yourself
]
, in which twisting your hands generates
visual feedback contradictory to your body schema. In the crossed hands illusion, this
leads to movement errors, and in the preceding techniques leads to the sense of being
momentarily disconnected from our own movements.
Some of our best information on the body schema has been from patients who have had
limbs amputated. More than 90% of amputees with reporting an experience of a “phantom
limb”: they still experience sensations (sometimes pain) from an amputated body part. This
suggests that the brain represents some aspects of body position and sensation as an
internal model that does not entirely depend on sensory feedback. Further evidence is
provided by a rare disorder called
autotopagnosia:
despite the
patients having intact limbs, brain injury (particularly to the left parietal lobe
[
Tour the Cortex and the Four Lobes
]
) causes a loss of spatial knowledge about the body so severe that they
are unable to even point to a body part when asked.
These disorders suggest that the brain’s system for representing body schema can
operate (and be damaged) independently from the sensory feedback provided by the body
itself. Sensory feedback must play a role of course, and it seems that it is used to
update and correct the model to keep it in check with reality. In some situations, like
the ones in the previous exercises, one type of sensory feedback can become out of sync
with the others, leading to the experience of mild confusion of the body schema.
Ramachandran and Rogers-Ramachandran applied an understanding of the relationship
between sensory feedback and the body schema to create a novel method to help people with
phantom-limb pain.
2
They used a mirror to allow people who were experiencing a phantom limb to
simulate visual experience of their amputated hand. In the same way as the earlier
exercise, the image of their amputated hand was simply a reflection of their remaining
hand, but this simulated feedback provided enough information to the brain so they felt as
if they could control and move their phantom limb. In some cases, they were able to “move”
their limb out of positions that had been causing them real pain.
An fMRI
[
Functional Magnetic Resonance Imaging: The State of the Art
]
study by Donna Lloyd
and colleagues
3
might explain why visual feedback of body position might have such a
dramatic effect. They scanned people while they were receiving tactile stimulation to the
right hand, either while they had their eyes closed or while they were looking directly at
their hand. When participants had the opportunity to view where they were being
stimulated, activation shifted dramatically, not only to the parietal area, known to be
involved in representing the body schema, but also to the premotor area, a part of the
brain involved in planning and executing movements. This may also explain why the earlier
exercises confuse our body schema enough to make accurate movement seem difficult or feel
unusual. Visual information from viewing our body seems to activate brain areas involved
in planning our next move.
- Ramachandran, V. S., & Blakeslee, S. (1998). Phantoms in the
Brain: Human Nature and the Architecture of the Mind. London: Fourth Estate. - Ramachandran, V. S., & Rogers-Ramachandran, D. (1996).
Synaesthesia in phantom limbs induced with mirrors.
Proceedings of the Royal
Society of London. Series B. Biological sciences, 263
(1369),
377–386. - Lloyd, D. M., Shore, D. I., Spence, C., & Calvert, G. A.
(2002). Multisensory representation of limb position in human premotor cortex.
Nature Neuroscience, 6
(1), 17–18.
- Tool use extends the body schema with its reach, altering the map the brain keeps
of our own body: Maravita, A., & Iriki, A. (2004). Tools for the body
(schema).
Trends in Cognitive Sciences, 8(
2), 79–86.
— Vaughan Bell
Experiments with tickling provide hints as to how the brain registers self-generated
and externally generated sensations.
Most of us can identify a ticklish area on our body that, when touched by someone else,
makes us laugh. Even chimpanzees, when tickled under their arms, respond with a sound
equivalent to laughter; rats, too, squeal with pleasure when tickled. Tickling is a curious
phenomenon, a sensation we surrender to almost like a reflex. Francis Bacon in 1677
commented that “[when tickled] men even in a grieved state of mind...cannot sometimes
forebear laughing.” It can generate both pleasure and pain: a person being
tickled might simultaneously laugh hysterically and writhe in agony. Indeed, in Roman times,
continuous tickling of the feet was used as a method of torture. Charles Darwin, however,
theorized that tickling is an important part of social and sexual bonding. He also noted
that for tickling to be effective in making us laugh, the person doing the tickling should
be someone we are familiar with, but that there should also be an element of
unpredictability.
As psychoanalyst Adam Phillips commented, tickling “cannot be reproduced in the absence
of another.” So, for tickling to induce its effect, there needs to be both a tickler and a
ticklee. Here are a couple of experiments to try in the privacy of your own home — you’ll need
a friend, however, to play along.
First, you can look at why there’s a difference between being tickled by yourself and
by someone else.
Try tickling yourself on the palm of your hand and notice how it feels. It might
feel a little ticklish. Now, ask a friend to tickle you in the same place and note the
difference. This time, it tickles much more.
When you experience a sensation or generate an action, how do you know whether it
was you or someone else who caused it? After all, there is no special signal from the
skin receptors to tell you that it was generated by you or by something in the
environment. The sensors in your arm cannot tell who’s stimulating them. The brain
solves this problem using a prediction system called a
forward
model
. The brain’s motor system makes predictions about the consequences of
a movement and uses the predictions to label sensations as self-produced or externally
produced.
Every time an action is made, the brain generates an
efference
copy
of the actual motor command in parallel. The efference copy is just
like a carbon copy, or duplicate, of the real motor command and is used to make a
prediction about the effect of the action, for example, the tickling effect of a finger
stroke. The predicted sensory effect of the efference copy and the actual sensory effect
of the motor command are compared (
Figure 6-3
). If there is a mismatch, the
sensation is labeled as externally generated.
Your accurate prediction of the consequences of the self-tickle reduces the sensory
effects (the tickliness) of the action, but this does not happen when someone else
tickles you. This explains why the sensation is usually more
intense when another person touches your arm compared with when you touch
your own arm.
distinguish between self-produced and externally produced sensations
Neuroimaging studies using a tickling machine (
Figure 6-4
) at University College London
1
suggest that the distinction between self and other is hardwired in the
brain. This device was used to apply a soft piece of foam to the participant’s left
palm. In one condition, the participant self-produced the touch stimulus with his right
hand, and in the other condition, the experimenter produced the stimulus. The
participant’s brain was scanned during the experiment to investigate the brain basis of
self-produced versus externally produced touch. Results show stronger activation of the
somatosensory cortex
and
anterior cingulate
,
parts of the brain involved in processing touch and pleasure, respectively, when a
person is tickled by someone else, compared with when they tickle themselves. The
cerebellum
, a part of the brain that is generally associated with
movement, also responds differently to self-produced and externally produced touch, and
it may have a role in predicting the sensory consequences of self-touch but not external
touch. (See
Get Acquainted with the Central Nervous System
for more about
these parts of the brain.)
One study used two robots to trick the brain into reacting to a self-tickle as if it
were an external tickle.
2
In the right hand, participants held an object attached to the first
robot. This was connected to a second robot, attached to which was a piece of foam that
delivered a touch stimulus to the palm of the left hand. Movement of the participant’s
right hand therefore caused movement of the foam, as if by remote control. The robotic
interface was used to introduce time delays between the movement of the participant’s
right hand and the touch sensation on the left palm, and participants were asked to rate
the “tickliness” (
Figure 6-5
).
When there was no time delay, the condition was equivalent to a self-produced tickle
because the participant determined the instant delivery of the touch stimulus by
movements of the right hand. Greater delay between the causal action and the sensory
effect (up to 300 ms) meant participants experienced the touch as more tickly. This
suggests that, when there is no time delay, the brain can accurately predict the touch
stimulus so that the sensory effect is attenuated. Introducing a time delay increases
the likelihood of a discrepancy between the predicted and actual sensory effect. As a
result, there is less attenuation of the tickly sensation, which tricks the brain into
labeling the stimulus as external. By making the consequences of our own action
unpredictable, therefore, the brain treats the self as another.
participant’s left palm
increased
You can see how we anticipate a stimulus and compensate for it, by attempting
to estimate a force and seeing whether you can get that right.
Use your right index finger to press down gently on the back of a friend’s hand.
Your friend should then use her right index finger to press down on the same spot on
your hand with the same force that she felt from your finger press. Continue taking
turns at this — reproducing the same force each time — and you may notice that after about
10 turns, the forces of your finger presses are getting stronger.
This predictive process may also be at the root of why physical fights tend to
escalate. Notice how tit-for-tat tussles between children (or indeed brawls between
adults) intensify, with each person claiming that the other hit him harder. In a recent study,
3
a motor was used to apply a brief force to the tip of each participant’s
left index finger. Participants were then asked to match the force they felt using their
right index finger to push down on their left index finger through a force
transducer.
Results showed that participants consistently applied a stronger force than that
which was applied to them. The authors suggest that, just as when we try to tickle
ourselves, the brain predicts the sensory consequences of the self-generated force and
then reduces the sensation. We can only predict the outcome of our own actions and not
of someone else’s, so an externally generated force feels more intense. As a result, if
you were to deliver a vengeful punch to match the force of your opponent’s blow, it is
likely that you would overestimate the strength of the opponent’s punch and strike back
harder.
Why have we evolved the inability to tickle ourselves? The force generation
experiment shows that sensations that are externally caused are enhanced. Similarly, our
reactions to tickling may have evolved to heighten our sensitivity to external stimuli
that pose a threat. Our sensory systems are constantly bombarded with sensory
stimulation from the environment. It is therefore important to filter out sensory
stimulation that is uninteresting — such as the results of our own movements — in order to
pick out, and attend to, sensory information that carries more evolutionary importance,
such as someone touching us. When a bee lands on your shoulder or a spider climbs up
your leg, the brain ensures that you attend to these potentially dangerous external
stimuli by ignoring feelings from your own movements. The predictive system therefore
protects us and tickling may just be an accidental consequence.
- Blakemore, S-J, Wolpert, D. M., & Frith, C. D. (1998). Central
cancellation of self-produced tickle sensation.
Nature Neuroscience,
1
(7), 635–640. - Blakemore, S-J, Frith, C. D., & Wolpert, D. W. (1999).
Spatiotemporal prediction modulates the perception of self-produced stimuli.
Journal of Cognitive Neuroscience, 11
(5), 551–559. - Shergill, S., Bays, P. M., Frith, C. D., & Wolpert, D. M.
(2003). Two eyes for an eye: The neuroscience of force escalation.
Science,
301
(5630), 187.
- Weiskrantz, L., Elliot, J., & Darlington, C. (1971). Preliminary
observations of tickling oneself.
Nature, 230
(5296),
598–599. - Wolpert, D. M., Miall, C. M., & Kawato, M. (1998). Internal models in the
cerebellum.
Trends in Cognitive Sciences, 2
(9), 338–347.
— Suparna Choudhury and Sarah-Jayne Blakemore