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Hack 64. Mold Your Body Schema
Your body image is mutable within only a few minutes of judiciousand misleadingvisual 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. Proprioceptiongenerated by sensory receptors located in our joints and muscles that feed back information on muscle stretch and joint positionis 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. In Action 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. Figure 6-2. Matt confuses his body schema using a mirror and curtain rail (being in dire need of a haircut isn't essential for the experiment)
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 andwith identical movements using her other handto 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. How It Works 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 [Hack #63], 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 [Hack #8]) 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 [Hack #4] study by Donna Lloyd and colleagues3 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. End Notes 1. Ramachandran, V. S., & Blakeslee, S. (1998). Phantoms in the Brain: Human Nature and the Architecture of the Mind. London: Fourth Estate. 2. 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. 3. 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. See Also · 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 |
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Hack 65. Why Can't You Tickle Yourself?
 
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 homeyou'll need a friend, however, to play along.
6.5.1. Tickle Predicting
First, you can look at why there's a difference between being tickled by yourself and by someone else.
6.5.1.1 In action
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.
6.5.1.2 How it works
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.
Figure 6-3. Forward model: an internal predictor uses information about movements to distinguish between self-produced and externally produced sensations
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.
Neuroimaging studies using a tickling machine (Figure 6-4) at University College London1 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 [Hack #7] for more about these parts of the brain.)
Figure 6-4. Tickling machine: this device was used to apply a soft piece of foam to the participant's left palm
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 left 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 right hand. Movement of the participant's left 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 left hand and the touch sensation on the right palm, and participants were asked to rate the "tickliness" (Figure 6-5).
Figure 6-5. Tickling robots: participants found the stimulus more tickly as the time delay increased
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 left 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.
6.5.2. Force Prediction
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.
6.5.2.1 In action
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 thisreproducing the same force each timeand you may notice that after about 10 turns, the forces of your finger presses are getting stronger.
6.5.2.2 How it works
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 uninterestingsuch as the results of our own movementsin 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.
6.5.3. End Notes
1. Blakemore, S-J, Wolpert, D. M., & Frith, C. D. (1998). Central cancellation of self-produced tickle sensation. Nature Neuroscience, 1(7), 635-640.
2. 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.
3. 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.
6.5.4. See Also
· 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
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Hack 66. Trick Half Your Mind
When it comes to visual processing in the brain, it's all about job delegation. We've got one pathway for consciously perceiving the worldrecognizing what's whatand another for getting involvedusing our bodies to interact with the world out there.
The most basic aspects of the visual world are processed altogether at the back of your brain. After that, however, the same visual information is used for different purposes by two separate pathways. One pathway flows forward from the back of your brain to the inferior temporal cortex near your ears, where memories are stored about what things are. The other pathway flows forward and upward toward the crown of your head, to the posterior parietal cortex, where your mental models of the outside world reside. Crudely speaking, the first pathway (the "ventral" pathway) is for recognizing things and consciously perceiving them, whereas the second (the "dorsal" pathway) is for interacting with them. (Well, that's according to the dual-stream theory of visual processing [Hack #13] .)
The idea was developed by David Milner and Melvyn Goodale in the 1990s, inspired in part by observation of neurological patients with damage to one pathway but not the other. Patients with damage to the temporal lobe often have difficulty recognizing thingsa toothbrush, saybut when asked to interact with the brush they have no problems. In contrast, patients with damage to the parietal lobe show the opposite pattern; they often have no trouble recognizing an object but are unable to reach out and grasp it appropriately.
Since then, psychologists have found behavioral evidence for this separation of function in people without neurological problems, using visual illusions.
6.6.1. In Action
In the mid-'90s, Salvatore Aglioti1 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 (hand-eye) 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.
6.6.1.1 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 6-7A. 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
6.6.1.2 Part 2
Tell your friend to look away again, get rid of the first line, and draw another one of identical length. (You could use another length if you think your friend might suspect what's going onit just makes comparing estimates easier if you use the same length twice.) This time, draw the disk at the end so that it overlays the line, as in Figure 6-7B. Now do exactly as before: get your friend to stand with her toes at the line start and guess the length verbally from where she is, blindfold her, and ask her to walk the same length as she thinks the line is.
6.6.1.3 Part 3
You should find that your friend's spoken estimate of the second line is less than her estimate of the first, even though both lines were the same length. That's the visual illusion. (If you used different length lines, this difference will be in relative terms.) And yet her walked-out estimates should be pretty much the same (i.e., not tricked by the illusion), or at least you should find she underestimates the second line's length far less when walking. That is, her conscious judgment should be tricked more by this illusion (a version of a famous illusion called the Muller-Lyer illusion), than her walked-out estimate, controlled by her dorsal stream.
6.6.2. How It Works
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 [Hack #62] 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.
6.6.3. End Notes
1. Aglioti, S. et al. (1995). Size contrast illusions deceive the eye but not the hand. Current Biology, 5, 679-685.
2. Franz, V. H. (2001). Action does not resist visual illusions. Trends in Cognitive Sciences, 5, 457-459.
3. Milner, D., & Dyde, R. (2003). Why do some perceptual illusions affect visually guided action, when others don't? Trends in Cognitive Sciences, 7, 10-11.
Christian Jarrett
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Hack 67. Objects Ask to Be Used
When we see objects, they automatically trigger the movements we'd make to use them.
How do we understand and act upon objects around us? We might perceive the shape and colors of a cup of coffee, recognize what it is, and then decide that the most appropriate movement would be to lift it by the handle toward our mouth. However, there seems to be something rather more direct and automatic going on. In the 1960s, James Gibson developed the idea of object affordances. Objects appear to be associated with (or afford) a particular action or actions, and the mere sight of such an object is sufficient to trigger that movement in our mind. There are obvious advantages to such a system: it could allow us to respond quickly and appropriately to objects around us, without having to go to the bother of consciously recognizing (or thinking about) them. In other words, there is a direct link between perceiving an object and acting upon it. I don't just see my cup of coffee; it also demands to be picked up and drunk.
6.7.1. In Action
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 Ellis1 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 rightevoking a right-hand graspthey 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 [Hack #56] 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 Stucchi2 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.
6.7.2. How It Works
Brain imaging has helped us to understand what is happening when we see action-relevant objects. Grèzes and Decety3 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 [Hack #66] : 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 todayprehistoric 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 Ellis4 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.
One point to remember from this research is that objects will exert a constant "pull" on people to be used in the ways that they afford. Don't be surprised if people who are tired, in a hurry, or simply not paying attention (or who just have a lack of respect for how you wanted the object to be used) end up automatically responding to the actions the object offers. One practical example: if you don't want something to be used by accident (e.g., an ejector seat), don't have it triggered by the same action as something else that is used constantly without much thought (e.g., have it triggered by a twist switch, rather than by a button like the ignition).
T. S.
6.7.3. End Notes
1. Tucker, M., & Ellis, R. (1998). On the relationship between seen objects and components of potential actions. Journal of Experimental Psychology: Human Perception and Performance, 24, 830-846.
2. de'Sperati, C., & Stucchi, N. (1997). Recognizing the motion of a graspable object is guided by handedness. NeuroReport, 8, 2761-2765.
3. Grezes, J., & Decety, J. (2002). Does visual perception of object afford action? Evidence from a neuroimaging study. Neuropsychologia, 40, 212-222.
4. Tucker, M., & Ellis, R. (2004). Action priming by briefly presented objects. Acta Psychologica, 116, 185-203.
Ellen Poliakoff
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Date: 2015-12-11; view: 628
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