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Figure 7-1. Each card has a letter on one side and a number on the reverse
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Many people turn over A and 2but that's not quite right. While turning over A will tell you whether "one side" of the rule is true (if vowel, then even number), turning over 2 won't tell you any more. It doesn't matter whether 2 has a K or an A on its reversethe rules doesn't specify either being true. Along with A, the other card you need to turn over is 7. If 7 has an A on its reverse, then the rule is disproved no matter what the A has on its reverse. You need to turn over A and 7.
Very few people solve this riddle on the first try. It shows that humans do not possess an innate set of abstract logic rules. Yet somehow we manage to get by without those rules. Try this similar puzzle, in Figure 7-2.
Figure 7-2. Four people sit at a bar drinking beer or cola, the cards show age on one side and beverage on the otherwho's breaking the rules?
Say there's a rule that you must be over 21 to drink beer. Whose drinks and ages would you need to check to see if this bar is flouting the rules?
By simply swapping drinks and ages for cards A, K, 2, and 7, it's obvious this time around that there's no point checking what the 21 year old (think 2 card) is drinkingit wouldn't make any difference to the rule if she were drinking cola or beer, whereas the 16 year old's (think 7 card) drink is of much more interest.
How It Works
Why are logic problems so much easier when they're expressed as real-life situations rather than in abstract terms? One early hypothesis called memory cuing proposed that we solve logic problems by drawing on personal experience, without using any deductive reasoning. We've all experienced the problems of drinking ages enough times that we don't even have to think about who should be drinking what, unlike playing with letter and number cards.
Despite the substantial evidence behind memory cuing,2,3 many scientists believe that in practice we use more than just experiencethat there is in fact some thinking involved. Instead, researchers such as Cheng and Holyoak4 think that, while we might not be so good at pure logic, we're excellent at the logic we need in real liferules, permissions, and obligations. This type of logicdeontic logicis what helps us solve everyday logic problems, by developing what they call "pragmatic reasoning schemas." Therefore, it shouldn't be surprising that our ability with logic is domain-specific, that is, limited to analyzing the complex web of permissions and obligations we encounter in life.
It's been suggested by Cosmides,5 a leading light of evolutionary psychology (the study of how evolution may have shaped the way we think6), that the reason we seem to possess domain-specific logic is because it's been selected for by evolution over countless generations. Cosmides argues that the really important parts of Cheng and Holyoak's pragmatic reasoning schemas are those about people. In other words, we are all born with the mental logic required to understand the costs, benefits, and social contracts involved in dealing with other people. It's a compelling argument, since the ability to make beneficial deals is a valuable survival trait. However, Cosmides' theory can't be the whole story, since we have no problem in solving many logic problems that have nothing to do with costs, benefits, or indeed other people at all. For example, the rule "If you're going to clean up spilt blood, then you need to wear rubber gloves" is easily understood and applied even though it doesn't concern other people.
Before resigning yourself to a life without logic, it's worth remembering that along with the countless other skills that we aren't born with, we can understand logic the hard wayby learning it. Even if you don't, you can still console yourself with the knowledge that you're as good as any philosopher in the everyday logic that really matters.
End Notes
1. Syllogisms: some C are B, no A is B, therefore some C are not A. Deductive: reasoning in which the conclusion necessarily follows from true premises (e.g., if X, then Y). Inductive: the sort of reasoning that Sherlock Holmes might use, in which he draws a conclusion (which might be wrong) based on possibly incomplete or irrelevant information.
2. Johnson-Laird, P. N., Legrenzi, P., & Sonino-Legrenzi, M. (1972). Reasoning and a sense of reality. British Journal of Psychology, 63, 395-400.
3. Manktelow, R. I., & Evans, J. St. B. T. (1979). Facilitation of reasoning by realism: Effect or non-effect? British Journal of Psychology, 70, 477-488.
4. Cheng, P. W., & Holyoak, K. J. (1985). Pragmatic reasoning schemas. Cognitive Psychology, 17, 391-416.
5. Cosmides, L. (1989). The logic of social exchange: Has natural selection shaped how humans reason? Studies with the Wason Selection task. Cognition, 31, 187-276.
6. Evolutionary psychology is the study of how evolution may have shaped the way we think, and often controversial. "Evolutionary Psychology: a Primer" (http://www.psych.ucsb.edu/research/cep/primer.html), by Leda Cosmides and John Tooby, provides an introduction.
Adrian Hon
Many of the unpleasant phenomena associated with injury and infection are in fact produced by the brain to protect the body. Medical assistance shifts the burden of protection from self to other, which allows the brain to reduce its self-imposed unpleasantness.
Injury or infection triggers a coordinated suite of physiological responses involving the brain, hormones, and immune system. The brain generates pain and fever, stress hormones mobilize energy from fat, and immune cells cause local swelling and redness. These processes are collectively known as the acute phase response because they occur rapidly and tend to subside after a few days. Medical assistance can help these unpleasant signs and symptoms to subside more quickly, even when that assistance is completely bogussuch as a witch doctor waving a rattle at you or a quack prescribing a sugar pill. This is known as the placebo effect.
7.5.1. In Action
It's hard to invent a placebo and try it on yourself, because the effect relies crucially on the sincerely held belief that it will work. Several experiments have shown that pure placebos such as fake ultrasound produce no pain relief when they are self-administered. So unless you can fool yourself that other people are caring for you when they are not, your experiments with placebos will have to involve other people.
Moreover, you will also probably have to lie. The placebo effect depends not just on other people, but also on the belief that those people are providing bona fide medical assistance. If you don't believe that the assistance provided by those around you is going to help you recover, you won't experience a placebo effect.
Sometimes a placebo effect seems to be triggered despite the absence of other people and the absence of deception. If you have ever felt better after taking a homeopathic remedy, for example, or after applying dock leaves to the pain caused by a stinging nettle, that was almost certainly a placebo effect, because it has been scientifically proven that such treatments are completely bogus. The essential factor, however, must still be presenta belief that this kind of treatment will help. Once you discover the truth about such bogus treatments, therefore, they cease to be capable of producing placebo effects.
Because it is hard (some might say impossible) to deceive yourself into believing something that you know to be false, deception is important for most placebo experiments. This plays a central role in many psychological experiments, and raises serious ethical problems. In universities and other research environments, an ethics committee must, quite rightly, approve experiments before they are allowed to proceed. It is therefore advisable to conduct the following experiment in the privacy of your own home, where ethics committees have no jurisdiction.
First, take an old medicine bottle and clean it thoroughly. Then fill it with a solution of tap water, sugar, and food coloring. The next time someone you know gets a headache or is stung by a stinging nettle, tell her that you have a special remedy that will help. If she asks what it is, tell her that it is a special solution of water and sugar and food coloring, and say that you have read somewhere (in this book) that this will help her feel better (that way, you won't even be lying!). Give her the colored water and ask her to drink a teaspoonful (if she has a headache) or to rub a small amount onto the affected area (if she has been stung by a nettle). See if it helps her feel better.
It will, if she believes it willand if there's nothing really wrong with her (be careful here; don't delay medical treatment for someone who is hurt because you want to see if you can placebo-cure her).
Studies have shown that for some people in some situations the placebo effect can be as strong as morphine. In one particularly striking study,1 patients who had undergone tooth extraction were treated with ultrasound to investigate whether this would reduce the postoperative pain. Unknown to both doctors and patients, however, the experimenters had fiddled with the machine, and half the patients never received the ultrasound. Since ultrasound consists of sound waves of very high frequencyso high, in fact, that they are inaudible to the human earthere was no way for either the doctors or the patients to tell whether the machine was emitting sound waves; the test was truly double-blind. After their jaws were massaged with the ultrasound applicator, the patients were asked to indicate their level of pain on a line with one end labeled "no pain" and the other "unbearable pain." Compared with a group of patients who were untreated, all those treated with the ultrasound machine reported a significant reduction in pain. Surprisingly, however, it didn't seem to matter whether the machine had been switched on or not. Those who had been massaged with the machine while it was turned off showed the same level of pain reduction as those who had received the proper treatment. In fact, when the ultrasound machine was turned up high, it actually gave less pain relief than when it was switched off.
Other studies have shown that placebo medicines are more effective if delivered in person by doctors and that it helps more if the doctors are wearing white coats. Red pills give a bigger placebo effect than white pills, and placebo injections are more powerful still.
7.5.2. How It Works
Nobody knows for sure yet how the placebo effect works, but one theory is that the brain is very sensitive to the presence of social support during the process of recovery from injury and infection. The various components of the acute phase response are all designed to promote recovery and prevent further injury while recovery is taking place. Pain, for example, makes you guard the wounded area. But these measures also have costs; high levels of pain, for example, can actually lengthen the healing process. The brain makes a trade-off between the risks of further damage to the injured area and the delay to the healing process. The presence of social support during recovery shifts the balance between these competing risks because some of the burden of preventing further damage is transferred from the sick person to those around them. The sick person can therefore reduce his own costly self-protective measures, such as pain, and allow the healing process to progress more rapidly.
Another suggestion is that the placebo effect works by means of conditioning (see also [Hack #92] ). Conditioning is a very general kind of learning process in which one stimulus is substituted for another. The classic example is Pavlov's dogs, which learned to salivate on hearing a bell after Pavlov had trained them to associate the sound of the bell with the arrival of food. In technical terms, an unconditioned stimulus (the sight of the meat), which leads naturally to a certain unconditioned response (salivating at the sight of the meat), is repeatedly paired with a conditioned stimulus (the sound of the bell). Eventually, the dogs learn the conditioned response of salivating at the sound of the bell. Pavlov's students showed that immune responses can also be conditioned, and others have gone on to suggest that this is what lies behind the placebo response. The unconditioned stimulus is a real drug or some other medical treatment that works even if you have never tried it before and don't believe in it. The unconditioned response is the improvement you feel after receiving the treatment. The conditioned stimuli are all the things that are repeatedly paired with the treatmentthe size, shape, and color of the pill, for example. If you then take a pill that has the same size, shape, and color as the real one, but which lacks the active ingredient, you may still experience some improvement because your immune system has been conditioned to respond to such stimuli.
Placebos won't cure the vast majority of medical conditions. It is much easier and quicker to list the things that placebos can influencepain, swelling, stomach ulcers, some skin conditions, low mood, and anxietythan the things they don't. Everything else is probably not placebo-responsive. That said, placebos are able to help in the management of nearly all illnesses because nearly all illnesses involve pain, low mood, and/or anxiety.
7.5.3. End Note
1. Hashish I., Harvey, W., & Harris, M. (1986). Anti-inflammatory effects of ultrasound therapy: Evidence for a major placebo effect. British Journal of Rheumatology 25, 77-81.
7.5.4. See Also
· Evans, D. (2003). Placebo: Mind over Matter in Modern Medicine. London: HarperCollins.
Dylan Evans
People don't like change. If you really want people to try something new, you should just coerce them into giving it a go and chuck the idea of persuading them straight off.
By default, people side with what already is and what happened last time. We're curious, as animals go, but even humans are innately conservative. Like the Dice Man, who delegates all decisions to chance in Luke Rhinehart's classic 1970s novel of the same name, was told: "It's the way a man chooses to limit himself that determines his character. A man without habits, consistency, redundancyand hence boredomis not human. He's insane."1
In this hack we're going to look at our preference for the way things are and where this tendency comes from. I'm not claiming that people don't changeobviously this happens all the time and is the most interesting part of lifebut, in general, people are consistent and tend toward consistency. Statistically, if you want to predict what people will do in a familiar situation, the most useful thing you can measure is what they did last time. Past action correlates more strongly with their behavior than every other variable psychologists have tried to measure.2 If you're interested in predicting who people will vote for, what they will buy, what kind of person they will sleep with, anything at all really, finding out what tendencies they've exhibited or what habits they've formed before is the most useful information at your disposal. You're not after what they say they will donot what party, brand, or sexual allegiance they tick on a formnor the choice they think they're feeling pressured into making. Check out what they actually did last time and base your prediction on that. You won't always be right, but you will be right more often by basing your guess upon habit than upon any other single variable.
This bias is the result of a number of factors, not least the fact that people's previous choice is often the best one or the one that best reflects their character. But also we have mental biases,3 like the mental biases we have about numbers [Hack #70], which produce consistent habits and an innate conservativism.
Biases in reasoning are tendencies, not absolutes. They make up the mental forces that push your conclusions one way or the other. No single force ever rules completely, and in each case, several forces compete. We're mostly trying to be rational so we keep a look out for things that might have biased us so we can discount them. Even if we know we can't be rational, we mostly try to be at least consistent. This means that often you can't give the same person the same problem twice if it's designed to evoke different biases. They'll spot the similarity between the two presentations and know their answers should be the same.
I'm carelessly using the word "rational" here, in the same way that logicians and people with a faith in pure reason might. But the study of heuristics and biases should make us question what a psychological meaning of "rational" could be. In some of the very arbitrary situations contrived by psychologists, people can appear to be irrational, but often their behavior would be completely reasonable in most situations, and even rational considering the kind of uncertainties that normally accompany most choices in the everyday world.
T.S.
But some biases are so strong that you can feel them tugging on your reason even when the rational part of your mind knows they are misleading. These "cognitive illusions" work even when you present two differently biased versions of the choice side by side. The example we're going to see in action is one of these.
7.6.1. In Action
I'm going to tell you in advance that the two versions of the problem are logically identical, but I knowbecause your brain evolved in the same way mine didthat you'll feel as if you want to answer them differently despite knowing this. If your supreme powers of reason don't let you feel the tug induced by the superficial features of the problem (the bit that conveys the bias), take the two versions and present them to two different friends.
Here we go . . .
7.6.1.1 Version 1
A lethal disease is spreading through the city of which you are mayor. It is expected to kill 600 people. Your chief medical adviser tells you that there is a choice between two treatment plans. The first strategy will definitely save 200 people, whereas the second strategy has a one-third chance of saving 600 people and a two-thirds chance of saving no one. Which strategy do you choose?
7.6.1.2 Version 2
A lethal disease is spreading through the city of which you are mayor. It is expected to kill 600 people. Your chief medical adviser tells you that there is a choice between two treatment plans. The first strategy will definitely kill 400 people, whereas the second strategy has a one-third chance that nobody will die and a two-thirds chance that 600 people will die.
Do you feel it? The choices feel different, even though you know they are the same. What's going on?
7.6.2. How It Works
At least two things are going on here. The first is the effect of the way the choice is presentedthe framing effect. Anyone who has ever tried to persuade someone of something knows the importance of this. It's not just what you say, but how you say it, that is important when presenting people with a choice or argument. The second thing is a bias we have against risking an already satisfactory situationwe're much more willing to take risks when we're in a losing position to begin with. In the examples, the first frame makes it look like you stand to gain without having to take a riskthe choice is between definitely saving 200 people versus an all-or-nothing gamble. The second frame makes it appear as though you start in a losing position (400 people down) and you can risk the all-or-nothing gamble to potentially improve your standing. In experimental studies of this dilemma, around 75% of people favor not gambling in the first frame, with the situation reversed in the second.4
So why do we gamble when we think we might lose out, but have a bias to avoid gambling on gains? I'm going to argue that this is part of a general bias we have toward the way things are. Let's call it the "status quo bias." This is probably built into our minds by evolutionnature's way of saying "If it ain't broke, don't fix it."
With habits, it is easy to see why the status quo bias is evolutionary adaptive. If you did it last time and it didn't kill you, why do it differently? Sure, you could try things differently, but why waste the effort, especially if there's any risk at all of things getting worse?
7.6.3. In Real Life
There's a way to hack this habit bias, and it's well-known to advertisers. If people generally stick with what they know, the most important thing you can do is get them to start off with your product in the first place (hence the value of kids as a target market). But you can make use of the bias: people choose based on what they did before, so it is more effective to advertise to influence what they choose rather than how they feel about that choice. Even if there's no good reason for someone using your product in the first place, the fact that they did once has established a strong bias for them doing so again. A computer user may prefer one browser, but if another one comes bundled with her new operating system, we can bet that's what she'll end up relying on. You may have no rational reason for choosing Brand A over Brand B when you buy jam, but if the manufacturers of Brand B can get you to try it (maybe by giving you a free sample or a special offer), they've overcome the major barrier that would have stopped you from buying it next time.
Status quo bias works for beliefs as well as behaviors. In many situations we are drawn to confirm what we already know, rather than test it in a way that might expose it to be false [Hack #72] .
It's an experience I've had a lot when debugging code. I do lots of things that prove to me that it must be the bug I first think it is, but when I fix that bug, my code still doesn't work.
It's not just me, right?
T.S.
Another manifestation of our preference for the way things are is the so-called endowment effect,5 whereby once we have something, however we acquired it, we give it more value than we would give up to obtain it. In one study, students were given a mug with their university emblem, worth $6. In a trading game they subsequently wanted an average of around $5 to give up their mug, whereas students without mugs were willing to offer an average of only around $2 to buy a mug. The mere sense of ownership that came with being given the mug was enough to create a difference between how the two groups valued the object. This is just one of the ways in which human behavior violates the rationality supposed by classical economic theory.
So we can see that if you want people to give something up, you shouldn't give it to them in the first place, and if you want to introduce something new, you should make people try it before trying to persuade them to accept it. If you can't do this, you should at least try and introduce the new change elements as part of the familiar experience.
7.6.4. End Notes
1. Rhinehart, L. (1971). The Dice Man.
2. Ajzen, I. (2002). Residual effects of past on later behavior: Habituation and reasoned action perspectives. Personality and Social Psychology Review, 6, 107-122. See also: Ouellette, J. A., & Wood, W. (1998). Habit and intention in everyday life: The multiple processes by which past behavior predicts future behavior. Psychological Bulletin, 124, 54-74.
3. The Wikipedia has an enjoyable, if unstructured, list of cognitive biases (http://en.wikipedia.org/wiki/List_of_cognitive_biases). A good introduction to cognitive biases and heuristics is Nicholls, N. (1999). Cognitive illusions, heuristics, and climate prediction. Bulletin of the American Meteorological Society, 80(7), 1385-1397 (http://ams.allenpress.com/pdfserv/i1520-0477-080-07-1385.pdf).
4. Tversky, A., & Kahneman, D. (1981). The framing of decisions and the psychology of choice. Science, 211, 453-458.
5. Kahneman, D., Knetch, J. L., & Thaler, R. H. (1991). Anomalies: The endowment effect, loss aversion, and status quo bias. Journal of Economic Perspectives, 5(1), 193-206. A reverse of the endowment effect is the windfall effect in which people value less highly money they didn't expect to come to them (like lottery wins and inheritance).
We group our visual perceptions together according to the gestalt grouping principles. Knowing these can help your visual information design to sit well with people's expectations.
It's a given that we see the world not as isolated parts, but as groups and single objects. Instead of seeing fingers and a palm, we see a hand. We see a wall as a unit rather than seeing the individual bricks. We naturally group things together, trying to make a coherent picture out of all the individual parts. A few fundamental grouping principles can be used to do most of the work, and knowing them will help you design well-organized, visual information yourself.
8.2.1. In Action
Automatic grouping is such second nature that we really notice only its absence. When the arrangement of parts doesn't sit well with the grouping principles the brain uses, cracks can be seen. Figure 8-1 shows some of these organizational rules coming into play.1
Figure 8-1. Two groups of triangles that point different ways and a middle triangle that can appear to point either way, depending on which group you see it being part of 2
You don't see 17 triangles. Instead, you see two groups of eight and one triangle in the middle. Your similarity drive has formed the arrangement into rows and columns of the shapes and put them into two groups: one group points to the bottom left, the other points off to the right.
Each group belongs together partly because the triangles are arranged into a pattern (two long rows pointing in a direction) and partly because of proximity (shapes that are closer together are more likely to form a group). The triangle in the middle is a long way from both groups and doesn't fall into the same pattern as either. It's left alone by the brain's grouping principles.
You can, however, voluntarily group the lone triangle. By mentally putting it with the left-hand set, it appears to point down and left along with the other triangles. You can make it point right by choosing to see it with the other set.
8.2.2. How It Works
The rules by which the brain groups similar objects together are called gestalt grouping principles in psychology. Although there's no direct German-to-English translation, "gestalt" means (roughly) "whole." When we understand objects and the relationships between them in a single, coherent pattern rather than as disconnected items, we understand the group as a gestalt. We have a gestalt comprehension of each of the sets of triangles in Figure 8-1, for instance.
Four of the most commonly quoted grouping principles are proximity, similarity, closure, and continuation. An example of each is shown in Figure 8-2.
Figure 8-2. The four most quoted gestalt grouping principles
Proximity
We preconsciously group items that are close together, so in the picture you see columns rather than rows or a grid. This principle is the cause of the triangles in the original diagram coming together into two sets and the reason the lone triangle didn't feel part of either of them.
Similarity
We prefer to group together objects of the same kind. In the example, you see alternating rows of circles and squares rather than columns of mixed shapes.
Closure
There's a tendency to complete patterns. There's no triangle in the example pattern, but we see one because the arrangement of the three Pac Man shapes would be completed if one were there.
Continuation
Just as we like to see completed patterns, we like seeing shapes that continue along the same path, smoothly. We see two lines crossing in the example, rather than two arrowheads touching at their points or four lines meeting together.
When none of these principles apply, it's still possible to mentally group items together. When you put the middle triangle in Figure 8-1 with one group or the other, it picks up the orientation of the group as a whole. It's a voluntary grouping that modifies how you see.
Gestalt principles exist in visual processing not because they are always right, but because on average, they are useful. They're good rules of thumb for making sense of the world. It's not that similar things can't be separate; it's more that most of the time they aren't. Although random coincidences can happen, they are vastly outnumbered by meaningful coincidences.
The world isn't a mess of disconnected parts, and it's useful to see the connectionsif you're hunting an animal, it makes sense to see it as a single gestalt rather than a paw here and a tail there.
8.2.3. End Notes
1. The gestalt grouping principles are interesting, but are they really useful? Good print or web page design involves easy comprehension, and knowing how the principles can conflict or mislead in your layout helps along the way. James Levin has applied the gestalt grouping principles to web design (http://tepserver.ucsd.edu:16080/~jlevin/gp).
2. Illustration inspired by Fred Attneave's demonstrations as used in "How the Mind Works" by Steven Pinker.
8.2.4. See Also
· Max Wertheimer was the first to identify the principles in his 1923 paper "Laws of Organization in Perceptual Forms" (http://psy.ed.asu.edu/~classics/Wertheimer/Forms/forms.htm).
· As well as the basic grouping principles, which work looking at static objects, others deduce grouping from behavior over time [Hack #76] .
We tend to group together things that happen at the same time or move in the same way. It's poor logic but a great hack for spotting patterns.
It's a confusing, noisy world out there. It's easier to understand the world if we perceive a set of objects rather than just a raw mass of sensations, and one way to do this is to group together perceptions that appear to have the same cause. The underlying assumptions involved manifest as the gestalt grouping principles, a set of heuristics used by the brain to lump things together (see [Hack #75] for the simplest of these, used for vision).
Perhaps the most powerful of these assumptions is termed common fate. We group together events that occur at the same time, change in the same way, or move in the same direction. Imagine if you saw, from far off, two points of light that looked a bit like eyes in the dark. You might think they were eyes or you could just put it down to a coincidence of two unrelated lights. But if the points of light moved at the same time, in the same direction, bounced with the characteristic bounce of a person walking, you'd know they were eyes. Using behavior over time allows you to stringently test spatial data for possible common cause. If the bouncing lights pass the common fate test, they're almost certainly a single object. Visual system tags this certainty by providing you with a correspondingly strong perceptual experience; if some things move together, it is almost impossible to see them as separate items instead of a coherent whole.
8.3.1. In Action
"IllusionMotion CaptureGrouping" (http://psy.ucsd.edu/chip/illu_mot_capt_grpng.html; a Real video requiring Real Player) demonstrates just how completely your perception of a single item is altered by global context and common fate. Watch the video for at least 30 seconds. At first you see just a dot blinking on and off next to a square. But then other dots are added in the surrounding area, and as the first dot blinks off, they all shift right. Now your unavoidable impression is of the first dot moving behind the square. The appearance of the other dots, and their behavior, gives your visual system correlations that are just too strong to ignore. The single dot is still blinking on and offyou just can't see it like that any more.
"A Time Gestalt Principle Example: Common Fate" (http://tepserver.ucsd.edu/~jlevin/gp/time-example-common-fate; a Java applet),1 shown in Figure 8-3, is an interactive demonstration of how your visual system deduces the shape of objects from movement, without any color or shading clues to help out.
Figure 8-3. When the circle hidden in the pattern is moving, it's clearly seen; printed like this, it's invisible
You see a shape with a static-like texture moving across a similarly randomized background. Click anywhere in the image to start and stop the demo. Frozen, there is no pattern to see; you see just a random mess. This is the real force of common fate. The correlations exist only across time, in movementit's only when the demo is moving that you can see an object among the noise.
8.3.2. How It Works
The gestalt grouping inferences are so preconscious and automatic that it's hard to imagine perceiving a world that the brain hasn't organized into objects. There's something very clever going on here; we are taking in very little information (only how the pattern changes over time), yet, in combination with an assumption that accidental correlations of visual patterns are unlikely, we construct a compelling perception of an object. In these demos, you just can't ignore the object. You are utterly unable to make yourself see a moving collection of dots instead of the shape in motion because the construction of the object is happening before the level of consciousness.
Common fate can lead to some sophisticated inferences. "Kinetic Depth" (http://www.biols.susx.ac.uk/home/George_Mather/Motion/KDE.HTML; a QuickTime video), just from a collection of moving lights, allows you to see an object with three-dimensional depth moving in a particular way. In this case, the pattern of dots causes you to see a sphere rotating on its axis.
What's really cute about this video is that there's an ambiguity in the visual informationyou can see the sphere rotating in one of two ways. Your visual system makes a choice for you, and you see some of the dots moving behind some of the others, which move in the opposite direction. The set you see as "in front" determines the direction in which you see the sphere rotating. If you watch for a while, your perception will switch and you'll see it reverse. You don't need to make any effort to for this to happen; it occurs naturally, probably due to some kind of adaptation process. Since you see the sphere rotating in one particular direction, the neurons that represent that perception will be active. Over time, they actively tune down their response, and the neurons that code for the other apparent rotation can now dominate. This kind of gain control [Hack #26] plays a similar role in motion aftereffects [Hack #25], in which neurons that are active for particular directions of movement down-regulate after being consistently stimulated and neurons active for the opposing direction take over and dominate our perception when the consistent moving stimulus is removed.
All these demonstrations show just how effective correlations over time are in molding our perception. And not just perceptionsynchronizing stimuli can actually alter your body image, where your brain believes your hands are [Hack #64], for instance. The heart of the thing is similarif two things correlate exactly, our perception treats them as part of the same object. For our brains, isolated inside the skull, perceived correlation is the only way we've ever had for deducing what sensations should be associated together as part of the same object.
Common fate can also draw inferences from points of light moving in much more complex ways than rotating spheres. For the case of biological motion [Hack #77], the visual system is specifically prepared to fit moving points into a schema based upon the human body to help perception of the human form. Alais et al. have suggested that the importance of common fate reflects a deeper principle of the brain's organization.2 Neuroscientists talk about the binding problem, the question of how the brain correctly connects together all the information it is dealing with: all the things that are happening in different parts of the world, detected by different senses, whose component parts have properties represented in different cortical areas (such as color, contrast, sounds, and so on), all of which have to be knitted together into a coherent perception. The suggestion is that common fate reflects synchronization of neuron firingand that is this same mechanism that may underlie the brain's solution to the binding problem.
8.3.3. End Notes
1. Part of Jim Levin's "Gestalt Principles & Web Design" (http://tepserver.ucsd.edu:16080/~jlevin/gp). Applet developed by Adam Doppelt.
2. Alais, D., Blake, R., & Lee, S. (1998). Visual features that vary together over time group together over space. Nature Neuroscience. 1(2), 160-164.
Lights on the joints of a walking person are enough to give a vivid impression of the person, carrying information on mood, gender, and other detailsbut only while the person keeps moving.
Visual perception has special routines for grouping things that move along together into single objects [Hack #76] . That's why we see cars as cars and not a collection of wheels, glass, and side-view mirrors just happening to travel along in the same direction. That's all well and good, but humans live not just in a world of objects like trees and cars, but a world full of people. Given how social we are, and how tricky other people can be, it's not surprising we also have specialized routines for grouping things that move like people together into single objects too. Looking at only a constellation of moving points of light attached to knees, elbows, and other parts of the body, we a get vivid perception of a person, a perception that doesn't exist at all when the points of light are still.
8.4.1. In Action
Open up your browser and point it at http://www.lifesci.sussex.ac.uk/home/George_Mather/Motion/BM.HTML1 or http://www.at-bristol.org.uk/Optical/DancingLights_main.htm (both are QuickTime movies). What do you see?
Both are just points of light moving in two dimensions. Yet the first is clearly a person walking, and the second obviously two people dancing, fighting, and otherwise performing.
As with the common fate demos [Hack #76] of how we group objects by their behavior over time, you can remove the effect by pausing the movies. This information only makes sense when it is moving (shame we can't have animations in the book, really), which is why Figure 8-4 (a frame of the first movie) looks more like a random star constellation than a human figure.
Figure 8-4. If this were moving, it'd look like a person walking
The vivid impression of a walking human shows that we are able to integrate the correlations of the light points and match them to some kind of template we have developed for moving humans. It is orientation-specific, by the way. Watch the video upside down (it's easier if you have a laptop), and you won't see anything resembling human motion at all.
And we don't perceive just abstract form from the moving lights. The demo at http://www.bml.psy.ruhr-uni-bochum.de/Demos/BMLwalker.html, shown in Figure 8-5, allows you to vary the gender, direction, weight, mood, and energy levels of the walker using the sliders on the left.
Figure 8-5. A happy heavyset man, as represented by points of light
You can tell if the moving lights are from a heavyset man who's happy or if they are from a medium-build woman who is slightly afraid. All just from way the lights move.
8.4.2. How It Works
The effect is obvious. That we can perceive the human formeven mood and genderjust from moving lights demonstrates that we automatically extract underlying patterns from the normal human forms we see every day.
Through a combination of experience and specialized neural modules, we have learned the underlying commonalities of moving human formsthe relationships in time and space between the important features (the joints) of the human body. Our brain can then use this template to facilitate recognition of new examples of moving bodies. Being able to do this provides (for free) the ability to perceive a whole just from abstracted parts that move in the right way. A similar process underlies the perception of expressions in emoticons [Hack #93] . It's the reason cartoonists and caricaturists can make a livingshowing just the essentials is as expressive, maybe even more expressive, than the full image with all its irrelevant details.
Given our brains are so good at detecting human forms, it's surprising that emoticons are so common and stick people aren't. Perhaps it's because posture is secondary to facial expression, and anyway you'd need to articulate the limbs to get the full effect. Mind you, that's not to say you can't have dancing stick people in plain text online chat (http://bash.org/?4281).
T.S.
Perceiving biological motion from moving lights isn't something that falls out of other, normal, visual processes.2 Brain imaging studies show that the process involves various brain regions, not only those normally involved with vision are brought to bear, but also those involved in object memory, spatial processing, and even motor processes.3,4
Even better, when the lights give the impression of a fearful person, the part of the brain (the amygdala) that normally responds to fearful expressions on faces is evoked.5 Our specialized mechanisms for recognizing biological motion link direct to our emotions.
The algorithm for perceiving biological motion doesn't always get it right. A light-point walker can actually appear to move in two directions at once. True light-point walkers are based on real people, and you can tell which direction they're walking in. The "chimeric walker" QuickTime movies (http://www.kyb.tuebingen.mpg.de/bu/demo/chimericwalker) have been edited to superimpose two walkers moving in opposite directions, one to the left and one to the right. Your biological motion detection kicks in, and you see a person moving, as normal, but you're really looking at only one set of the superimposed dotswith a little effort you can see the person going the other way instead. With a little more effort you can flip between seeing the two walkers, voluntarily. The detection algorithm's been fooled; you would never see this particular moving dot configuration in the wild.6
8.4.3. In Real Life
If you are a cyclist, you can use our specialized adaptation to the perception of biological motion to your advantage. Fluorescent safety markings that are positioned to tap into this biological motion detection system, by being placed on the joints, have been shown to make you more conspicuous to motorists.7
8.4.4. End Notes
1. Movie on George Mather's Motion Perception tutorial pages (http://www.lifesci.sussex.ac.uk/home/George_Mather/Motion/index.html).
2. Neri, P., Morrone, C., & Burr, D. C. (1998). Seeing biological motion. Nature, 395, 865-866.
3. Pelphrey, K. A., Mitchell, T. V., McKeown, M. J., Goldstein, J., Allison, T., & McCarthy, G. (2003). Brain activity evoked by the perception of human walking: Controlling for meaningful coherent motion. Journal of Neuroscience, 23(17), 6819-6825.
4. Giese, M. A., & Poggio, T. (2003). Neural mechanisms for the recognition of biological movements. Nature Reviews Neuroscience, 4, 180-192.
5. Hadjikhani, N., & de Gelder, B. (2003). Seeing fearful body expressions activates the fusiform cortex and amygdala. Current Biology, 13(24), 2201-2205.
6. Thornton, I.M Vuong, Q.C., & Bulthoff, H.H. (2003). "Last But Not Least: a Chimeric Point Walker." Perception, 32, 377-383 (http://www.perceptionweb.com/perc0303/p5010.pdf).
7. Kwan, Irene, & Mapstone, James (2004). Visibility aids for pedestrians and cyclists: A systematic review of randomised controlled trials. Accident Analysis and Prevention, 36(3), 305-312.