1 Seeing isn't believing

TAKE a moment to observe the world around you. Scan the horizon with your eyes. Tilt your head back and listen. You're probably getting the impression that your senses are doing a fine job of capturing everything that is going on. Yet that is all it is: an impression.

Despite the fact that your visual system seems to provide you with a continuous widescreen movie, most of the time it is only gathering information from a tiny patch of the visual field. The rest of the time it isn't even doing that. Somehow from this sporadic input it conjures up a seamless visual experience.

What is going on? Bang in the middle of your retina is a small patch of densely crowded photoreceptors called the fovea. This is the retina's sweet spot, the only part of the eye capable of seeing with the rich detail and full colour we take for granted. This tiny spot - which covers an area of our visual field no bigger than the moon in the sky - feeds your visual system almost all of its raw information.

To build up a big picture, your eyes constantly dart about, fixating for a fraction of a second and then moving on. These jerky movements between fixations are called saccades, and we make about three per second, each lasting between 20 and 200 microseconds.

The curious thing about saccades is that while they are happening we are effectively blind. The brain doesn't bother to process information picked up during a saccade because the eyes move too rapidly to capture anything useful. All in all, your visual system works like a man blundering around in the dark waving around a flickering torch with a very narrow beam.

Despite the fact that you don't normally notice saccades, you can catch them in action. Look at your eyes close-up in the mirror and flick your focus back and forth from one pupil to another. However hard you try you cannot see your eyes move - even though somebody watching you can. That's because the motion is a saccade, and your brain isn't paying attention. Now pick two spots in the corners of your visual field and flick your gaze from one to the other and back again. If you're lucky you'll notice, just barely, a brief flash of darkness. This is your visual cortex clocking off.

So how does your brain weave such fragmentary information into a seamless movie? This remains something of a mystery. The best explanation, according to Andrew Hollingworth of the University of Iowa in Iowa City, is that your short-term and long-term visual memories retain information from previous fixations and integrate them into a here-and-now visual experience (Visual Cognition , vol 14, p 781).

There is also some guesswork going on. You can get a feel for this from the frozen-time illusion - the sensation that you sometimes get when you look at a clock and the second hand appears to freeze momentarily before tick-tocking back into action.

This happens because of saccades. To compensate for the temporary shut-down of vision, your brain makes a guess at what it would have seen, but it does so retrospectively. So the 100 or so milliseconds of blindness gets back-filled with the image that appears after the saccade is over. If your eyes happen to alight on the clock just after the second hand has moved, your brain assumes that the hand was in that location for the duration of the saccade too. The "second" then lasts about 10 per cent longer than normal, which is enough for you to notice.

The weirdness isn't confined to vision. Your auditory system is also full of gaps and glitches that the brain cleans up so we can make sense of the world. This is especially true of speech.

In everyday life we encounter lots of situations that obscure or distort people's voices, yet most of the time we understand effortlessly. This is because our brain pastes in the missing sounds, a phenomenon called phonemic restoration. It is so effective that it is sometimes hard to tell that the missing sounds are not there.

A good demonstration of this effect was published last year by Makio Kashino of NTT Communication Science Laboratories in Atsugi, Japan. He recorded a voice saying "Do you understand what I'm trying to say?" then removed short chunks and replaced them with silence. This made the sentence virtually unintelligible. But when he filled the gaps with loud white noise, the sentence miraculously becomes understandable (Acoustic Science and Technology , vol 27, p 318).

"The sounds we hear are not copies of physical sounds," Kashino says. "The brain fills in the gaps, based on the information in the remaining speech signal." The effect is so powerful that you can even record a sentence, chop it into 50-millisecond slices, reverse every single slice and play it back - and it is perfectly intelligible. You can listen to Kashino's sound files [HERE] .

Another demonstration of the brain's ability to extract meaning from distorted signals is a form of synthesised speech called sine-wave speech. When you first hear a sentence in sine-wave speech it sounds alien and unintelligible, somewhat reminiscent of whistling or birdsong. But if you listen to the same sentence in normal speech and then return to the sine-wave version, it suddenly snaps into auditory focus. Try as you might, you cannot "unhear" the words that you didn't even realise were words the first time you heard them (listen to demos [HERE] and [HERE] ).

According to Matt Davis of the UK Medical Research Council's Cognition and Brain Sciences Unit in Cambridge, this happens because the brain has circuits that respond to speech, but doesn't switch them on unless it detects spoken language (Hearing Research , vol 229, p 132). Sine-wave speech isn't speech-like enough to trigger the circuits, but once you know it is speech they spring into action. "It's an example of top-down influence," says Davis. "What you know about what you're hearing changes the way you hear it."

Given the tricks that your visual and auditory systems play, it probably comes as no surprise that when they get together, fights can break out. A good demonstration of this is the McGurk effect, in which listening to a series of identical syllables such as "ba ba ba ba" while watching somebody mouth "ba da la va" makes you hear "ba da la va". Try it for yourself.

Until recently, psychologists believed that the visual system always trumps the other senses, but in 2000 a team of psychologists at the California Institute of Technology in Pasadena proved that this isn't the case. They showed volunteers a single flash on a computer screen. If they accompanied the flash with two very short beeps, the volunteers saw two flashes - in other words, this time the auditory system wins (Nature , vol 408, p 788). See the illusion [HERE] .

2 This is not my nose

YOU may know the crossed-hands illusion. Hold your arms out in front of you and cross them over, rotate your hands so your palms face each other, then mesh your fingers together. Now slowly rotate your hands up between your arms so you're staring at your knuckles. Ask someone to point to one of your index fingers, then attempt to move it. Did you move the wrong one?

If so, you've just experienced a minor failure of your body schema - your mental representation of the location, position and boundaries of your body. Your brain builds this up by drawing on data from vision, touch and a body-wide network of proprioceptive sensors that monitor position. Your body schema is a critical part of self-awareness, which is why it feels so odd when it goes wrong.

In the crossed-hands illusion, the schema fails because of a confusing visual input. You don't normally see your hands in this convoluted position; the finger you move is the one that is pointing in the direction that the correct one would be pointing if you had simply clasped your hands as if in prayer.

An even odder way of disturbing your body schema is an illusion that taps straight into your sense of body ownership. Known as the rubber-hand illusion, it fools you into thinking a rubber hand - or even a piece of wood, or a table - is part of your body.

To experience the illusion, get hold of a model hand (it doesn't have to be very realistic) and put it on the table in front of you. If it is a left hand, put your actual left hand somewhere you can't see it, in the same pose as the rubber hand. Now get someone to touch and stroke your unseen hand and the rubber hand with identical movements. If you concentrate on the rubber hand, you will probably get the uncanny feeling that it is your own.

What this illusion shows is that your sense of body ownership is less anchored in reality than you think. Your brain will happily override information from proprioception to conjure up an incorrect yet coherent body schema based on vision and touch.

In fact, your mental body map is an absolute sucker for visual information. This year Frank Durgin of Swarthmore College in Pennsylvania set up the illusion as described above but instead of touching the rubber hand he merely "stroked" it with light from a laser pointer, leaving the unseen hand alone. Two-thirds of 220 subjects reported a sense of ownership of the rubber hand and said they had the sensation of heat and even touch from the laser pointer (Psychological Science , vol 18, p 152). "It's obvious the hand is rubber - no one is fooled at all," says Durgin. "But if your brain decides it's your hand, all the conscious awareness in the world won't change it."

If you can't get hold of a fake hand, there are other (though less reliable) ways to experience the illusion. Some people can be fooled into believing a piece of wood has replaced their hand. Around half of people can even be made to feel a table top is part of their body. Sit at a table and put your hand out of sight underneath. Get someone to tap and stroke this hand while doing exactly the same to the table top directly above. If you watch the table top, you may experience the illusion that the table has become part of your body.

Proprioception may be the junior partner to vision and touch in creating your body schema, but it still plays a key role. You can demonstrate this with an illusion that taps into proprioception alone. This Pinocchio illusion is hard to do without a specialist piece of equipment called a physiotherapy vibrator, but if you can get hold of one, try this. Close your eyes, touch the tip of your nose and then get somebody to apply the vibrator at about 100 hertz to skin at the very top of your bicep. This creates the strong sensation that you are straightening your elbow, and that your nose is simultaneously growing longer and longer, like Pinocchio's.

Vibrating the skin above a tendon excites stretch receptors in the muscle, creating a powerful sensation that the muscle is stretching and the joint is extending. This confuses your proprioceptors, which revise your body schema accordingly. The result is rather like having a phantom limb: the sensed position of your arm in space doesn't correspond to its actual position.

If you're touching your nose at the same time, this leads to a weird sensation that it is growing. Your brain integrates the touch sensation from your fingers with the "movement" of your arm and comes to the erroneous conclusion that your nose must be growing to fill the gap.

The Pinocchio illusion is an important tool for understanding how the brain calculates the size and shape of our bodies. This isn't just an academic question. When it goes wrong, such as in body dysmorphic disorder, anorexia and phantom limb, the results can be devastating (PLoS Biology , vol 3, p e412).

3 A brain of two halves

WOULD you consider yourself to be logical and analytical or creative and empathic? According to popular psychology you're one or the other, and it's all down to which half of your brain you use the most: the rational and calculating left or the intuitive, artistic right.

It's a myth, of course, but like all good ones it contains a grain of truth. Your cerebral cortex - the outer layer of your brain that deals with higher functions - is indeed split into two halves. They are connected by a flat bundle of nerve fibres called the corpus callosum, but work in subtly different ways - and these differences occasionally flicker into your conscious awareness.

The left-brain/right-brain myth arose from experiments done in the early 1970s on people who had had their corpus callosum cut as a last-ditch treatment for epilepsy. These "split-brain" patients showed some strikingly odd responses to information that was preferentially sent to one side of the brain or the other by presenting it to the extreme left or right of their visual field. This works because the right visual field is monitored by the right eye, which routes straight into the left brain, and vice versa.

For example, when a word or picture is presented to their right brain, split-brain patients are often unable to read or recognise it. This and similar experiments led to the idea that the left side of the brain deals with logic and facts while the right side is more intuitive and interpretive. We now know that this dichotomy is too simplistic, but its essence holds true. The latest view is that the two hemispheres have subtly different styles of information processing: the left has a bias towards detail, the right a more holistic outlook. You can watch a video of a split-brain experiment at YOUTUBE.

Most people, of course, have a functional corpus callosum that shunts information between the hemispheres. Even so, subtle left-right differences exist. One task where the hemispheres operate differently is face recognition. When most of us see a face, our right cerebral hemisphere does the lion's share of the work recognising its gender and decoding its expression. And because the right hemisphere is fed by the left visual field, that means we have a notable left-sided bias in our judgement of faces.

Look at this pair of faces (left). Which appears happier? Chances are you chose the bottom one. The two faces are, however, identical apart from being mirror images of one another. The picture is called a chimeric face and is made by taking two pictures of the same face, one with a neutral expression and the other smiling, chopping the pictures in half and joining the two mismatched pieces. Our general bias towards the left side of the face (as we look at it) makes us see the faces as different even though they are essentially equivalent.

It isn't just visual processing that is lateralised. There is some evidence that emotion is too, with the right side of the brain more specialised for negative emotions and the left for positive ones. Amazingly, simply activating one or other hemisphere by moving parts of your body can noticeably change your emotional state.

You can experience this by repeating an experiment first done in 1989 by Bernard Schiff and Mary Lamon of the University of Toronto in Canada (Neuropsychologia , vol 27, p 923). They asked 12 volunteers to perform a "half smile", lifting one corner of their mouths and holding it for a minute. Left-smilers reported feeling sadder afterwards, while right-smilers felt more positive.

Other researchers have reproduced the effect simply by getting people to contract the muscles of their left or right hand a few times. More recent research has suggested that motivation is similarly affected: people who performed right-sided muscle contractions became more assertive and spent longer trying to crack an impossible maths puzzle.

Unsurprisingly, these claims are controversial, with some teams failing to replicate the results. Last year, however, Eddie Harmon-Jones of Texas A&M University in College Station used EEG to confirm that flexing the hand muscles produces changes in emotion, but only when it is preceded by activation of the opposite cortex (Psychophysiology , vol 43, p 598). The left-brain/right-brain legend, it appears, is alive and well.

4 Probe your subconscious

IT WAS a ground-breaking investigation into the nature of consciousness and free will. In 1983, psychologist Benjamin Libet of the University of California, San Francisco, hooked five volunteers up to an EEG machine and asked them to make voluntary movements, such as lifting a finger, whenever they felt like it. Watching the electrical activity in their brains, he discovered that his subjects only became consciously aware of their intention to act a few hundred milliseconds after their brain had initiated the movement. Libet was forced to conclude that what feels like a conscious decision may in fact be nothing of the sort (Brain , vol 106, p 623).

This experiment was the first demonstration of what is now an established theory in neuroscience: a major proportion of your thoughts and actions - even things you believe you are in conscious control of - actually take place in your unconscious. Most of the time you are essentially flying on autopilot.

Libet's experiment involved equipment that you're unlikely to have at home, but you can tap into a similar phenomenon using what is known as the "ideomotor effect". Make a pendulum out of a paper clip and a piece of thread and dangle it over a cross drawn on a piece of paper. Ask yourself a simple yes/no question, such as "am I at home?" or "do I have a cat?", and tell yourself that if the pendulum swings clockwise, the answer is yes, while anticlockwise means no. Spookily, the pendulum will generally start rotating in the direction of the correct answer.

It looks supernatural, but it's not. The reason it works is that, as soon as you ask the question, your unconscious brain fires up motor preparation circuits in anticipation of the answer it expects to see. These circuits initiate subtle muscle movements that you are not normally aware of - except when they are amplified by a pendulum (or dowsing stick or Ouija board). This is your unconscious brain in action.

A different aspect of your mental underworld is reflected in your "implicit assumptions". Your subconscious mind isn't just planning and executing actions, it also spends a great deal of time analysing the world, looking for patterns and relationships that can help you navigate through life. The conclusions it comes to are called implicit assumptions - subtle prejudices about people and events. For example, if you hear on the radio that a teenage boy has been shot dead in a car park near his home, it's almost impossible not to make assumptions about his family background and the area where he lived.

"Everybody has implicit assumptions," says Brian Nosek, a psychologist at the University of Virginia in Charlottesville who played a big part in their discovery. "They're a necessary part of how the brain operates and they generally serve us very well."

But not always. Nosek and colleagues argue that because we are not in control of our implicit assumptions, and are seldom aware of them, it is possible to develop unconscious prejudices that your conscious mind would find unappealing or even abhorrent - such as associating men with science and women with the arts, preferring thin people to fat people or assuming that blonde women are stupid. "You may think you're egalitarian, yet your associations are often quite different," says Nosek.

Nosek and colleagues have devised a way to access these implicit assumptions (take the test [HERE]). The tests are based on the idea that people find it easier to recognise pairs of stimuli that fit their unconscious assumptions - white people and positive words or black people and negative words, for example. People often find the results of their tests "provocative", says Nosek. "The most common implicit associations are race and age - they're quite profound."

Maybe sometimes it is better to ignore your unconscious mind.

5 Pay attention!

IMAGINE you are walking down the street and a passer-by asks you for directions. As you talk to him, two workmen rudely barge between you carrying a door. Then something weird happens: in the brief moment that the passer-by is behind the door, he switches places with one of the workmen. You are left giving directions to a different person who is taller, wearing different clothes and has a different voice. Do you think you would notice?

Of course you would, right? Wrong. When researchers at Harvard University played this trick on 15 unsuspecting people, eight of them failed to spot the change.

What this demonstrates is a phenomenon called "change blindness". It happens because of a chronic shortage of a crucial mental resource: attention. You are blithely unaware of most of what is going on around you, to the point where you can fail to notice "obvious" changes in your surroundings.

Attention is not well understood, but whatever it is, we have a limited amount. Of all the information entering or being generated by your brain at any one time - sights, sounds, memories, ideas and so on - only a tiny fraction enters your consciousness. Object-tracking studies suggest that the maximum number of items we can attend to at any one time is around five or six (see demos [HERE] ) .

Scientists studying attention spend a lot of time playing with change blindness because it provides direct access to the attentional system. In the door experiment, the subjects fail to see the change because their attention is elsewhere and the door conceals what would otherwise be attention-grabbing motion.

You can experience the same thing by watching "flicker images". These consist of two consecutive images that differ only in one key feature - two cowboys who swap heads, say. If the images are flashed up in quick succession with a brief blank screen between them (which acts like the door), most people take an astonishingly long time to spot the difference (see demos [HERE] , or try flicking your attention between the two images in the diagram below).

Similarly, we often fail to notice blatant continuity errors when films cut from one scene to another. We also usually fail to detect gradual changes to a static scene, such as the addition of a large building (see demos[HERE]and [HERE] ).

"Basically, the explanation is that attention is needed to see change," says psychologist Ronald Rensink of the University of British Columbia in Vancouver, Canada. "Attention is drawn automatically to the motion signals that accompany a change. But if these are swamped, then the observer can't rely on automatic control, but needs to hunt around with their attention."

A similar phenomenon is motion-induced blindness, in which concentrating on a moving pattern causes what should be very prominent static objects - such as bright yellow dots - to disappear (see demos [HERE] ). Motion-induced blindness was only discovered in 2001 and it is still unclear why it happens, but most researchers think it has something to do with attentional resources.

There is a related and even more counter-intuitive demonstration of our limited capacity for attention. If you are deliberately concentrating on something, it can render you oblivious to other events that you would normally have no trouble noticing. This "inattention blindness" is probably the reason why motorists sometimes collide with objects such as pedestrians and buses that they simply "didn't see".

The most famous demonstration of inattention blindness was staged in 1999 by Daniel Simons and Christopher Chabris of the University of Illinois at Urbana-Champaign. It involves a game of basketball. Chances are you've seen it or read about it before. If not, have a look [HERE] . The task is to count the number of passes made by the team in white. You won't believe your brain.

6 Made-up memories

A FEW years ago, the actor Alan Alda visited a group of memory researchers at the University of California, Irvine, for a TV show he was making. During a picnic lunch, one of the scientists offered Alda a hard-boiled egg. He turned it down, explaining that as a child he had made himself sick eating too many eggs.

In fact, this had never happened, yet Alda believed it was real. How so? The egg incident was a false memory planted by one of UC Irvine's researchers, Elizabeth Loftus.

Before the visit, Loftus had sent Alda a questionnaire about his food preferences and personality. She later told him that a computer analysis of his answers had revealed some facts about his childhood, including that he once made himself sick eating too many eggs. There was no such analysis but it was enough to convince Alda.

Your memory may feel like a reliable record of the past, but it is not. Loftus has spent the past 30 years studying the ease with which we can form "memories" of nonexistent events. She has convinced countless people that they have seen or done things when they haven't - even quite extreme events such as being attacked by animals or almost drowning. Her work has revealed much about how our brains form and retain memories.

While we wouldn't want to plant a memory of a nonexistent childhood trauma in your own brain, there is a less dramatic demonstration of how easy it is to form a false memory called the Deese-Roediger-McDermott paradigm. Read the first two lists of words and pause for a few minutes. Then read list 3 and put a tick against the words that were in the first two. Now go back and check your answers...

Mind Hacks: Tips and tools for using your brain by Tom Stafford and Matt Webb (O'Reilly, 2005)