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Mind Tricks

Mind Tricks



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Published by: Simon on Dec 20, 2007
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Mind tricks: Six ways to explore your brain
How does your brain work? Brain imaging, transcranial magnetic stimulation, andsimilar advanced techniques have given neuroscientists huge insights into thisquestion. Yet studying the brain doesn’t have to be such a high-tech enterprise.Simple experiments can still probe the inner workings of the brain, and many of these are easy to set up at home or are available on the internet.Try them on yourself and you will experience first-hand some of its strangest, mostamazing workings – facets of brain function that scientists are only just starting tounderstand. You’ll see aspects of perception, memory, attention, body image, theunconscious mind – and the curious consequences of your brain being split in two.
1 Seeing isn't believing
TAKE a moment to observe the world around you. Scan the horizon with youreyes. Tilt your head back and listen. You're probably getting the impression thatyour senses are doing a fine job of capturing everything that is going on. Yet thatis all it is: an impression.Despite the fact that your visual system seems to provide you with a continuouswidescreen movie, most of the time it is only gathering information from a tinypatch of the visual field. The rest of the time it isn't even doing that. Somehowfrom 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 denselycrowded photoreceptors called the fovea. This is the retina's sweet spot, the onlypart of the eye capable of seeing with the rich detail and full colour we take forgranted. This tiny spot - which covers an area of our visual field no bigger than themoon 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 arecalled saccades, and we make about three per second, each lasting between 20and 200 microseconds.The curious thing about saccades is that while they are happening we areeffectively blind. The brain doesn't bother to process information picked up duringa 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 arounda flickering torch with a very narrow beam.Despite the fact that you don't normally notice saccades, you can catch them inaction. Look at your eyes close-up in the mirror and flick your focus back and forthfrom 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 yourvisual field and flick your gaze from one to the other and back again. If you'relucky you'll notice, just barely, a brief flash of darkness. This is your visual cortexclocking off.So how does your brain weave such fragmentary information into a seamlessmovie? This remains something of a mystery. The best explanation, according toAndrew Hollingworth of the University of Iowa in Iowa City, is that your short-termand long-term visual memories retain information from previous fixations andintegrate them into a here-and-now visual experience (
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 andthe second hand appears to freeze momentarily before tick-tocking back intoaction.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 soretrospectively. So the 100 or so milliseconds of blindness gets back-filled with theimage that appears after the saccade is over. If your eyes happen to alight on theclock just after the second hand has moved, your brain assumes that the hand wasin that location for the duration of the saccade too. The "second" then lasts about10 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 andglitches that the brain cleans up so we can make sense of the world. This isespecially true of speech.In everyday life we encounter lots of situations that obscure or distort people'svoices, yet most of the time we understand effortlessly. This is because our brainpastes in the missing sounds, a phenomenon called phonemic restoration. It is soeffective 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 voicesaying "Do you understand what I'm trying to say?" then removed short chunksand replaced them with silence. This made the sentence virtually unintelligible. Butwhen he filled the gaps with loud white noise, the sentence miraculously becomesunderstandable (
)."The sounds we hear are not copies of physical sounds," Kashino says. "The brainfills in the gaps, based on the information in the remaining speech signal." Theeffect 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 perfectlyintelligible. You can listen toKashino's sound filesathttp://asj.gr.jp/2006/data/kashi/index.html.
Another demonstration of the brain's ability to extract meaning from distortedsignals is a form of synthesised speech calledsine-wave speech. When you firsthear a sentence in sine-wave speech it sounds alien and unintelligible, somewhatreminiscent of whistling or birdsong. But if you listen to the same sentence innormal speech and then return to the sine-wave version, it suddenly snaps intoauditory focus. Try as you might, you cannot "unhear" the words that you didn'teven realise were words the first time you heard them (listen to demos atwww.mrc-cbu.cam.ac.uk/~mattd/sine-wave-speechandwww.lifesci.sussex.ac.uk/home/Chris_Darwin/SWS).According to Matt Davis of the UK Medical Research Council's Cognition and BrainSciences Unit in Cambridge, this happens because the brain has circuits thatrespond to speech, but doesn't switch them on unless it detects spoken language(
). Sine-wave speech isn't speech-like enough totrigger the circuits, but once you know it is speech they spring into action. "It's anexample of top-down influence," says Davis. "What you know about what you'rehearing changes the way you hear it."Given the tricks that your visual and auditory systems play, it probably comes asno surprise that when they get together, fights can break out. A gooddemonstration of this is the McGurk effect, in which listening to a series of identicalsyllables 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 atwww.faculty.ucr.edu/~rosenblu/lab-index.html.Until recently, psychologists believed that the visual system always trumps theother 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 asingle flash on a computer screen. If they accompanied the flash with two veryshort beeps, the volunteers saw two flashes - in other words, this time theauditory system wins (
).See the illusionatwww.cns.atr.jp/~kmtn/soundInducedIllusoryFlash2/index.html.
2 This is not my nose
YOU may know the crossed-hands illusion. Hold your arms out in front of you andcross them over, rotate your hands so your palms face each other, then mesh yourfingers together. Now slowly rotate your hands up between your arms so you'restaring at your knuckles. Ask someone to point to one of your index fingers, thenattempt to move it. Did you move the wrong one?If so, you've just experienced a minor failure of your body schema - your mentalrepresentation of the location, position and boundaries of your body. Your brainbuilds 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.

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