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Change blindness: so you think you can see? RITA CARTER This article first appeared in . . . . . THE NOTICE ON THE DOOR invited volunteers to take part in a short, simple, psychological experiment. Just go inside, it said, and sign up at the reception desk. Encouraged by the reward of a book token, a steady stream of students entered the campus building and told the person at the desk they wanted to volunteer. Each one was handed a consent form to fill in. When they had completed it, the receptionist asked them to wait a moment while they filed the form in an area behind a screen. Then the receptionist returned and directed them to another room where they were greeted by a psychologist. At this point the volunteers discovered an unsettling fact. The experiment was already over. It had taken place in the minute or so during which they signed up. And the results of it gave three out of four of them reason – literally – to doubt their own eyes. What had happened was this: when the student approached the reception desk they were greeted by a man with blonde hair, wearing a yellow shirt. This person handed out the consent form, waited while the student filled it in, then took it back and disappeared around the back of the screen, ostensibly to file it. In fact, while hidden from view, the blonde man swapped places with a colleague – a man with darker hair and wearing a different coloured shirt. This other person then returned to the student and directed them to the other room. Astonishingly, most of the would-be volunteers failed to notice the switch. Only when the psychologist showed them a video of the entire event did they believe what had happened. That experiment, carried out at Harvard University in the late 90s, was one of the first to demonstrate a phenomenon known as “change blindness” – our ability to miss massive changes that occur in front of our eyes, provided the “before” and “after” scenes are separated by a short gap or visual interruption. As well as failing to notice when one person takes over from another, experiments have shown that vast chunks of a visual landscape can be removed between glimpses without most of us noticing. In one series of experiments none of the subjects noticed when a large building, smack in the middle of the picture, shrunk by a quarter between glimpses. None saw that a mountain range disappeared. And more than 90% failed to spot the sudden extinction of 30 puffins on an otherwise uninhabited ice floe.
Inattentional and change blindness demonstrate that what we see is not at all what we think we see. We seem to observe the world in all its richness, but in fact we only register consciously a tiny handful of elements – those that catch our attention. We are good at spotting changes if they happen while we are looking because change involves movement and movement grabs attention. But we cannot spot changes if there is a gap between the before and after because to do so we would have to hold a complete memory of the “before” scene to compare with the “after” one. And our memories are only as complete as our perceptions. Magicians have always exploited change and inattention blindness. When a sleight-of-hand artist makes a big deal of directing your attention to what is happening in his left hand you can be sure that the real business is happening in his right. But inattention blindness is not just an amusing curiosity. Our tendency to neglect things that we are not deliberately attending to has real life implications. Drivers, and pilots, for example, frequently make dangerous errors, not because they are not attending, but because their attention has got locked on to the wrong thing. In one case, for example, a plane crashed because the pilot and co-pilot were focussing so hard on a dodgy instrument that they failed to notice the ground rushing up to meet them. And the attention-grabbing effect of talking on a mobile phone while driving is reckoned to increase a driver’s risk of an accident fourfold. Attention is largely an unconscious faculty – most of the time it is “grabbed” by events rather than deliberately directed, so it is very difficult to avoid zooming in on some things to the detriment of others. Our best defence may be simply to remember that at any time we are only seeing a tiny bit of the picture. The whole thing may look very different indeed. To see examples of change blindness click here © Rita Carter,1999/2007 - ritacarter.co.uk |
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