Now you see it, now you don’t: ’Change blindness’ isn’t magic

A team of scientists at UCL (University College London) has discovered why we often miss major changes in our surroundings – such as a traffic light turning green when we’re listening to the radio. Our inability to notice large changes in a visual scene is a phenomenon often exploited by magicians – but only now can scientists put their finger on the exact part of the brain that is so often deceived.

The UCL team shows, in a research paper published in the September issue of the journal Cerebral Cortex (which goes online on 24th August) that the part of the brain called the parietal cortex, the area responsible for concentration, is also critical to our ability to detect changes. The exact critical spot lies just a few centimetres above and behind the right ear – the area many people scratch when concentrating.

Using Transcranial Magnetic Stimulation (TMS), the team switched off the parietal cortex part of the brain temporarily by applying magnetic stimulation to the head via a magnetic coil which produces small electrical currents to the brain. Without help from this region of the brain, subjects failed to notice even major visual changes– in this case a change of a person’s face.

In previous experiments using brain scanning (functional magnetic resonance imaging or fMRI), the team led by Professor Nilli Lavie at the UCL Department of Psychology, discovered that detection of visual changes was not only correlated with activity in conventional visual areas of the brain but also with activity in the parietal cortex.

But, until this experiment, when the team actually switched off the parietal cortex using TMS, they didn’t know that noticing change critically depends on activity in the parietal cortex. When that region of the brain was effectively switched off, ’change blindness’ (a failure to notice large changes in a visual scene) occurred.

Professor Lavie said: “Because the parietal lobe is not part of the visual cortex it was at first surprising to find that activity in the parietal lobe is critical for visual awareness. We have always known that the parietal cortex was responsible for concentrating. But it was a surprise to find out it is also important for detecting visual changes in a scene. The finding that this region of the brain has both these functions, concentration and visual awareness, explains why we can be so easily deceived by, say, a magicians’ trick. When we’re concentrating so hard on something that our processing capacity is at its limits, the parietal cortex is not available to pay attention to new things and even dramatic changes can go unnoticed. If you’re concentrating on what the magician’s left hand is doing, you won’t notice what the right hand is doing.”