Now You See It, Now You Don't: 'Change Blindness' Isn't Magic

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

UsingTranscranial Magnetic Stimulation (TMS), the team switched off theparietal cortex part of the brain temporarily by applying magneticstimulation to the head via a magnetic coil which produces smallelectrical currents to the brain. Without help from this region of thebrain, subjects failed to notice even major visual changes– in thiscase a change of a person's face.

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

But, until this experiment, whenthe team actually switched off the parietal cortex using TMS, theydidn't know that noticing change critically depends on activity in theparietal cortex. When that region of the brain was effectively switchedoff, 'change blindness' (a failure to notice large changes in a visualscene) occurred.

Professor Lavie said: "Because the parietal lobeis not part of the visual cortex it was at first surprising to findthat activity in the parietal lobe is critical for visual awareness. Wehave always known that the parietal cortex was responsible forconcentrating. But it was a surprise to find out it is also importantfor detecting visual changes in a scene. The finding that this regionof the brain has both these functions, concentration and visualawareness, explains why we can be so easily deceived by, say, amagicians' trick. When we're concentrating so hard on something thatour processing capacity is at its limits, the parietal cortex is notavailable to pay attention to new things and even dramatic changes cango unnoticed. If you're concentrating on what the magician's left handis doing, you won't notice what the right hand is doing."