Scientists say they have pinpointed the brain region involved in a curious phenomenon called "change blindness".
Most of us know what it is like to look at something but fail to see the obvious, such as a traffic light turning green.
UK researchers at University College London, along with US colleagues from Princeton University, have located the brain's parietal cortex as key.
Switching this area off causes change blindness, Cerebral Cortex reports.
There has been increasing evidence from brain scan studies to suggest that awareness of what we see is not only down to the part of the brain that processes visual information - the visual cortex - but also other brain regions.
Professor Nilli Lavie and colleagues at UCL focused on an area called the parietal cortex, which is involved with concentration.
"Even dramatic changes can go unnoticed"
Using a process called transcranial magnetic stimulation, which delivers currents to the brain, they were able to temporarily switch off the parietal cortex in nine healthy volunteers.
Visual trickery
When they did this, the volunteers failed to notice big changes in visual scenes, such as when one of four faces on a video screen was replaced by another face.
The exact critical spot in the parietal cortex lies just a few centimetres above and behind the right ear - the area many people scratch when concentration.
The researchers believe their findings explain change blindness, a phenomenon often exploited by magicians.
Professor Lavie said: "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."
Medical Research Council scientist Dr John Duncan said: "Doubtless, many other parts of the brain are involved."
He said findings such as these might help shed light on medical conditions that can affect a person's perception and attention.
For example, brain damage due to stroke can sometimes mean the individual will completely ignore one side of their body.
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