Focusing on the detail of something we have just seen diverts our attention away from things happening around us and results in an effect known as "inattentional blindness", The Telegraph reported.
While the eyes continue to see things in their path, the visual messages seem not to reach the brain when people are concentrating on something else because its ability to process information is limited, researchers from the University College London, said.
The new study showed that even without the distraction of several moving objects in front of us, we can still become "blinded" simply by trying to remember an image.
Researchers showed a group of volunteers images containing different coloured squares and asked them to hold them in their mind, and told to expect a flash of light.
The study found they were less likely to detect the flash when they were concerned with trying to remember the image than when their mind was unoccupied.
Scans of the participants' brains as they carried out the task revealed a lower level activity in the brain region which processes incoming visual information while the patients were trying to recall the image.
"An example of where this is relevant in the real world is when people are following directions on a sat nav whilst driving," Professor Nilli Lavie, who led the study, said.
"Our research would suggest that focusing on remembering the directions we've just seen on the screen means that we're more likely to fail to observe other hazards around us on the road, for example an approaching motorbike or a pedestrian on a crossing, even though we may be 'looking' at where we're going," he added.
The study was published in the Journal of Cognitive Neuroscience.
