Scientists explain the phenomenon of inattentional blindness in their recent study.
It's been known for some time that when our brains are focused on a task, we can fail to see other things that are in plain sight.
This phenomenon, known as 'inattentional blindness', is exemplified by the famous 'invisible gorilla' experiment in which people watching a video of players passing around a basketball and counting the number of passes fail to observe a man in a gorilla suit walking across the centre of the screen.
"An example of where this is relevant in the real world is when people are following directions on a sat nav while driving," said Professor Nilli Lavie from UCL Institute of Cognitive Neuroscience, who led the study.
"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 explained.
Participants in the study were given a visual memory task to complete while the researchers looked at the activity in their brains using functional magnetic resonance imaging.
The findings revealed that while the participants were occupied with remembering an image they had just been shown, they failed to notice a flash of light that they were asked to detect, even though there was nothing else in their visual field at the time.
The participants could easily detect the flash of light when their mind was not loaded, suggesting that they had established a 'load induced blindness'.
At the same time, the team observed that there was reduced activity in the area of the brain that processes incoming visual information - the primary visual cortex.
Professor Lavie said: "The 'blindness' seems to be caused by a breakdown in visual messages getting to the brain at the earliest stage in the pathway of information flow, which means that while the eyes 'see' the object, the brain does not."
The findings have been published in the 'Journal of Cognitive Neuroscience'.