The harder you focus on something, the less well you may actually see it, US researchers have discovered.
Sustained attention can actually worsen perception
This paradox, described in Nature Neuroscience, might explain how it is possible to miss visual cues.
Volunteers asked to pay close attention to black and white stripes were less able to discern them as a result.
The New York University team believe prolonged attention to an unchanging image effectively 'exhausts' vision after helping it initially.
Researchers know that it is easier to see objects when there is more contrast between their lighter and darker areas.
Previous work has suggested that paying closer attention to or focusing on an object makes it easier to see by effectively increasing this contrast.
Although focusing your sight on an object is helpful initially, investigators Samuel Ling and Marisa Carrasco found that this benefit soon fades.
Dr Carrasco explained: "Usually when we attend to something performance is better. But not always.
"If, for example, you are monitoring a screen and all the time you attend to a particular location on that screen then you are not going to be sensitive after a while.
"It's really paradoxical because you would think you are doing your best by focusing your attention."
The researchers believe this paradox may have an evolutionary advantage.
For example, tuning out visual information that you have already processed frees up the brain's limited resources to detect changes in other parts of the environment - a necessary ability for animals preyed upon in the wild.
But this can also backfire, Dr Will Curran, from the School of Psychology at Queen's University of Belfast, pointed out.
Predators can take advantage of this too, staying still for prolonged periods between intermittent advances on their prey until they are within striking distance, he said.
Professor Peter McOwan, professor of Computer Science at Queen Mary, University of London, said: "These results show a fascinating new kind of attentional illusion - the longer you 'look' the less you 'see'.
"Discovering and examining illusions like these will really help us understand how human perception works.
"Visual attention helps us decide what's important, and understanding how this works in humans may allow us to build smarter computer vision systems that know what to look for."
The researchers stressed that their findings should not be any cause for real concern in everyday life.