Researchers at The Johns Hopkins University say that the brain does so by way of a mechanism in a region of the visual cortex called V2.
The researchers recorded the activity of nerve cells in the V2 region in the brain of macaques, whose visual systems are much like that of humans.
They found that this region identifies "figure" and "background" regions of an image, provides a structure for paying attention to only one of the two regions at a time, and assigns shapes to the collections of foreground "figure" lines that a person sees.
"What we found is that V2 generates a foreground-background map for each image registered by the eyes. Contours are assigned to the foreground regions, and V2 does this automatically within a tenth of a second," Nature Neuroscience quoted Rudiger von der Heydt, a neuroscientist, professor in the university's Zanvyl Krieger Mind/Brain Institute and lead author on the paper, as saying.
He says that the foreground- background "map" generated by V2 also provides the structure for conscious perception in humans.
"Our experiments show that the brain can also command the V2 mechanism to interpret the image in another way," said von der Heydt.
"This explains why, in Escher's drawings, we can switch deliberately" to see either the white birds or the dark birds, he added.
The researcher says that the mechanism identified by the study is part of the system that allows people to search for objects in cluttered scenes, which is why one can turn ones attention to a particular object and derive a certain interpretation from it.
"We can do all of this without effort, thanks to a neural machine that generates visual object representations in the brain," von der Heydt said.
"Better yet, we can access these representations in the way we need for each specific task. Unfortunately, how this machine' works is still a mystery to us. But discovering this mechanism that so efficiently links our attention to figure-ground organization is a step toward understanding this amazing machine," he said.
He believes that the understanding of how this brain function works may help scientists unravel the causes of visual disorders like dyslexia, and identify viable treatments for the condition.