How do we make sense of what we see?( M.C. Eschers ambiguous drawings tran...),Health

M.C. Eschers ambiguous drawings transfix us: Are those black birds flying against a white sky or white birds soaring out of a black sky? Which side is up on those crazy staircases?

Lines in Escher's drawings can seem to be part of either of two different shapes. How does our brain decide which of those shapes to "see?" In a situation where the visual information provided is ambiguous -- whether we are looking at Escher's art or looking at, say, a forest -- how do our brains settle on just one interpretation?

In a study published this month in Nature Neuroscience, researchers at The Johns Hopkins University demonstrate that brains do so by way of a mechanism in a region of the visual cortex called V2.

That mechanism, the researchers say, identifies "figure" and "background" regions of an image, provides a structure for paying attention to only one of those two regions at a time and assigns shapes to the collections of foreground "figure" lines that we see.

What we found is that V2 generates a foreground-background map for each image registered by the eyes," said Rudiger von der Heydt, a neuroscientist, professor in the university's Zanvyl Krieger Mind/Brain Institute and lead author on the paper. "Contours are assigned to the foreground regions, and V2 does this automatically within a tenth of a second.

The study was based on recordings of the activity of nerve cells in the V2 region in the brain of macaques, whose visual systems are much like that of humans. V2 is roughly the size of a microcassette and is located in the very back of the brain. Von der Heydt said the foreground-background map generated by V2 also provides the structure for conscious perception in humans.

Because of their complexity, images of natural scenes generally have many possible interpretations, not just two, like in Eschers drawings, he said. In most cases, they contain a variety of cues that could be used to identify fore-
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