M.C. Escher's ambiguous drawings transfix us: Are
those black birds flying in front of the
white ones, or are the white ones soaring ahead of the
black?
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 in November in Nature
Neuroscience, researchers at Johns Hopkins
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 neuoroscientist Rudiger von der Heydt,
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 10th 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 those 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 Escher's drawings,"
he said. "In most cases, they contain a variety
of cues that could be used to identify fore- and
background, but oftentimes these cues contradict
each other. The V2 mechanism combines these cues
efficiently and provides us immediately with a
rough sketch of the scene."
Von der Heydt called the mechanism "primitive" but
generally reliable. It can also, he said, be
overridden by decision of the conscious mind.
"Our experiments show that the brain can also command
the V2 mechanism to interpret the
image in another way," he said. "This explains why, in
Escher's drawings, we can switch deliberately" to
see either the white birds or the dark birds.
The mechanism revealed by this study is part of a
system that enables us to search for objects
in cluttered scenes, so we can attend to the object of our
choice and even reach out and grasp 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."
Understanding how this brain function works is more
than just interesting: It also could assist
researchers in unraveling the causes of — and perhaps
identifying treatment for — visual disorders such
as dyslexia.
Other authors are Fangtu T. Qiu and Tadashi Sugihara,
both of the Zanvyl Krieger Mind/Brain
Institute. Funding for the research was provided by the
National Institutes of Health.
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