As we look at the world around us, images flicker into our brains
like so many disparate pixels on a computer screen that change every
time our eyes move, which is several times a second. Yet we don't
perceive the world as a constantly flashing computer display.
Why not?
Neuroscientists at The Johns Hopkins University think that part of
the answer lies in a special region of the brain's visual cortex
which is in charge of distinguishing between background and
foreground images. Writing in a recent issue of the journal Neuron,
the team demonstrates that nerve cells in this region (called V2) are
able to "grab onto" figure-ground information from visual images for
several seconds, even after the images themselves are removed from our sight.
"Recent studies have hotly debated whether the visual system uses a
buffer to store image information and if so, the duration of that
storage," said Rudiger von der Heydt, a professor in Johns Hopkins'
Zanvyl Krieger Mind-Brain Institute, and co-author on the paper. "We
found that the answer is 'yes,' the brain in fact stores the last
image seen for up to two seconds."
The image that the brain grabs and holds onto momentarily is not
detailed; it's more like a rough sketch of the layout of objects in
the scene, von der Heydt explains. This may elucidate, at least in
part, how the brain creates for us a stable visual world when the
information coming in through our eyes changes at a rapid-fire pace:
up to four times in a single second.
Rudiger von der Heydt is a professor in Johns Hopkins University's Zanvyl Krieger Mind-Brain Institute.
(Photo Credit: Will Kirk)
The study was based on recordings of activity in nerve cells in the
V2 region of the brains of macaques, whose visual systems closely
resemble that of humans. Located at the very back of the brain, V2 is
roughly the size of a wristwatch strap.
The macaques were rewarded for watching a screen onto which various
images were presented as the researchers recorded the animals' brain
nerve cells' response. Previous experiments have shown that the nerve
cells in V2 code for elementary features such as pieces of contour
and patches of color. What is characteristic of V2, though, is that
it codes these features with reference to objects. A vertical line,
for instance, is coded either as the contour of an object on the left
or as a contour of an object on the right. In this study, the
researchers presented sequences of images consisting of a
briefly-flashed square followed by a vertical line. They then
compared the nerve cells' responses to the line when it was preceded
by a square on the left and when it was preceded by a square on the
right. The recordings revealed that the V2 cells remember the side on
which the square had been presented, meaning that the flashing square
set up a representation in the brain that persisted even after the
image of the square was extinguished.
Von der Heydt said that discovering memory in this region was quite a
surprise because the usual understanding is that neurons in the
visual cortex simply respond to visual stimulation, but do not have a
memory of their own.
Though this research is only a small piece of the "how people see and
process images" puzzle, it's important, according to von der Heydt.
"We are trying to understand how the brain represents the changing
visual scene and knows what is where at any given moment," von der
Heydt said. "How does it delineate the contours of objects and how
does it remember which contours belong to each object in a stream of
multiple images? These are important and interesting questions whose
answer may someday have very practical implications. For instance,
how we function under conditions that strain our ability to process
all relevant information - whether it be driving in city traffic,
surveying a large crowd to find someone, or something else, may
depend in large part on what kind of short-term memory our visual system has."
Understanding how this brain function works is more than just
interesting. Because this study shows how the strength and duration
of the memory trace can be directly measured, it may eventually be
possible to understand its mechanism and to identify factors that can
enhance or reduce this important function. This could assist
researchers in unraveling the causes of - and perhaps even
identifying treatment for - disorders such as attention deficit
disorder and dyslexia.
