When a human looks at a number, letter or other shape,
neurons in various areas of the brain's visual center
respond to different components of that shape, almost
instantaneously fitting them together like a puzzle to
create an image that the individual then "sees" and
understands, researchers at Johns Hopkins report.
A team from the university's
Zanvyl Krieger Mind/Brain
Institute describes the complex but speedy process in
detail in a recent issue of the journal Neuron.
The question of how the brain sees, recognizes and
understands objects is one of the most intriguing in
neuroscience, associate professor and paper co-author
"This may not even seem like a scientific question to
some people, because seeing is so automatic and we are so
good at it — far better than the best computer vision
systems yet devised," Connor said. "That is because a large
part of the human brain is devoted to interpreting objects
in our world so that we have the necessary information for
interacting with our environment.
"Vision doesn't happen in the eye," Connor said. "It
happens at multiple processing stages in the brain. We
study how objects are signaled or encoded by large
populations of neurons at higher-level stages in the
object-processing part of the brain."
The report, based on recordings of nerve cells in the
visual cortex of macaque monkeys, reveals that neurons in
the higher-level visual cortex at first respond to a visual
stimulus "somewhat indiscriminately," signaling all the
individual features within a shape to which they are
sensitive. For instance, a particular neuron may respond to
objects with either a concave fragment at the top or a
convex fragment at the bottom. At this point, the neural
signals are ambiguous; the brain doesn't know whether the
concavity, the convexity or both are present.
Milliseconds later, however, neurons begin to react
exclusively to combinations of shape fragments rather than
to individual fragments. In other words, the brain begins
to put the pieces together to form larger sections, in the
same way that an artisan might fasten discrete shards of
stained glass to create a design.
"Humans do a rough categorization of objects very
quickly," Connor said. "For instance, in just a 10th of a
second, we can recognize whether something we see is an
animal or not. Our results show that this immediate rough
impression probably depends on recognizing just one or more
individual parts of what we see. Fine discriminations
— such as recognizing individual faces — take
longer to happen, and our study suggests that this delay
depends upon emerging signals for combinations of shape
fragments. In a sense, the brain has to construct an
internal representation of an object from disparate
In the long term, understanding exactly how the brain
processes information may lead to neural prostheses —
artificial replacements for lost sensory, motor and perhaps
even memory and cognitive functions. In the short term,
such work is driven by curiosity about one of the
fundamental mysteries: how the brain works.
"Our ability to see is one of the great evolutionary
accomplishments of the human brain," Connor said. "We still
don't know how the visual system accomplishes this marvel
of information processing. Such experiments are beginning
to reveal how large networks of neurons in the brain
extract meaning from the eye image."
Funding for this research was provided by the National
Institutes of Health. The article appeared in the Jan. 5
issue of Neuron.