Sarah Allred is researching how the brain keeps a stable perception of an objects color in different lighting conditions.

CAMDEN — How the brain perceives color is one of its more
impressive tricks. It is able to keep a stable perception of an object’s color as
lighting conditions change.

Sarah Allred, an assistant professor of psychology at
Rutgers–Camden, has teamed up with psychologists from the University of
Pennsylvania on groundbreaking research that provides new insight into how this
works.

Allred conducted the research with Alan L. Gilchrist, a professor
of psychology at Rutgers–Newark, and professor David H. Brainard and
post-doctoral fellow Ana Radonjic, both of the University of Pennsylvania.
Their research will be published in the journal Current Biology.

“Although we recognize easily the colors of objects in many
different environments, this is a difficult problem for the brain,” Allred says.
“For example, consider just the gray scale that goes from black to white. A
white piece of paper in bright sunlight reflects thousands of times more light
to the eye than a white piece of paper indoors, but both pieces of paper look
white. How does the brain do this?”

The process of seeing an object begins when light reflected off
that object hits the light-sensitive structures in the eye. The perception of
an object’s lightness (in terms of color shade) depends on the object’s
reflectance. Objects that appear lighter reflect a larger percentage of light
than those that appear darker.

Allred says the brain processes perceptual differences between
black and white objects even when illumination of the object changes. If the
brain did not do this, it would fail to distinguish color shade in different
light.

 In general, white
objects reflect about 90 percent of the light that hits them, and black objects
reflect about three percent, a ratio of 30-to-1, she explains.

“However, if you look at the intensities of light that
enter the eye from a typical scene, like a field of lilies, that ratio is much
higher, usually somewhere between 10,000-to-1 and a million-to-1,” Allred says.

This happens because in addition to having objects with
different reflectance, real “scenes” also have different levels of
illumination. One example might be a shadowed area under a tree. Allred and her
research colleagues wanted to determine how the brain maps a large range of
light intensity onto a much smaller reflectance range.

One long-time hypothesis is that the brain segments
scenes into different regions of illumination and then uses ratio coding to
decide what looks white.

To test if this hypothesis was true, the researchers
conducted an experiment where participants viewed images that had a very large
range of light intensities. Participants were asked to look at a 5x5
checkerboard composed of grayscale squares with random intensities spanning the
10,000-to-1 range. They were asked to report what shades of gray a target
square looked like by selecting a match from a standardized gray scale.

If the visual system relied only on ratios to determine
surface lightness, then the ratio of checkerboard intensities the participants
reported should have had the same ratio as that of the black and white samples
on the reflectance scale, about 100-to-1.

Instead, the researchers found that this ratio could be
as much as 50 times higher, more than 5,000-to-1.

“This research is important because we have falsified
the ratio hypothesis, which is currently the most widely invoked explanation of
how we perceive lightness,” Allred says. “We also were able to reject several
similar models of lightness. We were able to do this because we measured
lightness in such high-range and relatively complex images.”

She continues, “In addition, even though we used
behavioral rather than physiological measures, our results provide insight into
the neural mechanisms that must underlie the behavioral results.”

A Philadelphia resident, Allred received her undergraduate degree
from Brigham Young University and her graduate degree from the University of
Washington. She is also conducting research on color memory and perception
through a five-year grant from the National Science Foundation.