April 15, 2004
A Flash of Insight into Visual Processing
Howard Hughes Medical Institute scientists at The Salk Institute
have discovered a new class of optical illusion that they have studied
in detail to show that humans use both the timing and spatial context
of a visual stimulus to judge brightness.
The researchers said the discovery of the illusion, which they call
the “temporal context effect,” suggests that the human
brain has separate, parallel circuitry to process brightness. One
circuit pathway adapts to a stimulus that is constant in intensity,
while the other assigns a brightness to an object and does not adapt,
they said. The researchers said their findings offer an experimental
approach for teasing out new information about how the brain processes
information about an object's brightness.
“The question is how the brain integrates flash intensity and duration to produce a brightness.”
Terrence J. Sejnowski
The researchers, led by Howard Hughes Medical Institute investigator
Terrence
J. Sejnowski, published their findings in the April 15, 2004, issue
of the journal Nature. Sejnowski and his colleagues at The Salk
Institute for Biological Studies collaborated on the studies with
researchers from the University of Texas and the University of
California, San Diego.
According to Sejnowski, previous work in the field, some of which
was done in his laboratory, had concentrated on the influence of the
spatial context in people's judgment of the brightness of an object.
Prior to the new studies it was known, for example, that a flash of
brief duration looks dimmer than a physically identical flash of longer
duration when the two flashes started at the same time.
“No one had ever compared a brief and a long flash that ended
at the same time, so it wasn't clear which would be brighter,”
Sejnowski said. “The question is how the brain integrates flash
intensity and duration to produce a brightness, which is the perception
of intensity.”
In their experiments, the researchers presented subjects with two
flashes of light — one brief and one long. They asked the subjects to
report on which flash appeared brighter. When both short and long
flashes began at the same time, the subjects in the study reported that
the brief flash looked dimmer. But when the flashes ended at the same
time, the brief flash looked brighter, according to the subjects.
“It's a very dramatic percept, in the sense that you don't
have to average a lot of trials to see the effect,” said
Sejnowski. “You see it on the first try, which means it's a big
effect and not a small one.
“And that immediately tells you that there's something funny
going on, because how could it be that just moving the timing of the
short flash relative to the long one influences your judgment of the
brightness? There must be something odd going on having to do with the
representation of the long one.”
In additional experiments with various arrangements of flashes and
variations in the long flash, the researchers confirmed that it was
specifically the brightness of the brief flash that subjects were
perceiving as changing with temporal context — rather than some change
in perception of the long flash.
The researchers found that the effect was due to higher visual
processing in the brain and not at the level of the retina or initial
visual processing. They confirmed this by presenting the brief flash to
one eye and the long flash to the other. Subjects still reported seeing
the effect. This information suggested that the effect must occur in
the primary visual cortex or later, beyond where binocular information
from the two eyes converges in processing, said Sejnowski. Also, the
researchers performed experiments indicating that the effect was not
due to shifts in the subjects' attention from one flash to another.
“These findings will help us to understand more about the
cortical representation of brightness,” said Sejnowski.
“It's known that many neurons in the cortex respond vigorously at
the onset of a stimulus and then adapt down to a lower rate of firing.
One possible explanation is that somewhere in the circuitry there is a
trace of the original absolute representation of intensity despite the
fact that the firing rates of the neurons are adapting.
“People are still very sensitive to whether a stimulus is
changing up or down. So, you have to come up with some explanation in
which there are two different channels involved — one channel that
gives you the percept of a stimulus that does not change in brightness
and another channel that adapts to a constant stimulus. And somehow,
the brief stimulus is compared with the adapting channel. Our
experiments are able to probe those two channels.”
Sejnowski and his colleagues will next turn their human studies to
exploring more complex patterns of stimuli to delve deeper into the
phenomenon of the temporal context effect. He said that experimental
neurophysiologists in other laboratories would begin
electrophysiological studies to attempt to trace neuronal circuitry to
distinguish the two processing pathways.
In general, said Sejnowski, these kinds of studies will bring a new
understanding of the role of time-dependence on visual processing in
the brain. “I think we know the least about how time is
represented in the visual system,” he said. “How does our
impression of the world change when we're making comparisons in time as
well as in space? Studying the perception of moving objects can't
distinguish the two kinds of effects, since the objects are moving in
both time and space. However, our experiments enable us to sort out the
differences in processing in space and in time,” he said.
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