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Brain Functions During Visual Searches
updated: Jul 16, 2012, 3:11 PM
Source: UCSB
You're headed out the door and you realize you don't have your car keys. After a
few minutes of rifling through pockets, checking the seat cushions and scanning
the coffee table, you find the familiar key ring and off you go. Easy enough,
right? What you might not know is that the task that took you a couple seconds
to complete is a task that computers -- despite decades of advancement and
intricate calculations -- still can't perform as efficiently as humans: the
visual search.
"Our daily lives are comprised of little searches that are constantly changing,
depending on what we need to do," said Miguel Eckstein, UC Santa Barbara
professor of psychological and brain sciences and co-author of the recently
released paper "Feature-Independent Neural Coding of Target Detection during
Search of Natural Scenes," published in the Journal of Neuroscience. "So the
idea is, where does that take place in the brain?"
A large part of the human brain is dedicated to vision, with different parts
involved in processing the many visual properties of the world. Some parts are
stimulated by color, others by motion, yet others by shape.
However, those parts of the brain tell only a part of the story. What Eckstein
and co-authors wanted to determine was how we decide whether the target object
we are looking for is actually in the scene, how difficult the search is, and
how we know we've found what we wanted.
They found their answers in the dorsal frontoparietal network, a region of the
brain that roughly corresponds to the top of one's head, and is also associated
with properties such as attention and eye movements. In the parts of the human
brain used earlier in the processing stream, regions stimulated by specific
features like color, motion, and direction are a major part of the search.
However, in the dorsal frontoparietal network, activity is not confined to any
specific features of the object.
"It's flexible," said Eckstein. Using 18 observers, an MRI machine, and hundreds
of photos of scenes flashed before the observers with instructions to look for
certain items, the scientists monitored their subjects' brain activity. By
watching the intraparietal sulcus (IPS), located within the dorsal
frontoparietal network, the researchers were able to note not only whether their
subjects found the objects, but also how confident they were in their finds.
The IPS region would be stimulated even if the object was not there, said
Eckstein, but the pattern of activity would not be the same as it would had the
object actually existed in the scene. The pattern of activity was consistent,
even though the 368 different objects the subjects searched for were defined by
very different visual features. This, Eckstein said, indicates that IPS did not
rely on the presence of any fixed feature to determine the presence or absence
of various objects. Other visual regions did not show this consistent pattern of
activity across objects.
"As you go further up in processing, the neurons are less interested in a
specific feature, but they're more interested in whatever is behaviorally
relevant to you at the moment," said Eckstein. Thus, a search for an apple, for
instance, would make red, green, and rounded shapes relevant. If the search was
for your car keys, the interparietal sulcus would now be interested in gold,
silver, and key-type shapes and not interested in green, red, and rounded
shapes.
"For visual search to be efficient, we want those visual features related to
what we are looking for to elicit strong responses in our brain and not others
that are not related to our search, and are distracting," Eckstein added. "Our
results suggest that this is what is achieved in the intraparietal sulcus, and
allows for efficient visual search."
For Eckstein and colleagues, these findings are just the tip of the iceberg.
Future research will dig more deeply into the seemingly simple yet essential
ability of humans to do a visual search and how they can use the layout of a
scene to guide their search.
"What we're trying to really understand is what other mechanisms or strategies
the brain has to make searches efficient and easy," said Eckstein. "What part of
the brain is doing that?"
Research on this study was also conducted by Tim Preston, Koel Das, Barry
Giesbrecht, and first author Fei Guo, all from UC Santa Barbara.
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