Brain Networks May Be Key To Odd Attention Deficit Produced By Some Strokes
Human Brain Networks
The first direct analysis of the interactions between two brain networks that govern visual attention may help researchers working to develop treatments for stroke patients with a condition known as spatial neglect.
These patients have difficulty focusing on or paying attention to stimuli in the left half of their visual field or from the left side of the body.
"Soon after a stroke, these patients may forget to shave the left side of their face, fail to eat food on the left side of a plate or seem to be unaware that their left arm belongs to them," explains Maurizio Corbetta, M.D., the Norman J. Stupp Professor of Neurology and senior author of the paper. "But if you explicitly tell them to pay attention to the left side, then for a brief time they can do so."
Corbetta and his colleagues believe spatial neglect is caused by disruptions of the interactions between two systems in the brain that control two different types of attention.
"If you imagine seeing a painting in a museum for the first time, your attention will initially focus on the largest, brightest and most colorful object in the painting, and this is because of stimulus-driven orienting," Corbetta explains.
Stimulus-driven control of attention also prepares the brain to respond to novel or unexpected situations, such as the sound of a fire alarm or an explosion.
"These kinds of stimuli lead us to immediately drop whatever subject we are focused on and prepare to respond to the new stimulus," Corbetta explains. "We call this stimulus-driven reorienting, because the attention has to be reoriented toward a novel event."
The second major type of attention control is known as voluntary orienting.
"For an example of voluntary attention, go back to the painting in the museum and imagine that a guide had told you earlier to look for a particular detail in the painting," Corbetta says.
Based on earlier research, Corbetta and his colleagues theorized a few years ago that voluntary attention is controlled by networks of neurons near the top of the brain they call the dorsal attention system, and stimulus-driven attention is controlled by neurons on the side of the brain near the ear known as the ventral attention system. But they didn't know whether the ventral attention system was controlling the ability to attend to bright objects or to reorient to surprising events.
To directly study the activity of these brain areas, scientists exposed volunteers in a magnetic resonance imaging (MRI) scanner to a 3 x 3 grid like a tic-tac-toe board. Subjects were asked to watch for the brief appearance of a letter in one of the board's spaces, and to identify whether the letter was a T or an L.
To vary the types of orienting and attention triggered by the exercise, scientists altered a cue screen that appeared prior to the letter. In one set of trials, scientists presented multiple colored squares, one of which "popped out" because its bright color naturally attracted attention. The location of the brightly colored square had no connection to where the letter was going to appear, but it made volunteers tend to focus on the spot. The goal was to create a response similar to the stimulus-driven orienting that leads the first-time viewer's eye to the brightest area of a painting.
In another set of trials, the same grid of colored squares was presented, but this time none of the squares had a color that made it stand out visually. Subjects were told that the left or right side of a diamond appearing in the center of the grid would be thickened and that this thickening would likely predict the side of the grid where the target would appear.
"We know from prior studies that when people expect something to happen at a particular location, they voluntarily shift their attention there before the event happens, using the same parts of the brain as when they voluntarily inspect different parts of a painting," explains Gordon Shulman, Ph.D., research scientist in neurology and an author of the study.
In a variation of this second set of trials, the letter occasionally appeared at a location other than that predicted by the thickening of one side of the diamond. This caused stimulus-driven reorienting, the subjects had to adjust their attention to cope with an unexpected stimulus.
Scientists used the MRI scanner to track blood flow in the brain as the subjects performed the task. Researchers have long recognized increased blood flow to a particular brain area during a mental task as an indicator that the area is involved in the task.
As predicted, when subjects were given prior knowledge of where the target might appear, activity increased in the dorsal attention system. However, researchers were surprised to find that activity also increased in some of those same areas in trials where visual attention was involuntarily directed by the brightly colored square.
The ventral attention system had its biggest activity increase in trials where the target appeared at an unexpected location, and subjects had to reorient their attention.
"We were surprised to see that subjects use the dorsal attention system both when they know where they want to look and when a bright object involuntarily makes them look at it," Corbetta says. "We were able to confirm that the ventral attention system works like an alarm bell, telling people something unexpected and important has just occurred and is worthy of their attention."
Corbetta and his colleagues compared the areas activated in their research with prior studies of brain damage in stroke patients with spatial neglect. They found that lesions in stroke patients with spatial neglect closely matched the location of the ventral attention areas activated during stimulus-driven reorienting.
"This system is mostly centered in the right hemisphere of the brain, and about 90 percent of the time, patients with spatial neglect will have damage to the right hemisphere," Corbetta says. "We think these lesions are damaging the interactions between these two systems, and the next challenge will be to understand how this happens."
Corbetta and his colleagues currently are carrying out experiments in patients with spatial neglect to determine the nature of those interactions as a prelude to developing better treatments.
"For ten years now we have built information based on the normal brain, and now we can begin to take those ideas and start testing what happens in stroke patients," he says. "If we can understand the pathology, then we can devise a treatment."
Kincade JM, Abrams RA, Astafiev SV, Shulman GL, Corbetta M. An event-related functional magnetic resonance imaging study of voluntary and stimulus-driven orienting of attention. The Journal of Neuroscience, May 4, 2005.
Funding from the National Institutes of Mental Health and the McDonnell Foundation supported this research.
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