DURHAM, N.C. -- By asking subjects to direct their attention to particular areas in space while their brains were being scanned by MRI, researchers have mapped brain regions active in the high-level neural control of attention. Like an initial satellite reconnaissance of new terrain, this first mapping represents a key step toward understanding the detailed topography and function of brain regions involved in high-level "executive" control of attention.
The researchers reported their findings in an article in the March Nature Neuroscience. They are postdoctoral fellow Joseph Hopfinger and Associate Professor Michael Buonocore of the University of California at Davis, and George R. Mangun, professor of cognitive neuroscience and psychology at Duke University. Their research was sponsored by the National Institute of Mental Health, the Human Frontier Science Program and the National Science Foundation.
According to Mangun, basic understanding of attentional control could provide insights into the pathology of such problems as Attentional Deficit Hyperactive Disorder (ADHD), schizophrenia and disorders of attention following brain damage from stroke. Such understanding could also allow measurements of the therapeutic activity of drug treatments in improving attentional functioning.
"Before we can understand how such patients are different in their attentional control, we have to know how the process functions normally," said Mangun, who is director of the Duke Center for Cognitive Neuroscience. "With this finding, we are laying some important basic groundwork in mapping the areas involved in attentional control and ultimately understanding their computational structure."
Mangun added that the study of such "executive control" in the brain constitutes an important new direction for cognitive neuroscience, which has often focused mainly on how the brain processes sensory input during attention.
Mangun and his colleagues used an analytical technique known as "event-related functional MRI" to distinguish brain regions active during attentional control. MRI uses harmless magnetic fields to map the brain, detecting regions of increased blood flow that reflect increased activity of brain cells called neurons.
In their experiments, the researchers placed subjects in an MRI machine and had them watch for an arrow to pop up on a video screen. Depending on whether the arrow pointed left or right, after a pause they were to direct their attention, without moving their eyes, to a checkerboard to the left or right of the arrow. Occupying their attention was a task of determining whether the black-and-white checkerboard included any gray squares.
By integrating the results of large numbers of such trials, the scientists determined that certain discrete brain areas of the cortex invariably showed activity during the attentional tasks. Principal among these areas are the superior frontal, inferior parietal and superior temporal cortex. The cortex is the thin layer of brain tissue overlying the brain that is responsible for integrating sensory and motor information and for higher brain function.
While previous studies of the brain had implicated regions of the cortex in attentional control, Mangun said, the studies had not distinguished between the act of volitional orienting of attention and the subsequent selective processing of sensory inputs that are attended or ignored.
"We wanted to distinguish between the neural networks that activate when you initially tell someone to pay attention to something, from those involved in processing what happens as a result," Mangun said. "Thus, in our study, we were able to distinguish the brain regions involved with the initial command to pay attention from the orienting of attention to a spatial location.
"It's similar to the distinction in the brain's motor system between what happens when a person decides to reach out for an object and the subsequent neural signals to activate muscle contraction to actually reach out."
According to Mangun, the experiment had to be designed to separate the two tasks significantly in time, because of a slight lag between increased neuronal activity and the change in blood flow that would show up on MRI scans.
"When you measure blood flow changes, as we do in neuroimaging, the blood flow response to the cue may occur over seconds. And so, if presentation of the cue and the target are separated by only milliseconds, it is very difficult to distinguish the responses."
Such a lag meant that even with a 10-second separation of the elements of the experiment, careful analysis of the rise and fall of the "hemodynamic response" was still necessary to unequivocally reveal the brain regions strictly involved in attentional control.
Mangun emphasized that the new findings represent only the beginning of efforts to define the brain regions involved in attentional control. Further experiments will use more powerful functional MRI techniques to map the active regions at higher resolution, like distinguishing finer and finer objects in satellite images.
The researchers also plan to combine MRI mapping with a complementary technique of electrical recording of brain waves during attentional tasks, a method first reported by Mangun and his colleagues in 1994. While such electrical recording cannot distinguish active regions of the brain as well as MRI, it can offer far more precise measurement of the timing of brain region activity.
"Now, we can distinguish the brain regions that are active, but we need to understand in detail which ones are active first, second and third," he said. "Our objective is to distinguish the different mental operations involved, ultimately to understand the detailed computational process of attention."
According to Mangun, new experiments also are underway that vary the nature of the attentional task; for example, paying attention to color rather than a spatial location. Such experiments should yield further insight into the basic brain mechanisms of attentional control.
The Mangun paper was one of two complementary papers on attentional control published in the issue of Nature Neuroscience. The other paper was by Maurizio Corbetta and colleagues at Washington University School of Medicine in St Louis. In that report, the authors tested the idea that the junction between the temporal and parietal areas played a role in reorienting attention toward stimuli at unexpected locations; and that another region, called the intraparietal sulcus, is involved in voluntary orientation and maintenance of attention at cued locations.
While both papers investigated the two major components of attention -- the top-down attentional control processes, and the resulting modulations of perceptual processing -- the Mangun paper isolated and demonstrated the two components.
In a News and Views article on the paper, co-authors Roger Tootell and Nouchine Hadjhikhani of Massachusetts General Hospital wrote "... these two papers demonstrate the power of new imaging techniques to resolve complex cognitive operations into their component steps, and to reveal the structures involved in each step.
"They are likely to stimulate many future studies, and by combining ever-better imaging methods with other approaches such as patient studies and physiology of non-human primates, we can hope to gain a new depth of understanding of how the brain controls attention."
Note to editors: A high-resolution JPEG image is available at photo1.dukenews.duke.edu, file name Mangun.jpg. George Mangun may be reached at (919) 681-0814, E-mail: mangun@duke.edu.
Journal
Nature Neuroscience