A team of researchers led by cognitive scientist Elizabeth Bates, a professor at the University of California, San Diego, has developed a novel new brain imaging technique that produces maps that "light up" the relationship between the severity of a behavioral deficit and the voxels (similar to pixels in computer images) in the brain that contribute the most to that deficit.
Discovery of the new technique, known as Voxel-based Lesion-Symptom Mapping (VLSM), was reported in the April 21 issue of Nature Neuroscience. According to Bates, who is known for her research on the brain and how it is organized to process language, VLSM will give researchers an invaluable new tool for pinpointing the specific areas of the brain that are most crucial for normal functioning during critical brain activities, starting with the measures of language comprehension and production that were used for the first demonstration in Nature Neuroscience, but moving on to many different language and non-language functions. To view or download the paper, which includes color VLSM brain maps, please visit: http://www.nature.com/cgi-taf/dynapage.taf?file=/neuro/journal/vaop/ncurrent/index.html
"This is a new brain mapping technique to be used with structural rather than functional magnetic resonance imaging scans (fMRI) that locate brain damage for individual patients," said Bates. "It is an important breakthrough because it is a bridge, a tool, to bring two completely different traditions in brain research – lesion-behavior mapping and fMRI's -- into alignment."
Lesion-behavior mapping is the oldest technique in neuroscience, going back literally thousands of years. But, despite its many contributions, lesion analysis has not allowed researchers to examine functional networks in the brain. With functional brain imaging, the newest tool in cognitive neuroscience, researchers can identify the parts of the brain that are engaged when a patient undertakes such critical tasks such as language comprehension, speech, or music perception. However, up until now, says Bates, these two techniques were not used together because there was no direct and quantifiable method for comparing them.
The VLSM method applied in this study represents an approach to lesion-symptom matching which uses voxel-based procedures inspired by those used in the analysis of functional neuroimaging data, avoiding some of the limitations of traditional lesion analysis methods. It is an improvement on previous lesion-symptom mapping techniques because it does not require patients to be grouped either by lesion site or behavioral cutoff, but instead exploits continuous behavioral and lesion information.
"The differences between lesion-symptom mapping (what brain areas are crucial for a behavior to be carried out) and activation in normal brains (what brain areas are merely along for the ride when a behavior is carried out) are just as important and informative as the similarities. Now we are in a position to conduct those comparisons," said Bates.
In their study, the researchers applied VLSM to a group of 101 left-hemisphere damaged aphasic patients, using behavioral data from two well-studied tasks: language fluency and language comprehension. The VLSM maps for these variables confirm the anticipated contrast between anterior and posterior areas of the brain, while at the same time implicating interacting regions that also facilitate fluency and auditory comprehension and converge on new findings from modern brain imaging.
The VLSM technique is designed to match fMRI studies of the same behavioral functions in normal brains, which light up degrees of metabolic activity (by measuring blood flow) at different sites on a voxel-by-voxel basis. VLSM, which produces comparable pictures of the brain, shows the extent to which brain damage occupies each voxel. The researchers did a comparison of these patients that more accurately conveys how important each particular voxel is for the behaviour being measured.
"Put all those color-coded importance-level voxels together and you get lovely graded maps of degree-of-importance brain tissue for the behavior in question, maps that can be compared directly and quantitatively with the degree-of -activation maps that come out of functional brain imaging," explained Bates.
It has taken Bates and her team 3-4 years to develop and refine the VLSM technique. She is in the process of sharing the VLSM software with other researchers and plans to make it available through the Internet, at no cost. Researchers involved in the study, which was funded by the National Institute on Deafness and Communication Disorders, include: Martin Sereno, Ayse Pinar Saygin, and Frederic Dick, UCSD Cognitive Science Department; Stephen Wilson, UCLA; Nina Dronkers, VA Northern California Health Care System and UC Davis; and Robert Knight, VA Northern California Health Care System and UC Berkeley.
Future work, said Bates, will focus on quantitatively comparing VLSM maps with activation maps from functional imaging studies of normal subjects performing the same of similar tasks.
Bates, a professor in UCSD's Department of Cognitive Science in the Division of Social Sciences, is the director of the Center for Research in Language. Much of her work is interdisciplinary, involving large teams of neurologists, pediatricians, linguists, psychologists, computer scientist, and cognitive scientists. She is the author of numerous publications on brain organization and language, including Rethinking Innateness (MIT Press, 1996), From First Words to Grammar (Cambridge University Press, 1988), and The Cross-Linguistic Study of Sentence Processing (Cambridge University Press, 1989).
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