A team at the University of Illinois claims it's research is unique as it enlisted an extraordinary pool of volunteer participants: 182 Vietnam veterans with highly localised brain damage from penetrating head injuries.
"It's a significant challenge to find patients (for research) who have brain damage, and even further, it's very hard to find patients who have focal brain damage," said lead scientist Prof Aron Barbey.
Brain damage -- from stroke, for example – often impairs multiple brain areas, he said, complicating the task of identifying the cognitive contributions of specific brain structures.
But the very focal brain injuries analysed in the study allowed the researchers "to draw inferences about how specific brain structures are necessary for performance", Barbey said.
"By studying how damage to particular brain regions produces specific forms of cognitive impairment, we can map the architecture of the mind, identifying brain structures that are critically important for specific intellectual abilities," he added.
The scientists took CT scans of the participants' brains and administered an extensive battery of cognitive tests. They pooled the CT data to produce a collective map of the cortex, which they divided into 3,000 3D units called voxels.
By analysing multiple patients with damage to a particular voxel or cluster of voxels and comparing their cognitive abilities with those of patients in whom the same
structures were intact, the team was able to identify brain regions essential to specific cognitive functions, and those structures that contribute significantly to intelligence.
"We found that general intelligence depends on a remarkably circumscribed neural system. Several brain regions, and the connections between them, were most important for general intelligence," Barbey said.
These structures are located primarily within the left prefrontal cortex (behind the forehead), left temporal cortex (behind the ear) and left parietal cortex (at the top rear of the head) and in "white matter association tracts" that connect them, the 'Brain: A Journal of Neurology' reported.
The scientists also found that brain regions for planning, self-control and other aspects of executive function overlap to a significant extent with regions vital to general intelligence.