A new study conduced by scientists at the California Institute of Technology (Caltech) sheds new light on how the various factors that comprise an "intelligence quotient" (IQ) score depend on particular regions of the brain.
The researchers claim that they have carried out the most comprehensive brain mapping to date of the cognitive abilities measured by the Wechsler Adult Intelligence Scale (WAIS), the most widely used intelligence test in the world
Neuroscientist Ralph Adolphs and postdoctoral scholar Jan Gläscher compiled the maps using detailed magnetic resonance imaging (MRI) and computerized tomography (CT) brain scans of 241 neurological patients recruited from the University of Iowa's extensive brain-lesion registry.
The researchers revealed that all the subjects had some degree of cognitive impairment from events such as strokes, tumour resection, and traumatic brain injury, as assessed by testing using the WAIS.
The WAIS test is composed of four indices of intelligence, each consisting of several subtests, which together produce a full-scale IQ score.
The four indices are the verbal comprehension index, the perceptual organization index that involves visual and spatial processing, the working memory index that represents the ability to hold information temporarily in mind, and the processing speed index.
The researchers first transferred the brain scans of all 241 patients to a common reference frame, an approach pioneered by neuroscientist Hanna Damasio of the University of Southern California, a co-author of the study.
They later used a technique called voxel-based symptom-lesion mapping to correlate the location of brain injuries with scores on each of the four WAIS indices.
"The first question we asked was if there are any parts of the brain that are critically important for these indices or if they are very distributed, with intelligence processed globally in a way that can''t be mapped," Adolphs says.
With the exception of processing speed, which appears scattered throughout the brain, the lesion mapping showed that the other three cognitive indices really do depend on specific brain regions.
The researchers revealed that lesions in the left frontal cortex were associated with lower scores on the verbal comprehension index, lesions in the left frontal and parietal cortex (located behind the frontal lobe) were associated with lower scores on the working memory index, and those in the right parietal cortex were associated with lower scores on the perceptual organization index.
They observed that a large amount of overlap in the brain regions was responsible for verbal comprehension and working memory, indicating that these two now-separate measures of cognitive ability may actually represent the same type of intelligence, at least as assessed using the WAIS.
According to the research team, the details about the structure of intelligence provided by their study might prove useful in future revisions of the WAIS test, so that its various subtests are grouped on the basis of neuroanatomical similarity rather than on behaviour, as is the case presently.
The brain maps produced by the study could also be used as a diagnostic aid, and doctors ould combine the maps with their patients'' Wechsler test results to help localize likely areas of brain damage.
"It wouldn''t be sufficient to be diagnostic, but it would provide information that clinicians could definitely use about what parts of the brain are dysfunctional," Adolphs says.
The researchers believe using brain-scan results to predict the IQ of patients, as measured by the Weschler test, may also be possible.
Although the results wouldn''t be as clear-cut as they are in patients with brain lesions, Adolphs says, "you could take a large sample of healthy brains and measure the relative volumes of specific brain areas and draw some associations with these IQ factors."