Adolphs, Damasio, Tranel, and collaborators have published a fascinating open access study
in which they report results of having administered standard general intelligence tests to 241 neurologic patients who were being evaluated in connection with their enrollment in the Iowa Cognitive Neuroscience Patient Registry at the University of Iowa, over the course of approximately 2 decades. They found statistically significant associations between g and damage to a remarkably circumscribed, although distributed, network in frontal and parietal cortex, critically including white matter association tracts and frontopolar cortex. They suggest hat general intelligence draws on connections between regions that integrate verbal, visuospatial, working memory, and executive processes. In particular they found a region in the left frontal pole (white circles
in Figure 3) that is unique to g and not captured by other subtests. A clip from their discussion:
The largest overlap between WAIS subtests and g was found for Arithmetic, Similarities, Information, and Digit Span; the former two tests also exhibited the greatest conjunction with g. These subtests assess verbal knowledge about the world, verbal reasoning, and abstraction, as well as working memory capacity, and are associated with the left inferior frontal gyrus, the superior longitudinal/arcuate fascicule, and to some degree with parietal cortex (Fig. 3). This suggests that g draws on the combination of conceptual knowledge and working memory, and that the communication between areas associated with these capacities is of crucial importance. Such an interpretation is consistent with the Parieto-Frontal Integration Theory (P-FIT), which postulates roles for cortical regions in the prefrontal (Brodman Area 6, 9–10, 45–47), parietal (BA 7, 39–40), occipital (BA 18–19), and temporal association cortex (BA 21, 37). Our results emphasize the important role of white matter tracts in binding the proposed regions together into a unified system subserving g, in line with a recent study relating white matter integrity to intellectual performance: the study reported significant correlations between integrity of the superior fronto-occipital fasciculus and full-scale intelligence quotient (IQ) (a measure related, but not identical, to g).
One of the caveats they mention:
.... Spearman's g disregards theories of multiple intelligences and does not incorporate specific emotional abilities. Therefore we may have isolated an anatomical network important for processing external stimuli, which might operate in parallel with others that are more critical for stimulus reward processing and interoception.
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