Intelligence and brain size in 100 postmortem brains: sex, lateralization and age factors

Abstract

The neural basis of variation in human intelligence is not well delineated. Numerous studies relating measures of brain size such as brain weight, head circumference, CT or MRI brain volume to different intelligence test measures, with variously defined samples of subjects have yielded inconsistent findings with correlations from approximately 0 to 0.6, with most correlations approximately 0.3 or 0.4. The study of intelligence in relation to postmortem cerebral volume is not available to date. We report the results of such a study on 100 cases (58 women and 42 men) having prospectively obtained Full Scale Wechsler Adult Intelligence Scale scores. Ability correlated with cerebral volume, but the relationship depended on the realm of intelligence studied, as well as the sex and hemispheric functional lateralization of the subject. General verbal ability was positively correlated with cerebral volume and each hemisphere's volume in women and in right-handed men accounting for 36% of the variation in verbal intelligence. There was no evidence of such a relationship in non-right-handed men, indicating that at least for verbal intelligence, functional asymmetry may be a relevant factor in structure-function relationships in men, but not in women. In women, general visuospatial ability was also positively correlated with cerebral volume, but less strongly, accounting for approximately 10% of the variance. In men, there was a non-significant trend of a negative correlation between visuospatial ability and cerebral volume, suggesting that the neural substrate of visuospatial ability may differ between the sexes. Analyses of additional research subjects used as test cases provided support for our regression models. In men, visuospatial ability and cerebral volume were strongly linked via the factor of chronological age, suggesting that the well-documented decline in visuospatial intelligence with age is related, at least in right-handed men, to the decrease in cerebral volume with age. We found that cerebral volume decreased only minimally with age in women. This leaves unknown the neural substrate underlying the visuospatial decline with age in women. Body height was found to account for 1-4% of the variation in cerebral volume within each sex, leaving the basis of the well-documented sex difference in cerebral volume unaccounted for. With finer testing instruments of specific cognitive abilities and measures of their associated brain regions, it is likely that stronger structure-function relationships will be observed. Our results point to the need for responsibility in the consideration of the possible use of brain images as intelligence tests.

Gender differences in spatial perception of body tilt

“…Rather than a spatial-reasoning deficit, this is consistent with employment of a different perceptual strategy in which women more strongly attend to and utilize auditory information. This is consistent with other cognitivereasoning measures that indicate that women perform better on auditory-based tasks, such as language skills (Cahill, 2005;Witelson et al, 2006). In short, the findings are consistent with the conclusion that women do not generally have less spatial-reasoning ability but, rather, a different strategy for allocating resources that more heavily weights audition.…”

“…Numerous studies have supported gender differences in spatial processing and perception (Arthur, Hancock, & Chrysler, 1997;Cahill, 2005;Linn & Petersen, 1985;McFadden, Henselman, & Zheng, 1999;Ries et al, 2007;Seurinck, Vingerhoets, de Lange, & Achten, 2004;Sholl, 1989;Tremblay & Elliott, 2007;Witelson, Beresh, & Kigar, 2006;Wraga, Duncan, Jacobs, Helt, & Church, 2006;Zuidhoek, Kappers, & Postma, 2007). The general pattern appears to be that males perform better than females on tasks that emphasize visual spatial cues, such as reading maps, whereas, females perform better on tasks that emphasize verbal or other acoustic sensory information.…”

Evolving Knowledge of Sex Differences in Brain Structure, Function, and Chemistry

“…In adult men, volume loss in whole brain, frontal and temporal lobes increases with age, whereas in women, volume loss in hippocampus and parietal lobes increases with age (37). In adult men and women, global grey matter decreases linearly with age with a steeper decline in men (24;38), a finding that has been confirmed postmortem (39). The reasons for these differences are not clear but may be related to the female sex steroids.…”

Factors Influencing Intelligence Quotient

“…Specific features that may affect IQ include the size and shape of the frontal lobes, the amount of blood and chemical activity in the frontal lobes, the total amount of gray matter in the brain, the overall thickness of the cortex and the glucose metabolic rate. Well-functioning pathways correlate to better brain functioning, brain efficiency and information processing, which all point to better IQ scores [15,16]. It should be noted that the correlation with brain size is not simple.…”