Quick Thinking and Intelligence

http://www.npr.org/templates/story/story.php?storyId=102169531

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The Study:

The study is the first to analyze genetic and environmental factors that affect brain fiber architecture and its genetic linkage with cognitive function. It assessed white matter integrity voxelwise (voxel, is a volume element, representing a value on a regular grid in three dimensional space—analogous to a pixel) using diffusion tensor imaging at high magnetic field (4 Tesla), in 92 identical and fraternal twins. White matter integrity, quantified using fractional anisotropy (FA), was used to fit structural equation models (SEM) at each point in the brain, generating three-dimensional maps of heritability. The study visualized the anatomical profile of correlations between white matter integrity and full-scale, verbal, and performance intelligence quotients (FIQ, VIQ, and PIQ).

The quantifiable measure of white matter integrity related to cognition—fractional anisotropy (directional variability) of diffusion is higher in heavily myelinated fiber tracts, and increases with progressive myelination during development. Increases in myelination and larger axonal diameter are associated with increased neuronal conduction speed and may support better cognitive function. Fractional anisotropy correlates with intellectual performance in normal subjects and is reduced by degenerative processes that impair axonal fiber integrity.

The study was comprised of 92 twins, 23 pairs of identical (11 male pairs and 12 female pairs) and fraternal (10 male pairs and 13 female pairs). Each person was tested using the Wechsler Adult Intelligence Scale and then scanned using diffusion tensor imaging in order to create a spatially detailed map of white matter integrity.

Methods:

Using the Wechsler Adult Intelligence Scale, three verbal (information, arithmetic, and vocabulary) and two performance (spatial and object assembly) subtests were examined for the purposes of this study. Each subtests produced a raw score and verbal (VIQ), performance (PIQ) and full-scale (FIQ) intelligence quotient standardized scores were derived. In this study the IQ scores for identical and fraternal twins were not significantly different.

Diffusion tensor imaging (DTI) is a magnetic resonance imaging (MRI) technique that enables the measurement of the restricted diffusion of water in tissue in order to produce neural tract images instead of using this data solely for the purpose of assigning contrast or colors to pixels in a cross sectional image. The idea of using diffusion data to aid in the production of images of neural tracts curving through the brain.

More extended diffusion tensor imaging (DTI) scans derive neural tract directional information from the data using 3D or multidimensional vector algorithms based on three, six, or more gradient directions, sufficient to compute the diffusion tensor. The diffusion model is a rather simple model of the diffusion process, assuming homogeneity and linearity of the diffusion within each image-voxel. From the diffusion tensor, diffusion anisotropy measures such as the Fractional Anisotropy (FA), can be computed. Moreover, the principal direction of the diffusion tensor can be used to infer the white-matter connectivity of the brain (i.e. tractography; trying to see which part of the brain is connected to which other part).

The principal application is in the imaging of white matter where the location, orientation, and anisotropy of the tracts can be measured. The architecture of the axons in parallel bundles, and their myelin sheaths, facilitate the diffusion of the water molecules preferentially along their main direction.


When an IQ score was significantly correlated with FA ( with FDR <0.05), intelligence as well as estimate the genetic and environmental contributions to the correlations between FA and IQ in the same set of subjects. If the correlation between the voxel value of FA in one twin and the level of IQ in the other twin is greater in identical pairs than in fraternal pairs, the excess in the identical correlation over the fraternal correlation is then assumed to be attributed to common genetic factors that mediate both white matter integrity and intelligence.

Given the correlation between IQ scores and white matter integrity in similar regions, it is plausible that overlapping sets of genes may influence IQ measures and fiber architecture. A way to determine this is to use a measure of one trait in one twin to predict the level of the other trait in the other twin. If a prediction can be made with greater precision in identical twins tan fraternal, then a common setoff genes must be involved.

Findings:

-white matter integrity under strong genetic control, with highest heritability in parietal brain regions

White matter integrity (FA) was under strong genetic control in all posterior white matter regions and was highly heritable in bilateral frontal (a2 = 0.55, p = 0.04, left; a2 = 0.74, p = 0.006, right), bilateral parietal (a2 = 0.85, p < a2 =" 0.84," a2 =" 0.76," p =" 0.003)" p =" 0.04" p =" 0.01">

-white matter integrity linked to intellectual performance, with correlations as high as 0.3-0.4 between performance IQ and white matter integrity

-using cross-trait mapping, implicated the same genes as mediating the correlation between IQ and white matter integrity—suggesting a common physiological mechanism for both.

FA and FIQ, PIQ or OBJ scores were influenced by an overlapping set of genes in the cingulum and isthmus of the corpus callosum, the cerebral peduncles, the posterior limbs of the internal capsule and the left posterior thalamic radiation/optic radiation, the right superior fronto-occipital fasciculus and the anterior, superior and posterior corona radiate bilaterally. These correlations were mediated by common genetic factors. The fiber systems whose integrity was most tightly linked with IQ include several with critical roles in visuospatial processing. FA may reflect underlying levels of axonal myelination, which may account for differences in reaction times, processing speed and intellectual performance across subjects.

Issues and Questions:

Limited by age—narrow age range, not model influence of age on heritability

This kind of DTI scan can help to detect Alzheimer’s (slow down of neural pathways) and could also help to determine whether or not new medication for Alzheimer’s is working.

The question of measuring intelligence—in order for this study to stand one must accept the use and validity of standardized intelligence tests.