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Brain morphometry in reading-disabled twins

From the University of Denver (Drs. Pennington and Lefly, and J. Churchwell), CO; the University of California (Dr. Filipek), Irvine, CA; Harvard Medical School (Drs. Kennedy and Galaburda), Cambridge, MA; the University of Colorado School of Medicine (Drs. Simon and Filley), Denver, CO; Denver Veterans Affairs Medical Center (Dr. Filley), CO; the Cedars-Sinai Medical Center (Dr. Alarcon), Los Angeles, CA; and the University of Colorado (Dr. DeFries), Boulder, CO.

Address correspondence and reprint requests to Dr. Bruce F. Pennington, University of Denver, 2155 S. Race Street, Denver, CO 80208.

OBJECTIVE: To test for brain structure differences in reading disability (RD) by means of MRI-based morphometry.

BACKGROUND: Consensus is lacking on the brain structural correlates of RD. The current study reports on a wider set of structures in the largest sample yet studied, controlling for age, gender, IQ, and attention deficit hyperactivity disorder (ADHD).

METHODS: A case–control study was performed that was comprised of 75 individuals with RD (mean age, 17.43 ± 4.29 years) and 22 control subjects without RD (mean age, 18.69 ± 3.75 years), each a single member of a twin pair. The two groups were similar in age, gender, and handedness, but differed in full-scale IQ (FSIQ), with the RD group having a lower mean FSIQ (101.8 ± 9.9 versus 118.3 ± 10.3). Using three group-by-structure analyses of covariance, groups were compared in terms of volume (in cubic centimeters) of major neocortical subdivisions, subcortical structures, and midsagittal areas (in square millimeters) of three subdivisions of the corpus callosum.

RESULTS: Controlling for age, gender, and IQ, the authors found a significant group-by-structure interaction for the major neocortical subdivisions (p = 0.002), reflecting a different developmental pattern in the RD group, with the insula and anterior superior neocortex being smaller and the retrocallosal cortex being larger in the RD group. In contrast, they found no group main or interaction effects for the subcortical or callosal structures. The pattern of results was essentially the same in subjects without ADHD.

CONCLUSIONS: Most brain structures do not differ in size in RD, but cortical development is altered subtly. This study replicates in a larger sample previous findings of insular differences in RD and demonstrates further that those differences are not attributable to comorbid ADHD.

Key words: Dyslexia—Reading disability—Brain morphometry—Structural MRI.

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