Twin differences in lateral ventricle size accompany twin differences in phonological decoding.

Phonological processing skills are foundational for becoming a proficient reader and have only partially been linked to genetic and shared environmental effects in twin studies. This twin difference study of 88 twin pairs (age x̄ = 16.34 ± 1.66 years; 64% female; 65% monozygotic) was designed to examine brain structure and perinatal reasons for twin differences in a measure of phonological decoding accuracy. Diffeomorphic spatial normalization was used align T1-weighted images collected from the 176 participants to a common coordinate space. Jacobian determinant images that represent the amount of volumetric displacement to spatially normalize the T1-weighted images were then examined using voxel-based analyses to determine the extent to which twin differences in voxel-wise volumetric displacement were associated with twin differences in phonological decoding accuracy. Twins with larger lateral ventricles compared to their co-twin, particularly in the left hemisphere, had significantly poorer phonological decoding accuracy. This lateral ventricle effect depended on twins with relatively large differences in phonological decoding accuracy and white matter microstructure in fiber tracts adjacent to the lateral ventricles. Perinatal risk variables, such as slow fetal growth, were hypothesized to explain these twin differences but the current data did not provide clear perinatal explanations for the lateral ventricle and phonological decoding accuracy association. Together, the results suggest that increased lateral ventricle size is a marker for phonological decoding accuracy that is lower than expected based on common genetic and environmental influences on twin brain development.