Individuals differ in brain structure, particularly in the cerebral cortex. In this project, we are investigating such individual differences and how they relate to variations in language processing and reading skill. We have now collected neuroanatomical and behavioral data from 200 young adults. These data include measurements of brain volume, corpus callosum areas, the size and shape of numerous cortical regions, and left-right asymmetries in these areas. We also have demographic information, IQ and reading test scores, handedness measurements, and experimental data from 7 divided visual field tasks (word naming, nonword naming, lexical decision, masked word recognition, semantic decision, verb and category generation) from these 200 individuals. We have documented a large range of individual differences in all behavioral and neuroanatomical measures, and are now exploring the dimensions that may underlie these differences.
We will be analyzing these data for many years, but some of our initial findings are:
- Contrary to popular belief, male and female brains are quite similar in the size and asymmetry of regions relevant for language, when sex differences in overall brain size are taken into account. Similarly, there are no sex differences in lateralization for our word reading tasks, and we have not observed any evidence that females outperform males in our language and reading measures. Very little of the individual variation in our brain and behavioral data can be attributed to the sex of the individual.
- Handedness can account for some variation, but the important distinction is not between left- and right-handers, but between those with a very strong degree of hand preference (consistent handers) those with somewhat weaker hand preferences (mixed handers). About half of our sample show strong and consistent hand preference, and for these individuals it is advantageous to be strongly lateralized for language. That is, consistent handers with large visual field differences have better reading skill than those with small visual field differences. However, mixed handers do not show this relationship, suggesting that reading skill is not related to functional lateralization for this group.
- There are discernable subtypes of individuals who show similar performance profiles, yet cannot be differentiated by factors such as sex and handedness. Cluster analyses revealed 4 different behavioral subtypes, and a small set of individuals who could not be so classified (outliers). Interestingly, the outliers demonstrated some unusual neuroanatomical features suggesting that an atypical trajectory of brain development may be associated with behavioral outcomes that do not “fit” typical templates.
This project is funded by the National Institutes of Health (grant DC006957), and was conducted in collaboration with Dr. Christiana Leonard (University of Florida, Gainesville) and Dr. Ronald Otto (CDIC, Riverside).