In Proceedings of the National Academy of Sciences (PNAS), Professor Robert Dougherty and colleagues at Stanford University (Palo Alto, CA, US) reported that differences in the fibers connecting the two hemispheres of the brain are correlated with competence in phonological awareness. They examined the corpus callosa of 49 children using diffusion tensor imaging and related measures of reading to the differences in the features of the structures that they assessed.
The corpus collusum connects the cerebral hemispheres, allowing areas on one side of the brain to communicate with areas on the other side. Diffusion tensor imaging is a non-invasive method of measuring the direction and location of paths of nerons. Neural pathways can can be (a) along the same direction as the pathway, (b) up and down from it, or (c) side to side from it. Dougherty et al. found that high scores on phonological awareness tasks were associated with higher measures of diffusion in directions perpendicular to the main fiber direction, particularly in the neural pathways that connect the temporal lobes (where auditory processing occurs).
Temporal-callosal pathway diffusivity predicts phonological skills in children
Robert F. Dougherty, Michal Ben-Shachar, Gayle K. Deutsch, Arvel Hernandez, Glenn R. Fox, and Brian A. Wandell
Stanford Institute for Reading and Learning, and Department of Psychology, Stanford University, Stanford, CA 94305
The development of skilled reading requires efficient communication between distributed brain regions. By using diffusion tensor imaging, we assessed the interhemispheric connections in a group of children with a wide range of reading abilities. We segmented the callosal fibers into regions based on their likely cortical projection zones, and we measured diffusion properties in these segmented regions. Phonological awareness (a key factor in reading acquisition) was positively correlated with diffusivity perpendicular to the main axis of the callosal fibers that connect the temporal lobes. These results could be explained by several physiological properties. For example, good readers may have fewer but larger axons connecting left and right temporal lobes, or their axon membranes in these regions may be more permeable than the membranes of poor readers. These measurements are consistent with previous work suggesting that good readers have reduced interhemispheric connectivity and are better at processing rapidly changing visual and auditory stimuli.
Link to the PNAS page about the study.
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Dear Mr. Lloyd and Ms. Weiss,
Thank you for reporting the correlations in physiological properties and phonological-processing difficulties.
Question: Correlative or causative? Is there any chance your finding could be CAUSATIVE?
If not, what other factors/circumstances were common to your subjects?
Judy, thanks for the comment and the questions. I do not know enough to say much with certainty; we need an expert to help us.
I can note, however, that I have a hard time supposing that the differences Dougherty and colleagues found are consequences of reading competence. In the case of the changes in blood flow in brains, as revealed by Simos and colleagues for example, I suspect that good reading instruction caused the physiological changes. That doesn’t seem likely here; this is a cross-sectional study, not (yet) one that followed participants and would permit the researchers to see changes over time.
As I read the study, however, the differences are correlational: The differences in the pathways correspond with differences on scores on the PA measures.
In this study, the differences appear to be structural. If they are, then it is possible that they play a causal role; that is, if the collosal fibers are different from the beginning, then perhaps something about them inhibits (which could be good or bad) inter-nerve-cell communication, thus leading to poorer of better phonological awareness. Here’s an extended quote: