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:

The specificity of the phonological awareness effect to radial diffusivity of the temporal-callosal fiber tract may be explained in at least two ways. One possibility is that the membranes and myelin sheaths are more permeable to diffusing water in individuals with high phonological awareness. A second possibility, which we favor, is that there is a higher proportion of large axons in the good readers and thus a lower density of cell membranes in the perpendicular direction. This second interpretation is consistent with research that suggests that poor readers have difficulty processing visual and auditor y stimuli that change quickly over time (28 –30) because larger axons can conduct signals more quickly than smaller axons (31). Taken together with the lack of a correlation between reading and the overall size of the temporal-callosal segment (r 0.1), this axon size and density interpretation also suggests that good readers have fewer total callosal connections passing through this part of the callosum. This reduced connectivity is consistent with data that suggest that poor readers have reduced hemispheric specialization in temporal lobe language regions (32, 33) because hemispheric specialization is related to reduced interhemispheric communication (20, 21). It is also consistent with the hypothesis that callosal differences are related to other gross morphological differences in good and poor readers (34).

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?