Figure 3. A dual process model of white matter development.

(a) FA-development is illustrated for two cases. Throughout white matter and in all children FA is low at birth and generally increases with age. Tract development combines processes that increase and decrease FA, and the balance between these opposing processes differs between children. The balance can be modeled by classic homeostatic equations that govern development over time; the parameters of the developmental processes for the two curves were chosen to model the characteristics of the above-average readers (black curve) and below-average readers (gray curve). The model for the above-average group has synchronous development of the processes, while the below-average group is modeled with slightly asynchronous development. For both groups the FA-developmental rate is approximately linear during the age range of 7–15 years (shaded region) but the specific rate of development differs (dotted lines). (b) During development (7–15 years) FA increases in above-average readers and declines FA in below-average readers for both the left arcuate and inferior longitudinal fasciculus. Combining FA development rates from the arcuate and ILF into an additive model predicts reading proficiency.