While trajectories of growth curves provide a basis for estimating peaks in growth, the accuracy of the estimates is limited by temporal resolution of the measurements. The test-retest interval used in the large-scale longitudinal NIMH study of brain development, focused on ages 3.5 to 33 years old, is 2 years (Giedd et al., 1999; Giedd et al., 1996a). The cohort of Sowell and colleagues (e.g., Sowell et al., 2004a; Sowell et al., 2002) was also scanned at 2-year intervals and focused on development of regional cortical thickness and presented novel findings on continued gray matter thinning of cortical language areas associated with vocabulary scores, an index of language development (Sowell et al., 2004a). Likewise, the NIH MRI study of normal brain development uses 2 year intervals for ages 7 days to 18.3 years old but introduced intervals as close as 3 months for children ages 7 days to 4.5 years to improve modeling of development during these years of rapid change (Evans, 2006).
