The value of a neurophysiological index observed at a specific age represents the cumulative effect of developmental changes up to the age at which the observation was made. The time course of this cumulative effect is the trajectory of the developmental process. Since there is no explicit biological model for the developmental trajectory, no parametric form is attributed to it. In the varying coefficient model (Fan and Zhang, 2008) used in this study, the dependent variable, the development trajectory of the phenotype, is an implicit (non-parametric) function of age, and the effects of the independent variables (covariates) on the trajectory can vary with age. (In this study, the developmental trajectories are those of the natural logarithm of the theta ERO power values from the scalp locations mentioned above. The logarithmic transformation of the data was used to eliminate the skewed distribution found at every age. In the paper all uses of the word “log” as short for “logarithm” mean natural logarithm, including all text on figures.) The developmental trajectory s(t) is the mean of the phenotypic values at age of
