In examining the trajectory of a neurophysiological index, the interest is both in determining the value of that index at any particular age, and in determining how fast and in what direction it is changing. The value of the index observed at a particular 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. The instantaneous state of the developmental process at any specific age is given by the rate of change of the trajectory (developmental rate) at the age of interest. In this perspective, the state of the developmental process is not directly observable, but can only be estimated from the observable cumulative effects of the developmental process. Although there is a direct mathematical relation between the process and its cumulative effects, results are presented on both the process and its cumulative effects for a more comprehensive view of the phenomena studied. In addition, to understand the relation between the neurophysiological indices studied, the trajectories of their correlations are also determined, and results presented.
