Studies of local cerebral glucose metabolism by positron emission tomography (PET) have provided interesting information about the functional maturation of the brain. There are remarkable differences between the newborn and the mature individual. Neonatal values, adjusted for brain weight, are only one-third those of the adult; except for the primary sensorimotor cortex, they are confined to brainstem, cerebellum, and thalamus. During infancy, there is a progressive evolution in the pattern of glucose metabolism in the parietal, temporal, striate, dorsolateral occipital, and frontal cortices, in this order. Only by the end of the first year do the glucose metabolic patterns qualitatively resemble those of the normal young adult (Chugani).
Behavior during infancy and early childhood is also the subject of a substantial literature, contributed more by psychologists than neurologists. In particular they have explored sensorimotor performance in the first year and language and social development in early childhood. In the first 6 years of life, the infant and young child traverse far more ground developmentally than they ever will again in a similar period. From the newborn state, when the infant demonstrates a few primitive feeding and postural reflexes, there are acquired, within a few months, smiling and head and hand–eye control; by 6 months, the ability to sit; by 10 months, the strength to stand; by 12 months, the muscle coordination required to walk; by 2 years, the ability to run; and by 6 years, mastery of the rudiments of a game of baseball or a musical skill. On the perceptual side, the neonate progresses, in less than 3 months, from a state in which ocular control is tentative and tonic deviation of the eyes occurs only in response to labyrinthine stimulation to one in which he is able to fixate on and follow an object. (This last corresponds to the development of the macula.) Later, the child is able to make fine discriminations of color, form, and size. Gesell has provided a graphic summation of the variety and developmental sweep of a child’s behavior. He writes
At birth the child reflexly grasps the examiner’s finger, with eyes crudely wandering or vacantly transfixed . . . and by the sixth year the child adaptively scans the perimeter of a square or triangle, reproducing each form with directed crayon. The birth cry, scant in modulation and social meaning, marks the low level of language, which in two years passes from babbling to word formation that soon is integrated into sentence structure, and in six years to elaborated syntactic speech with questions and even primitive ideas of causality. In personality makeup … the school beginner is already so highly organized, both socially and biologically, that he foreshadows the sort of individual he will be in later years.
The early studies of Gesell and of others represent attempts to establish age-linked standards of behavioral development, but the difficulties of using such rating scales are considerable. The components of behavior that have been chosen as a frame of reference for neurological development are not likely to be of uniform physiologic value or of comparable complexity, and they have seldom been standardized on large populations drawn from different cultures. Also, the examinations at specified ages are cross-sectional assessments, which give a limited idea of the dynamics of behavioral development. As already stated, temporal patterns of behavior reveal an extraordinary degree of variation in their emergence, increment, and decrement, as well as marked variation from one individual to another.
Indeed, the predictive value of developmental assessment has been the subject of a lively dispute. Gesell took the position that careful observation of a large number of infants, with accurate recording of the age at which various skills are acquired, permits the establishment of norms or averages. From such a framework one can determine the level of developmental attainment, expressed as the development quotient (DQ = developmental age/chronologic age), and thus ascertain whether any given child has superior, average, or inferior performance. Furthermore, after examining 10,000 infants over a period of 40 years, Gesell concluded that “attained growth is an indicator of past growth processes and a foreteller of growth yet to be achieved.” In other words, the DQ predicts potential attainment.