Age, sex, and experience as related to the neural basis of cognitive development.

Abstract

Studies of rhesus monkeys raised from infancy without portions of the central nervous systems provide evidence regarding the timing of functional maturity of specific regions. Some findings have been presented which show that deficits following lesions of specific cortical areas emerge at that age when abilities dependent upon the cortical area in question take on adult form in unoperated monkeys. However, age is but a convenient abstraction for conditions that vary over time. One such condition is the presence and titer of gonadal hormones. The finding that orbital functions may develop at different times in males and females is of interest from a number of perspectives, but it is especially stimulating to consider the possibility that the development of cortical tissue may be regulated by neuroendocrine factors in a fashion analogous to that envisioned for differentiation of hypothalamic mechanisms. Finally, experiential factors may depend greatly on the maturational status of those brain regions designed to be recipients of that stimulation. The fact that young children acquire second languages with far greater ease than adults or conversely that language fails to develop before 18-28 months of age regardless of training are features of human experience consistent with the interdependence of experience and neurological maturation. The present finding that training at relatively early periods of development facilitated recovery from brain injury indicates further that brain-damaged individuals can be even more sensitive to the effects of previous experience than intact cases. Age, sex, and experience are factors that may be isolated for experimental purposes and for the purpose of discussion. However, it is precisely the complex interactions of these variables that constitute the subject matter of future research in neurobiology, to which studies of infant monkeys may contribute animal models of normal and disordered human development.