They performed experiments recording the activities of individual brain neurons after selectively depriving newborn animals of different types of visual stimulation. The evidence supporting the hypothesis that environmental experiences influence shape anatomical brain structures is powerful has been substantiated by neurophysiological experiments. For example, recent studies done on spinal cord paralysis and the role of a molecule called GAP-43, which turns on for cell growth and repair, suggests that the nervous system has critical periods of plasticity from a biochemical point of view as well as a structural one. This finding elegantly supports the behavioral observations of Piaget while offering an explanation for why children may recover from serious brain injuries where adults would have a very poor prognosis. What he observed was that in castrated animals, changing the levels of testosterone over a wide range produced little differences between pre-surgical aggression and that seen with differing amounts of chemical replacement.
Social conditioning seems to more than make up for much of the lower hormone levels.
The picture appears to be a complex interaction between environment, previous social experience and chemistry. In his recent book, Nobel laureate neurobiologist Gerald Edelman developed a hypothesis supported by extensive neuroanatomical and physiological studies. He argued that repeated common experiences strengthen key neuronal connections through frequent firings. The result is to develop and reinforce anatomically based concept maps which are formed from groups of neurons linked by common experiences, not unlike the categories proposed by Kant. These concept maps subsequently modify how sensory information is processed and organized.