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These increases in brain cholesterol and pituitary activity were clues that were rich in their implications, and in the late 1960’s a research team at the University of California at Berkeley began to look for specific differences in the neural structures of gentled and ungentled rats. They found that greater tactile stimulation resulted in the following differences: These animals’ brains were heavier, and in particular they had heavier and thicker cerebral cortexes. This heaviness was not due only to the presence of more cholesterol—that is, more myeline sheaths—but also to the fact that actual neural cell bodies and nuclei were larger. Associated with these larger cells were greater quantities of cholinesterase and acetylcholinesterase, two enzymes that support the chemical activities of nerve cells, and also a higher ratio of RNA to DNA within the cells. Increased amounts of these specific compounds indicates higher metabolic activity. Measurements of the synaptic junctions connecting nerve cells revealed that these junctions were 50% larger in cross-section in the gentled rats than in the isolated ones. The gentled rats’ adrenal glands were also markedly heavier, evidence that the pituitary-adrenal axis—the most important monitor of the body’s hormonal secretions—was indeed more active.34 Many other studies have confirmed and added to these findings. Laboratory animals who are given rich tactile experience in their infancy grow faster, have heavier brains, more highly developed myelin sheaths, bigger nerve cells, more advanced skeletal muscular growth, better coordination, better immunological resistance, more developed pituitary/adrenal activity, earlier puberties, and more active sex lives than their isolated genetic counterparts. Associated with these physiological advantages are a host of emotional and behavioral responses which indicate a stronger and much more successfully adapted organism. The gentled rats are much calmer and less excitable, yet they tend to be more dominant in social and sexual situations. They are more lively, more curious, more active problem solvers. They are more willing to explore new environments (ungentled animals usually withdraw fearfully from novel situations), and advance more quickly in all forms of conditioned learning exercises.35 Moreover, these felicitous changes are not to be observed only in infancy and early maturation; an enriched environment will produce exactly the same increases in brain and adrenal weights and the same behavioral changes in adult animals as well, even though the adults require a longer period of stimulation to show the maximum effect.36
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