disorders including posttraumatic stress disorder (PTSD) (Pitman et al., 1993). Therefore, 
conditioning procedures incorporating unconditioned stressors have occupied an important 
place in the study of anxiety. The neurocircuitry of the fear-potentiated startle response has 
been identified through an elegant series of investigations in rats (Davis, 1992); the continued 
application of pharmacological techniques to this model will almost certainly facilitate the 
design of new treatments for human anxiety disorders. 
EFFECTS OF EARLY EXPERIENCE 
Experiments on animals have confirmed, refined, and extended clinical observations on the 
long-lasting effects of infant experience. The demonstration of prolonged physiological as 
well as behavioral effects has motivated many significant efforts to enhance the beneficial and 
deter the detrimental effects of early childhood experiences (Hunt, 1961). 
Investigators (Riesen, 1975; Wiesel and Hubei, 1965) have shown that various forms of 
visual deprivation cause permanent deficits in the development of visual connections in the 
brain. As a result of this work, pediatricians pay far more attention to the very early detection 
and correction of visual defects in infants, thereby reducing the occurrence of irreversible 
defects in adult vision (Moses, 1975). 
Experimental studies with animals have also been key in demonstrating how the effects of 
early experience may be reversible. For example, Rosenzweig (1984) found that enriching the 
normally impoverished environment of infant rats produced more complex and elaborated 
play as well as the development of thick cortical brain layers. These thickened layers 
contained many more neural connections than those found in infant rats reared in an 
impoverished environment. These differences were discernible in adulthood. Enrichment 
works even in aged animals (Diamond and Connor, 1982) and can even reverse the effects of 
a genetic defect. Knockout mice lacking a receptor for an excitatory neurotransmitter in the 
hippocampus had many deficits in hippocampal-dependent cognition, yet environmental 
enrichment in these animals as adults overcame these deficits (Rampon et al., 2000). 
Some infants that experience psychosocial deprivation fail to thrive and in extreme cases even 
become dwarfs. Brief periods of separation of newborn rats from the mother cause 
deficiencies in growth hormone and receptor function. The critical social deficit was not only 
the mother's absence, but also a lack of physical contact with the mother, especially a lack of 
the "stroking" that infant rat pups receive when the mother licks them. Stroking with a 
paintbrush can prevent or reverse both the hormonal deficits and the inhibition of growth 
(Schanberg et al., 1984). This knowledge has been directly applied to the clinical treatment of 
premature human infants. The aseptic conditions of incubators and nurseries for premature 
infants approximate maternal deprivation, evidenced by a disproportionate number of these 
infants failing to thrive. 
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