Neurophysiologically, cortisol and norepinephrine are principal mediators of the stress response, and both influence memory function. In particular, cortisol improves memory at low levels but impairs memory at higher doses. Similarly, some evidence indicates that norepinephrine also enhances memory in low doses and either impairs or does not influence memory at high doses. Much of the experimental work in this area has been conducted with animals. Studies have recently begun to extend these paradigms to humans.