Elevated cortisol levels, which characterize some moderate-to-severe depressive states, may be associated with a reduction in hippocampal volume, which appears to be proportional to the duration of untreated depression. This process has been likened to a loss of neurons similar to that mediated by corticosteroids in animal models of stress and as suggested by magnetic-resonance-imaging studies that have reported lower levels of N-acetyl aspartate, a neuronal marker, in depression.