stressor that occurs, and then continues to yield stress into the immediate future. A chronic stressor involves exposure to a long-term stressor, and a distant stressor is a stressor that is not immediate. Neurochemical factors of stress on the brain Cortisol is the primary glucocorticoid in the human body. In the brain, it modulates the ability of the hippocampus and prefrontal cortex to process memories. Although the exact molecular mechanism of how glucocorticoids influence memory formation is unknown, the presence of glucocorticoid receptors in the hippocampus and prefrontal cortex tell us these structures are some of its many targets. It has been demonstrated that cortisone, a glucocorticoid, impaired blood flow in the right parahippocampal gyrus, left visual cortex and cerebellum. A study by Damoiseaux et al. (2007) evaluated the effects of glucocorticoids on hippocampal and prefrontal cortex activation during declarative memory retrieval. They found that administration of hydrocortisone (name given to cortisol when it is used as a medication) to participants one hour before retrieval of information impairs free recall of words, yet when administered before or after learning they had no effect on recall. They also found that hydrocortisone decreases brain activity in the above-mentioned areas during declarative memory retrieval. Therefore, naturally occurring elevations of cortisol during periods of stress lead to impairment of declarative memory. It is important to note that this study involved only male subjects, which may be significant as sex steroid hormones may have different effects in response to cortisol administration. Men and women also respond to emotional stimuli differently and this may affect cortisol levels. This was also the first Functional magnetic resonance imaging(fMRI) study done utilising glucocorticoids, therefore more research is necessary to further substantiate these findings. Consolidation during sleep It is believed that sleep plays an active role in consolidation of declarative memory. Specifically, sleep's unique properties enhance memory consolidation, such as the reactivation of newly learned memories during sleep. For example, it has been suggested that the central mechanism for consolidation of declarative memory during sleep is the reactivation of hippocampal memory representations. This reactivation transfers information to neocortical networks where it is integrated into long-term representations. Studies on rats involving maze learning found that hippocampal neuronal assemblies that are used in the encoding of spatial information are reactivated in the same temporal order. Similarly, positron emission tomography (PET) has shown reactivation of the hippocampus in slow-wave sleep (SWS) after spatial