Researchers have for the first time observed the effects of psychological stress on the human brain. In the study, scientists used functional magnetic resonance imaging (fMRI) to spy on the brains of subjects who were asked to complete challenging arithmetic exercises while being monitored. The team also took note of the participants' emotional responses and changes in their stress hormone levels and heart rate; many reported feeling flustered and upset by the task. The imaging results showed increased blood flow to the brain's prefrontal cortex, an area known to be associated with anxiety and depression, but also its ability to perform executive functions that allow humans to cope with environmental challenges and threats. Furthermore, the increased blood flow continued even after completion of the math problems. According to the team, the results suggest that although stress may enhance focus, too much of it may be damaging to mental health. The findings are to be published online this week by the Proceedings of the National Academy of Sciences.