It's not always easy to identify exactly why you feel moody. But now scientists say they have pinpointed a specific part of the brain associated with negative feelings. According to a study published online today by the Proceedings of the National Academy of Sciences, activity in a small area near the front of the brain correlates with feelings of distress, irritability, anxiety and anger.

David H. Zald of Vanderbilt University and his colleagues used positron emission tomography (PET) to track blood flow in the brains of 89 men and women. Before the scans, subjects filled out questionnaires describing the extent to which they had experienced disagreeable moods in the previous month. The scientists then used the responses to rate the participants using a negative affect scale, which describes unpleasant mood states. Previous research had shown that individuals who rate highly on the scale are at increased risk of developing depression or anxiety disorders. The researchers determined a positive correlation between activity in the brain's ventromedial prefrontal cortex (VMPFC) and high negative affect scores.

The scientists caution that the findings are preliminary. Though they discovered a relationship between moodiness and VMPFC activity, it is unclear whether the observed brain activity is the cause or the effect of negative moods. Says Zald, "such a connection does make sense, however, because animal studies show that this region of the brain controls heart rate, breathing, stomach acidity levels, sweating and similar autonomous functions that have a close connection to mood."