Monthly cycle changes women's brains.

Activity in a brain region that regulates emotions fluctuates over the course of a woman's menstrual cycle, according to a new brain-imaging study.

Up to half of all women experience a variety of emotional symptoms, such as mood swings, irritability, and depression, during the days leading up to their periods. Combined with physical complaints, including bloating and cramping, these symptoms are collectively called premenstrual syndrome (PMS).

Although the emotional symptoms of PMS are well documented, says David Silbersweig of Cornell University, researchers know little about how the menstrual cycle affects women's brains.

To investigate, Silbersweig and his colleagues recruited 12 healthy women between the ages of 22 and 35. The researchers specifically selected women who reported having none of the emotional symptoms of PMS.

One to 5 days before their periods, the women were asked to read words with connotations that were positive (such as "safe" or "delighted"), negative (such as "fault" or "death"), or neutral (such as "bookcase" or "clarinet"). They repeated the task 8 to 12 days after their periods started. During all these word-reading sessions, the researchers scanned the women's brains to measure activity.

During the premenstrual period, part of the volunteers' orbitofrontal cortex, an area involved in regulating emotion, was more active in response to negative words than to positive or neutral words. This difference decreased after the women's periods. The researchers report these results in the Nov. 1 Proceedings of the National Academy of Sciences.

Although the reason for this increased premenstrual brain activity isn't clear, Silbersweig suggests that it may be keeping the volunteers' emotions in check. In a future study, the researchers plan to compare these results with those obtained from women experiencing severe emotional PMS symptoms. --C.B.