Brain scans have recently revealed that mental processes can change across the menstrual cycle.

The orbitofrontal cortex (OFC) has been implicated in the representation of emotional stimuli, assignment of emotional valence/salience to stimuli, stimulus-reinforcement association learning, motivation, and socio-emotional control.

In the study, which appears in the online early edition of the Proceedings of the National Academy of Sciences, Scientists at Weill Medical College of Cornell University, New York, and The Rockefeller University, New York, used MRI scans to monitor the brain activity patterns of women as they were asked to read words with negative, neutral or positive connotations.

The reserachers recruited 12 women for the study, who were asked to perform the same task premenstrually - one to five days before their period was due - and then postmenstrually - eight to 12 days after their period.

During the premenstrual phase the women showed much greater activity in frontal brain regions that help control emotions when they were reading the emotive words. Postmenstrually, this increased brain activity had disappeared.

The researchers say it is possible that the brain changes might have allowed the women to maintain a consistent emotional state and compensate for the surging hormones that occur around menstruation, which some suggest are involved in PMS. Also, women who experience particularly severe emotional symptoms as part of PMS may lose this control.

Using functional magnetic resonance imaging in female subjects without premenstrual mood symptoms, the researchers found that OFC activity to emotional linguistic stimuli varies depending on the menstrual cycle phase.