Even if a woman doesn't seem moody in the days before her period begins, chances are her brain is working differently, according to a new study.

Researchers from Weill Medical College of Cornell University in New York found that women with no apparent premenstrual syndrome mood changes may use parts of their brains differently over the course of their menstrual cycles.

The findings are published in the online edition of the Proceedings of the National Academy of Sciences.

In the brain imaging experiment, 12 women were shown printed words with either negative, neutral, or positive connotations. They performed behavioral tasks while the researchers recorded blood oxygen patterns in their brains, which corresponds to increased brain region usage.

The women were tested one to five days before their periods began and eight to 12 days after their periods began.

The researchers found that in the premenstrual time period, the women's frontal brain regions -- which help control emotions -- showed greater activity. The opposite was found in the days after the women's periods.

But the changes weren't reflected in the women's outward emotional states. The researchers speculate that the greater frontal brain usage might allow the women to compensate for hormonal changes and maintain a consistent emotional state.

• Study Abstract