Ir ao cinema, não há bilhetes, vou fazer um aborto...

O texto abaixo explica a minha percepção do fenómeno nele analisado, pelo que a teoria nele exposta será por mim adoptada acriticamente até ao fim da minha vida e utilizado como prova evidente de que a culpa não é minha de nada que eu pudesse, possa ou venha a poder fazer ser alguma vez suficiente.

Assim, e sem mais delongas, do Economist:

Time of the month
The reward circuitry in a woman's brain shifts with her menstrual cycle

THAT some women may just possibly get the teensy-weensiest bit prickly before their monthly periods is widely acknowledged. Though it is less well known, a woman's menstrual cycle can also affect her susceptibility to addictive drugs. The connection between the two may not be immediately obvious, but is probably to do with the effect of fluctuating levels of two sex hormones, oestrogen and progesterone, on the mood-producing parts of the brain.

To find out a bit more about what is going on as “that time of the month” approaches, Karen Berman and her colleagues at America's National Institute of Mental Health, in Maryland, put 13 female volunteers in a brain scanner. Their conclusion, just published in the Proceedings of the National Academy of Sciences, is that mood shifts do, indeed, coincide with shifting patterns of brain activity.

Dr Berman and her collaborators used a technique called functional magnetic-resonance imaging (fMRI), which is sensitive to changes in blood flow—and thus oxygen consumption—to measure how active particular parts of their volunteers' brains were. Their interest was in the anticipation of, and response to, rewarding stimuli (in this case, money). They made the volunteers play slot machines (or, rather, representations of slot machines depicted on a screen inside the scanner), on the understanding that the volunteers could keep part of the notional winnings from their gambles. That created the expectation—though not the certainty—of a reward each time the virtual wheels spun.

Crucially, each volunteer was examined twice in a month: once early in her menstrual cycle, before she had ovulated, and once late in the cycle, after ovulation but before the onset of menstrual bleeding. In the first case, only oestrogen is abundant in the bloodstream. In the second, progesterone is circulating, too.

Previous experiments have established that many parts of the brain are involved in anticipating and enjoying rewards. These include structures such as the amygdala, which have a wider role in regulating emotions and mood. Dr Berman found that almost all of these reward-associated regions are more active early in the menstrual cycle than late in it. In other words, in the days leading up to ovulation a woman's brain looks as though it is both primed to receive rewards and more appreciative of them when they arrive. After ovulation, the brain's readiness for both desire and enjoyment seems to fall away. Hence, possibly, premenstrual grumpiness. Although the researchers could not prove that these shifts are caused by hormonal changes, they are certainly correlated with them.

Such fluctuations in reward-circuit activity may also explain why women seem to be more sensitive to drugs like cocaine early in their cycles than later on. Previous work done at Columbia University found that if female addicts are treated with progesterone, both their craving for and their response to cocaine damp down. Put together with Dr Berman's observations, this suggests that early in the cycle, when there is little progesterone around but oestrogen is abundant, either some aspect of the reward circuit is vulnerable to corruption or a protective effect is lacking.

The fMRI method may shed light on severe premenstrual symptoms, too. Indeed, looking at women who suffer such symptoms is the team's next project. What it cannot do is explain why fluctuations in the reward circuit evolved. One possibility is that an enhanced anticipation of reward as a woman is approaching ovulation, and thus the most fertile time of her cycle, would increase her drive for sex or other pleasures. Getting the experiments needed to test that hypothesis past an ethics committee might, however, be tricky.