High oestrogen levels in women while they are ovulating may be directly responsible for sluggishness or problems concentrating, a Canadian study has found.
Researchers at Concordia University's Center for Studies in Behavioral Neurobiology in Montreal linked high oestrogen levels in laboratory rats to an inability to pay attention and learn.
These high levels have also been shown to interfere with women's ability to pay attention, but the study, to be published in the journal Brain and Cognition, is the first to show "how this impediment can be due to a direct effect of the hormone on mature brain structures."
Both humans and rodents have similar brain physiology, although there is some debate over whether sex hormones have similar effects.
"Although oestrogen is known to play a significant role in learning and memory, there has been no clear consensus on its effect," says study lead author Wayne Brake.
"Our findings ... show conclusively that high oestrogen levels inhibit the cognitive ability in female rodents."
Researchers repeatedly exposed rats to a tone, with no consequences. Once they became used to it and ignored it, another stimulus was linked to the tone.
Rats with low levels of oestrogen quickly learned that the tone was associated with the new stimulus whereas those with higher levels of oestrogen took longer to form this memory.
"We only observed this effect in adult female rats," says Brake. "This and our other findings indicate that oestrogen directly affects the brain, perhaps by interfering with brain signalling molecules."
The next step, he says, will be to determine how this occurs.