New research published in the journal Human Brain Mapping provides evidence of changes in brain waves over the course of the menstrual cycle.

“First and foremost I am interested in developing ways that we can measure brain health in humans using non-invasive techniques such as EEG. The menstrual cycle is a major change in brain chemistry that occurs every month and due to a lack of suitable non-invasive techniques there is still a lot about its effect on the human brain that we have yet to learn,” said study author Rachael Sumner of The University of Auckland.

“I am particularly interested in first understanding the healthy menstrual cycle and hopefully using this knowledge to inform future research into menstrual cycle related neurological disorders.”

The study of 20 women aged 21–23 years used EEG to record a pattern of electrical activity in the visual cortex of the brain known as gamma oscillations or gamma waves. One brain scan session took place during the follicular phase while the other session took place in the mid-luteal phase of the menstrual cycle.

The researchers observed a higher frequency of gamma oscillation in the luteal phase, when progesterone and estradiol are highest.

“Every month, over the course of a natural menstrual cycle hormones are causing significant changes in brain form and function and in this study we found a way to measure the nature of this change more precisely and understand its consequences in humans,” Sumner told PsyPost. “Much of what has been done in electrophysiological research up to this point has been in rodents, and whilst valuable, it doesn’t capture all of the complexities of the human cycle.”

“This research will also add to findings that the menstrual cycle must be taken into account when scientists are researching females, as the shifting balance in cortical excitation and inhibition evidently has a substantial impact on measures such as brain oscillations,” she noted.

The implications of the findings are unclear at this point. But it provides a starting point for future research.

“It is important to note that we aren’t trying to say anything about brain function being better or worse at different points of the menstrual cycle. Just different,” Sumner said.

“However, we do know that there are many disorders and diseases that are either triggered by, or get worse, during the menstrual cycle. These include PMS, the more severe pre-menstrual dysphoric disorder (PMDD), and epilepsy (many females with epilepsy experience a worsening in their symptoms at certain points in their cycle, also known as catamenial epilepsy).”

“These disorders have already been strongly linked with hormone related changes to the brains electrophysiology,” she explained. “Researching these disorders in the future is where I think the value of this study and the methods we used will be especially realised, because it’s where animal models become particularly limited in what they can tell us.”

“We know that these disorders are more complex than simply different hormone levels,” Sumner added. “Going forward it is hoped that the model of the cortical microcircuit we used to explain the way hormones were interacting with the brain to produce changes in gamma oscillations could be used to understand the more detailed dynamics of how the brain might be interacting with and responding to hormones to cause specific disorders.”

The study, “Peak visual gamma frequency is modified across the healthy menstrual cycle“, was authored by Rachael L. Sumner, Rebecca L. McMillan, Alexander D. Shaw, Krish D. Singh, Fred Sundram, and Suresh D. Muthukumaraswamy.