How your Sex Influences your Brain – Morning Sign Out at UCI

Written by Panamdeep Thind and Edited by Josephine Chan

Govind Bhagavatheeshwaran, Daniel Reich, National Institute of Neurological Disorders and Stroke, National Institutes of Health

Throughout the years, many scientists have overlooked the influence of sex on their research based on the assumption that the biological differences between men and women are minimal. However, this is not the case for UCI researcher Dr. Larry Cahill, who refuses to overlook these sex differences in his research. Dr. Cahill has been a neuroscientist for forty years. He began studying the concept of sexual differences after conducting initial research on the separation of memory into distinct emotional states within the brain. This research was inspired by the James L. McGaugh Hypothesis, which proposes that memories are better recalled when one is under stress. When hormones flow, feedback travels to the brain and activates the amygdala—the part of the brain responsible for processing emotion—which increases memory storage in parts of the brain. His main experiments surrounding this hypothesis were conducted using male rats only because of the assumption that everything needed could be accessed from the male anatomy; females were avoided due to hormonal complications [1]. Eventually, the focus of Dr. Cahill’s research shifted to the effects of hormonal contraception on adolescent brains.

For the first twenty years of his career, Dr. Cahill, like many other neuroscientists, studied male animals because he did not feel the need to study female anatomy. However, this viewpoint changed when Dr. Cahill began testing the McGaugh hypothesis on humans. Through PET scans of the brain, he discovered that the amygdala’s response was not the same for men and women. He “reported significantly different patterns of amygdala responsiveness depending both on the sex of the subjects and on whether the right or left hemisphere amygdala was being studied” [2]. Based on these studies, he proposed the question of how the brains of men differ from those of women; however, he concluded that there are many similarities as well as differences. Sex difference was a very touchy subject at the time, and pursuing research based on it was a risky career choice, but Dr. Cahill chose to press on. He says, “At the root of the resistance to sex-influences research, especially regarding the human brain, is a deeply ingrained, implicit, false assumption that if men and women are equal, then men and women must be the same” [3]. He wanted to get to the bottom of the differences in the amygdala, not sex differences. However, his model based on rats making emotional memories was applied to humans and he began to focus deeper on sex differences.

Now aware of the differences between sexes, Cahill took his research in another direction, with his recent discoveries involving hormonal contraception. In his most recent publication, “How does hormonal contraception affect the developing human adolescent brain?” Cahill emphasizes the importance of understanding the effects that birth control pills have on a teenage girl. One of the largest driving forces for sex differences are sex hormones. According to Cahill, scientists are often easily prompted to control sex hormones through hormonal contraception, but there are no clear studies on the effects that hormonal contraception has on the human brain [4]. The human brain is not fully developed until the age of twenty, and most development takes place during adolescence. Knowing this, Cahill’s main goal is to analyze the result of stunting an adolescent’s sex hormones, especially on a developing mind. As stated in his article, “it appears highly plausible that sex hormone alterations during adolescence could have important cognitive/neural consequences both during and after adolescence, yet we almost know nothing about what those consequences are” [4]. Over the fifty years that women have been taking the pill, there is still little public knowledge on the pill and how it alters the development of the human brain. Cahill is devoting his research to understanding the good and bad influences of the pill on the developing female teenage brain. “I wasn’t looking for the sex difference issue; it found me,” says Dr. Cahill, who says he owes it all due to his initial findings of the differences and similarities between the amygdala responses in men and women.

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