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The Male and Female Brain Are More Similar Than Once Assumed

New study debunks widely-held belief that hippocampus size varies by gender.

This post is in response to
How Big Is Your Hippocampus? Does It Matter? Yes and No.
Wikimedia/Life Sciences Database
Hippocampus in red.
Source: Wikimedia/Life Sciences Database

Neuoroscientists currently believe that the male and female brain are much more similar than once assumed. Traditionally, differences observed in memory and spatial skills suggested that males and females have gender-specific differences in the structure and function of their hippocampus.

However, a new meta-analysis of 76 published papers, involving over 6,000 healthy individuals, has debunked the widely-held assumption that hippocampus size varies by gender. Meta-analysis is a statistical technique that allows researchers to combine the findings from many independent studies into a comprehensive review.

The hippocampus was given its name because it resembles the shape of a seahorse. Hippocampus comes from the Greek hippos, "horse," and kampos, "sea monster." Humans have two hippocampi, one in the left hemisphere of the brain, and one in the right hemisphere. The hippocampus plays an important role in the consolidation of learning from short-term memory to long-term memory and in spatial navigation.

The October 2015 paper, “The Human Hippocampus Is Not Sexually-Dimorphic: Meta-Analysis of Structural MRI Volumes,” was published in the journal NeuroImage. This study is the first meta-analysis of male–female differences in hippocampal volume based on published MRI studies of healthy participants of all ages to investigate whether the hippocampus is sexually dimorphic.

Lise Eliot, PhD, associate professor of neuroscience at Rosalind Franklin University of Medicine and Science, put together a team to conduct the meta-analysis of structural MRI volumes. The researchers found no significant difference in hippocampal size between men and women.

According to Dr. Eliot, author of Pink Brain, Blue Brain: How Small Differences Grow into Troublesome Gaps and What We Can Do About It, the general consensus among neuroscientists has been that women tended to have larger hippocampal volume than men. Her latest findings challenge the scientific basis of the assumption that larger hippocampi might explain the generalized observation that females tend to display, "greater emotional expressiveness, stronger interpersonal skills, and better verbal memory.” In a press release, Eliot said,

"Sex differences in the brain are irresistible to those looking to explain stereotypic differences between men and women. They often make a big splash, in spite of being based on small samples. But as we explore multiple datasets and are able to coalesce very large samples of males and females, we find these differences often disappear or are trivial. Many people believe there is such a thing as a 'male brain' and a 'female brain.' But when you look beyond the popularized studies—at collections of all the data—you often find that the differences are minimal."

According to Eliot, meta-analyses by other investigators have disproved other purported sex differences in the brain. She says that previous analyses have found that there is no difference in the size of the corpus callosum, which consists of white matter that allows the two sides of the cerebrum to communicate. Men and women also do not appear to differ in the way their left and right hemispheres of the cerebrum process language.

Source: Triff/Shutterstock

Another October 2015 paper, “Structural and Functional Neuroplasticity in Human Learning of Spatial Routes,” also published in the journal NeuroImage, reported that brief spatial navigation training changes a person's brain tissue and improves how that changed tissue communicates with other brain areas involved in spatial navigation for both men and women.

This discovery by reseachers at Carnegie Mellon University (CMU) establishes a critical link between structural size and functional connectivity brain alterations that occur in the hippocampus during spatial learning. Importantly, this study also illustrates that brain changes linked to spatial learning are related to how neural activity synchronizes the communication between the hippocampus and other regions necessary for navigation.

Neurogenesis and the Hippocampus

In a 2010 study, researchers found an association between physical fitness levels and the hippocampus size of 9 and 10-year-old boys and girls. The children who were more fit had a larger hippocampus and performed better on a test of memory than their less-fit peers, regardless of gender.

In a press release, Arthur Kramer Director of the Beckman Institute said, "This is the first study I know of that has used MRI measures to look at differences in brain between kids who are fit and kids who aren't fit." Kramer conducted this study with University of Illinois at Urbana-Champaign colleagues Laura Chaddock-Heyman and kinesiology and community health professor Charles Hillman.

When the researchers analyzed the MRI data, they found that the physically fit children tended to have bigger hippocampal volume. In fact, the hippocami were about 12 percent bigger relative to total brain size than their out-of-shape peers. The children who were in better physical condition also did better on tests of relational memory—the ability to remember and integrate various types of information—than their less-fit peers.

Other studies in older adults and in animals have also shown that exercise can increase the size of the hippocampus. A larger hippocampus is associated with better performance on spatial reasoning and other cognitive tasks. "In animal studies, exercise has been shown to specifically affect the hippocampus, significantly increasing the growth of new neurons and cell survival, enhancing memory and learning, and increasing molecules that are involved in the plasticity of the brain," Chaddock-Heyman said in a press release.

Conclusion: Brain Size and Functional Connectivity Are Ever-Changing

Regardless of the innate size or functional connectivity of your hippocampus at birth, neuroplasticity and neurogenesis (the growth of new neurons) make it possible to alter both the size and functional connectivity of your hippocampus via long-term daily lifestyle choices throughout your lifespan.

For example, getting enough sleep, or taking a power nap, have been found to help the hippocampus consolidate memories. Playing action video games has been shown to improve brain structure and function. Unfortunately, growing up in poverty has also been correlated to reduced gray matter volume of the total brain, frontal lobe, temporal lobe, and hippocampus. Also, chronic stress can damage brain size and functional connectivity in both male and female children and adults.

Practicing new skills that engage spatial navigation alter both the size and connectivity of specific regions of the hippocampus to other brain regions. Also, regular physical activity can optimize gray matter brain volumes in various brain regions as well as the functional connectivity and integrity of white matter tracts in both sexes.

Ultimately, the evidence is clear—lifestyle factors and long-term daily habits impact brain size and functional connectivity more than whether you are male or female.

If you'd like to read more on this topic, check out my Psychology Today blog posts,

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