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For people living in higher latitudes, distinct seasons are a fact of life. A verdant spring gives way to a hot and humid summer, which simmers to a picturesque fall filled with painted leaves, and finally leads to a cold, dark winter.

According to a recent study published in PLoS ONE, these broad shifts in weather may also be paired with a curious change in brain size.

Researchers primarily based out of the Olin Neuropsychiatry Research Center and Yale University analyzed brain scans of 3,279 healthy adults aged 18-65 taken between August 2003 and October 2018. Subjects mostly hailed from the Northeast United States. The scientists explored how the average volumes of subjects' brains changed depending upon the time of year.

Given the sheer amount of data, the scientists discerned numerous statistically-significant changes in various brain regions, but that was to be expected. In any study with a large sample size where lots of variables are being parsed, there will be significant effects simply by chance alone. What the researchers were really looking for is a clear and significant signal – do any regions of the brain clearly change in size as the seasons progress through their unceasing cycle?

The scientists spotted such a trend in the size of the cerebellum, a significant region in the hindbrain which "plays an important role in motor control and may also be involved in some cognitive functions such as attention and language as well as emotional control such as regulating fear and pleasure responses."

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Fig 2. Monthly trends in percent difference from annual mean in left- and right-cerebellum cortex, and left- and right-cerebellum white matter, in males and females.

As is fairly apparent from the above graph, the cerebellum tended to grow in warmer months and shrink in colder ones.

Numerous mental conditions, most notably seasonal affective disorder, are associated with changing seasons. It's possible that changes to brain size could play a role, the researchers say.

While fascinating, these results are decidedly preliminary. The researchers suggested how best to proceed with future research.

"Replication of the changes found in this analysis would be best tested using a single subject, or set of subjects, scanned daily throughout an entire year. Such data would confirm whether the effects seen in this study are a biological effect or are only found at a group-level in a heterogeneous group."

Source: Book GA, Meda SA, Janssen R, Dager AD, Poppe A, Stevens MC, et al. (2021) Effects of weather and season on human brain volume. PLoS ONE 16(3): e0236303. https://doi.org/10.1371/journal.pone.0236303

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This article originally ran on realclearscience.com.

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