The Cerebellum Flourishes in Spring but May Shrink in Winter

Source: Alberto Andrei Rosu/Shutterstock

Does your brain seem to work differently during seasonal transitions from hotter to colder months and vice versa? If you live somewhere with four seasons, does your brainpower seem to fluctuate as summer gives way to shorter and colder days or when winter fades and "April showers bring May flowers?"

In tandem with Pumpkin Spice Lattes making their return each autumn, September and October in the Northern Hemisphere often feel like a time when a back-to-school mindset takes hold in our brains. As carefree summer attitudes dissipate in the fall, the human brain seems to recalibrate in ways that foster more cerebral pursuits.

Interestingly, a recent neuroscience study (Lim et al., 2018) into the "seasonal plasticity of cognition" found that older adults tend "to have better thinking and concentration in the late summer and early fall than in the winter and spring."

Another recently published 15-year study (Book et al., 2021) of how weather and seasonal changes affect brain volume in a large cohort of people living in the Northeastern United States unearths some surprising discoveries. These open-access findings were published on March 24 in the peer-reviewed journal PLOS One.

In a significance statement, the authors write: "Changes in brain volume due to natural environmental variation have not been reported before and may have implications for weather-related and seasonal ailments."

For this exploratory cross-sectional analysis of the effect of season and weather on the human brain, researchers in Hartford, Connecticut, led by Gregory Book, conducted MRI brain scans of 3,279 healthy individuals during different months/seasons across a 15-year span.

For reasons that remain unclear to scientists, seasonal brain volume changes in regions of interest (ROIs) above and below the tentorium cerebelli's dividing line occur in opposite directions from January to June and July to December.

Source: Wikimedia Commons/Public Domain

Unlike Cerebral Brain Regions, the Cerebellum Shrinks as Winter Approaches

Notably, the researchers found that for people living in this New England state, numerous brain regions of interest (ROIs) in the cerebrum got bigger from late summer into winter and smaller as spring transitioned to summer. Conversely, brain volume in the cerebellum tended to move in the opposite direction; on average, the cerebellar cortex got bigger in the summertime and smaller in wintertime.

Much like the equator divides the Northern and Southern hemispheres on planet Earth, the tentorium cerebelli is a dividing line that delineates the cerebral and cerebellar hemispheres within the cranial globe. Book et al. found that infra-tentorial regions below this line, in the cerebellum, got noticeably smaller as New Engander's were heading into winter.

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. This figure demonstrates the trend peaks at different times of year but is only for illustrative purposes, and not all comparisons between months were statistically significant.

Source: Book et al., 2021 PLOS One/Open Access (CC BY 4.0)

On the flip side, supra-tentorial regions above this line, in the cerebrum, grew larger as the weather got colder and days became shorter, as depicted in this graph. The authors explain:

"Left- and right- cerebellum cortex increased in volume between January and June and decreased between July and December. The tentorium acted as a divider between effect direction, with changes from summer to winter months being positive for supra-tentorial regions of interest and negative for infra-tentorial ROIs."

Changes in Barometric Pressure May Be Linked to Brain Volume Changes

Notably, another animal study (Lázaro et al., 2021) of seasonal changes in the mammalian brain found that when bad weather is approaching and barometric pressure drops, supra-tentorial regions tend to gain volume; the approach of winter has a similar effect. However, brain volume in the cerebellum and brain stem (infra-tentorial regions) changed in the opposite direction.

"A possible explanation of brain volume changes is from a change in blood flow as previous studies have found a seasonal effect on ambulatory blood pressure. Blood flow associated with barometric pressure may also offer an explanation," Book et al. write. "However, this does not explain why the cerebellum follows a different pattern from the rest of the brain. Differences may be due to the blood vessels supplying those two regions of the brain, in that the cerebellum and brain stem have a different blood supply than the cerebrum."

In an October 2021 Discover article about this research, Book reiterated that weather-related changes in atmospheric pressure, typically higher in winter and lower in summer, may be linked to fluctuations in brain volume. However, he reiterates that barometric pressure alone doesn't seem to explain why the cerebellum follows a different volume-changing pattern than other parts of the brain.

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Circadian rhythm and vitamin D from sun exposure during daylight hours might also play a role in seasonal brain volume shifts. From an evolutionary perspective, the cerebellum is one of our oldest brain regions and has primordial functions that have been preserved since time immemorial. Seasonal changes in cerebellar volume might be an ancestral remnant held over from a bygone era.

In closing, Book and his coauthors note that replication of their analysis should be retested by scanning a single test subject, or group of subjects, on a daily basis throughout an entire year.

"Investigating the biological cause of such large volume changes may be clinically relevant, including why volume changes are observed in opposite directions in the supra-tentorium vs. infra-tentorium," Book et al. conclude. "Investigating these changes further may be informative for seasonal disorders or discover previously unknown seasonal effects on other diseases."