Seasonal variations play a major role in the animal kingdom—in reproduction, food availability, hibernation, even fur color. Whether this seasonality has such a significant influence on humans, however, is an open question. Its best-known association is with mood—that is, feeling down during the colder months and up in the summer—and, in extreme cases, seasonal depression, a phenomenon known as seasonal affective disorder (SAD).

A new study published in this week’s Proceedings of the National Academy of Sciences seeks to delve deeper into how human biology has adapted not only to day/night cycles (circadian rhythms) but to yearly seasonal patterns as well. Scientists have previously found seasonal variation in the levels and concentrations of certain compounds associated with mood (including dopamine and serotonin), conception and even mortality. Now for the first time, using functional MRI, “it’s [been] conclusively shown that cognition and the brain’s means of cognition are seasonal,” says neuroscientist Gilles Vandewalle of the University of Liège in Belgium, the study’s lead researcher. These findings come at a time when some scientists are disputing the links between seasonality and mental health.

Originally aiming to investigate the impact of sleep and sleep deprivation on brain function, Vandewalle and his fellow researchers placed 28 participants on a controlled sleep/wake schedule for three weeks before bringing them into the laboratory, where they stayed for 4.5 days. During this time they underwent a cycle of sleep deprivation and recovery in the absence of seasonal cues such as natural light, time information and social interaction. Vandewalle’s team repeated the entire procedure with the same subjects several times throughout the course of nearly a year and a half.

At each session, the researchers measured their subjects’ brain activity during two different tasks: The first probed sustained attention by having participants press a button as soon as a stopwatch started counting on a screen. The second assessed working memory, a more complex function that requires information storage and decision-making, by presenting a series of letters to participants and asking them to state whether each letter was the same as the letter that had appeared three turns earlier.

The researchers then used fMRI to look for any consistent seasonal changes in brain activity. They found that although overall performance on the tasks remained the same, the “cost of cognition,” or in other words, the neural resources available or used in cognition, varied with time of year. Most notably, brain activity involved in sustained attention (especially in the thalamus, amygdala and hippocampus, among other regions) was highest in the summer and lowest in the winter. This pattern shifted by three months for brain activity associated with working memory (including responses in the pulvinar and insula as well as the prefrontal and frontopolar regions), a higher executive function that allows us to recall and manipulate information, which was higher in the fall and lower in the spring.

Anna Wirz-Justice, a chronobiologist at the University of Basel in Switzerland who did not participate in this research, finds the results exciting. “What’s new about the fMRI study is that it’s moving from normal sort of psychological and behavioral measures to really looking at cognition and important higher functions of the brain,” she says. “We’re just like hamsters, really. Even though we live in an artificial environment with light cycles that are no longer seasonal, we have the programming in our brains to respond to seasonality,” she adds.

Vandewalle’s next goal is to study seasonality at the neuronal level and observe its manifestation in other cognitive functions and environments where more experimental control is not as high or, conversely, where more time is allowed for in-laboratory testing. “We showed that at the system level, brain areas are interacting with each other, and this in turn relates to performance,” he says. “But we don’t know what’s underneath.”

This research is particularly relevant as it relates to groups of people who are susceptible to the seasons, including those who suffer from SAD (and mental illnesses such as bipolar disorder and schizophrenia, which sometimes have shown seasonal fluctuations as well), a link that has recently come under contention. Last month in Clinical Psychological Science researchers from Auburn University at Montgomery published the results of a population-based study questioning whether there is any validity to linking major depression with seasonal variation.

“It’s difficult to establish seasonality,” Vandewalle says, “but here in our study we had the chance to strictly control protocol for almost five days, so I’m really confident in our results.”