To find out, Gilles Vandewalle and colleagues at the University of Liege in Belgium scanned the brains of 28 volunteers while they performed attention and working memory tests at different times of the year. To ensure the results were influenced by the seasons rather than the environmental conditions on the test day, the participants were confined to a lab for 4.5 days prior to the test, exposed to a constant light level and temperature.
Although their test scores didn’t change with the seasons, activity in some brain areas showed a consistent seasonal pattern among the volunteers: brain activity peaked in the summer on the attention task and in the autumn on the memory task.
Many seasonally changing factors could regulate such a pattern, including day length (known as photoperiod), temperature, humidity, social interaction and physical activity. Since these weren’t all controlled for in the study, it’s impossible to say what is responsible for the seasonal changes seen.
“In our data it seems that photoperiod, or the rate of change of photoperiod, was more likely to explain what we were seeing. But we can’t exclude all the others,” says Vandewalle.
Surprising seasonality
The results suggest that over the course of a year, the brain might work in different ways to compensate for seasonal factors that could affect its function, enabling it to maintain a stable performance. Vandewalle speculates that these mechanisms might not work as well in some people, for example, those vulnerable to the winter blues.
There have been many studies showing seasonal patterns in human behaviour, sometimes in surprising ways. Suicide rates peak in the spring in many countries, not when the days are shortest, as one might expect. Violence against others peaks in the late summer months in England, and about a month later in the US.
In light of these facts, it’s not too surprising that human cognition may vary across the seasons, says Russell Foster at the University of Oxford. “The problem with all these observations is that we have little or no idea regarding mechanism,” he says.
Even the well-known phenomenon of winter depression is murky. Seasonal affective disorder used to be considered a unique mood disorder by the Diagnostic and Statistical Manual of Mental Disorders, but the latest edition marks it as a factor in major depressive disorder. A recent study cast further doubt on its importance, finding that seasonality wasn’t a major factor in depression scores in a sample of 35,000 people in the US.
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