It's the second week of February, and for many of us in the Northern Hemisphere that means we're hitting the limits of our ability to cope with winter. It's hard to get excited about anything—even hot chocolate—when the cold and dark seem to roll on forever.
We know that our mood can ebb and flow with the seasons. About 20 percent of Americans suffer from winter blues or full-blown seasonal affective disorder. But it's not just our mood that changes. Depending on the time of year, our brains may need to work harder to sustain certain cognitive processes, a recent experiment indicates.
When scientists in Belgium and the United Kingdom asked people to perform tasks requiring them to pay attention or work with new information, they did just as well in any season. But the brain activity behind these abilities rose and fell at different points in the year, the researchers reported today in the journal Proceedings of the National Academy of Sciences.
"Our brain is not the same with season," said coauthor Gilles Vandewalle, of the University of Liège in Belgium. "For most people this is likely not perceived. But potentially for people more vulnerable to season…what we see may contribute to the negative impact of season on mood. Depression depends on many parameters and what we see may be one of them."
Vandewalle and his team asked 28 healthy, young people in Belgium to spend four and a half days in a laboratory, staggered at different points between May 2010 and October 2011. After several days cloistered away from any season-dependent cues like sunlight, the participants had their brains scanned with fMRI during different activities.
In one task, people had to hit a button whenever they saw a stopwatch appear on a computer screen; this tested how well they could pay attention. In another, the participants had to listen to a stream of consonants and report whether a particular sound was the same as one they'd heard three noises earlier. This tested their ability to store, update and compare information in their short-term memory.
How people did on these tests did not seem to have any connection to what season it was outside. But how their brains responded to these challenges did vary.
While engrossed in the attention-related test, certain regions in people's brains crackled with the most activity in June and the least near the winter solstice. Among these areas were the thalamus and amygdala (which are involved in alertness), the frontal areas and hippocampus (which play a role in executive control, or the regulation of various thinking processes) and the precuneus (which plays a role in visuospatial attention).
While trying to recall and compare consonants, people's brains followed a different pattern, showing more activity in fall and less near the spring equinox. As in the attention task, frontal areas and the thalamus were involved. The insula, which plays a role in attention, executive processes and mood regulation, was also more active in autumn.
The researchers didn't find any variations in the participants' hormones, alertness or sleep that would explain the seasonal patterns of brain activity. The team concluded that the brain doesn't draw on the same resources year-round.
When brain areas involved in attention fire up near the summer solstice, one possible explanation is that more resources are free to be tapped, making the task easier. By contrast, "Lower activation in winter can mean that the brain is less efficient and that it will be more difficult to do the task, and in a way more costly," Vandewalle said. In winter, the brain might have to pull from other resources to get the same performance.
It's not clear why this is, though. Different messenger molecules might be more or less available at some times of year. Some studies have found more serotonin in people's cerebrospinal fluid and blood in summer, and more dopamine in fall, the researchers noted. A protein called brain-derived neurotrophic factor, which is involved in learning, also gets a boost in fall.
"With fMRI we only have access the activity that fluctuates with the task," Vandewalle said. "We don't know if baseline brain activity changes...So other parameters of brain function that we did not measure may fluctuate as well."
The many ways that seasons affect us deserve more investigation, he added. Our brain's seasonal calendar can't make us migrate or hibernate, like some mammals do. But studies have found that seasons might play a role in when people conceive, die or commit suicide. Seasons also influence when our brains flip on certain genes and how much we eat. And though we might not notice it, seasons might cause shifts in our blood pressure and cholesterol.
"If the brain is seasonal, other aspects of physiology may also be," Vandewalle said.