Researchers at the GIGA-CRC-In Vivo Imaging Laboratory of the University of Liege, in collaboration with scientists at the Surrey Sleep Research Centre, in England, conducted a study on the seasonal aspect of human cognitive functions in strictly controlled conditions. This work, whose first author is Doctor Christelle Meyer, was published on February 8th 2016 by the American journal Proceedings of the National Academy of Sciences of the United States of America (PNAS) (2).

The researchers measured the brain activity of 28 healthy male and female volunteers around the age of twenty at different times of the year. During their visit to the laboratory, the participants were deprived of any seasonal bearings. Therefore they had no access to daylight, Internet, telephone and, more broadly, any information from the outside. The experiment lasted a total of four and a half days during which they lived their lives in normal conditions. The conditions were practically constant during the last two and a half days. “The light in the laboratory was dimmed while the temperature and other parameters that usually vary with the seasons were kept constant. The subjects slept for 8 hours and then had to stay in bed in a semi recumbent position for 42 hours during which they were deprived of sleep and they took a protein snack every two hours. Then they slept again for 12 hours before being taken by stretcher to a functional magnetic resonance imaging (fMRI) apparatus one hour after waking”, explains Gilles Vandewalle.

Solstices and equinoxes

After being allowed to recuperate from their 42 hour period without sleep, they were asked to complete two tasks during which their brain activity was recorded. The first task was a simple one called the Psychomotor Vigilance Task and was attention-based. The subjects had to press a button as quickly as possible after a chronometer was randomly triggered. In the second task, called 3-back, the participants were enunciated letters one after the other. When they heard a letter, they were required to indicate whether the letter was the same as the letter suggested three steps before. For example, when the fifth letter was enunciated, they had to say if it was the same as the second. This was therefore a memory task requiring the intervention of executive functions, high-level cognitive processes which enable us to adapt to our environment when routine actions are not sufficient. On a cognitive level, the second task was evidently more complex to perform than the first. The subjects had to remember previous information, to inhibit non-relevant data and to make comparisons…

The question was: would cerebral activity fluctuate according to a seasonal rhythm while the tasks were being performed? The response was positive. Although the performance level of the subjects remained constant during the year, their brain was significantly affected by the seasons. In the attention-based task, the variations were to be seen in the thalamus and the amygdala, regions that are involved in vigilance, as well as the frontal areas and the hippocampus, structures that intervene in executive control. Seasonal variations were also detected in the globus pallidus, the parahippocampal gyrus, the fusiform gyrus, the supermarginal gyrus and the temporal pole. “All the areas called on to contribute were affected in the same way with regard to their functioning”, explains Gilles Vanderwalle. “In other words, they presented the same variations in activity under the influence of the seasons”.

(2) Christelle Meyer, Vincenzo Muto, Mathieu Jaspar, Caroline Kussé, Érik Lambot, Sarah Laxhmi Chellappa, Christian Degueldre, Éveline Balteau, André Luxen, Benita Middleton, Simon N. Archer, Fabienne Collette, Derk-Jan Dijk, Christophe Phillips, Pierre Maquet et Gilles Vandewalle, Seasonality in human cognitive brain responses, PNAS 2016 Feb 8. http://www.pnas.org/content/early/2016/02/04/1518129113.abstract .