A similar phenomenon was observed during the memory task, where the researchers detected the impact of the seasons on the functioning of the thalamus, the prefrontal and fronto-polar cortex as well as the insula, a region concerned with executive processes, attention and regulation of the emotions.

However, there was an unexpected element: the seasonal rhythms demonstrated with regard to the cerebral resources used to carry out the two tasks were not the same. For the attention-based task, brain activity reached its maximum level in June, around the time of the summer solstice, and its minimum level in December around the time of the winter solstice; for the executive task (short-term memory), it reached its maximum level around the time of the autumn equinox (September) and its minimum level around the time of the spring equinox (March).

It is important to note that complementary analyses made it possible to rule out any connection between these results and neurophysiological variations in the waking state, the quality of sleep or endocrine fluctuations in the level of melatonin, the “circadian sleep hormone”.

More questions

The rhythm associated with the accomplishment of the attention-based task seems to correlate to the photoperiod, that is to say, the division, during the day, between the diurnal phase and the nocturnal phase. “It’s one of two things”, says Gilles Vandewalle. “Either we have a memory of the photoperiod which extends beyond several days, or human beings possess a veritable seasonal rhythm that is independent from the photoperiod and is endogenous similarly to the circadian rhythmicity which modulates different physiological or cognitive functions. In this second theory, external factors only serve to re-establish or re-entrain the rhythm. An example of the occurrence of this would be when we travel to the southern hemisphere where the seasons are inverted by comparison to our hemisphere”.

The performance to the executive task was not correlated to the photoperiod. On the other hand, it was strangely correlated to the daylight duration differential. The more quickly daylight duration diminishes or increases, the more cerebral activity reaches its optimal or minimum level. Could there be a causal element here? It remains a mystery, according to Gilles Vandewalle. “As for the photoperiod, however, factors such as the temperature of the air and humidity vary in parallel with the length of the day, to the extent that they can also contribute to the seasonal aspect of cognitive functions”, point out the authors of the article published by PNAS on February 8^th.

One of their theories was that the variations in seasonal rhythms are more marked in the case of a basic task such as the Psychomotor Vigilance Task than in a more complex cognitive task. In the latter case, the cognitive processes are based on an increased number of variables such as, for example, social interactions. The fact that a somewhat reduced number of areas of the brain are involved in seasonal response to the work memory task in relation to the attention-based task could add weight to this theory.

It remains to confirm these results on other populations, to test other cognitive functions and to more thoroughly address the relationship between performance and the “cost of cognition”, notably by means of more mentally demanding tasks, to study the influence of the subjects’ age on the nature and extent of seasonal rhythms, but also to gain an insight into the secrets and fine workings of this seasonal aspect to cognitive functions which seem to have been brought out of the shadows and into the full light of day by the researchers at the University of Liege.