Although light influences mood, it is not known what cerebral mechanisms are involved. The commonly-held theory is that the effect observed results from the direct impact of light on biological rhythms. What process is involved in this? It is known that light information is transmitted to the suprachiasmatic nucleus (the internal biological clock) by the ganglion cells. Once ‘stimulated’, this nucleus acts on different structures of the hypothalamus and on the pituitary, but also after following a complex pathway, on the pineal body, a structure that produces in the brain. Also called the ‘hormone of the night’, this facilitates the onset of sleep. Darkness and dim light cause it to be secreted, while a light level of middle-intensity (higher than around 100 lux) blocks it. In addition, melatonin changes the periodicity of our biological clock. During winter, the season with the longest periods of darkness, it increases the period, while it shortens it in summer. “This is still only a theory, however”, says Gilles Vandewalle.

When the brain prefers blue

In any event one question needs to be asked: could this indirect mechanism, which causes an individual to feel better in summer because his biological rhythms are better at that time, be the only mechanism by which light influences mood? Could an alternative (direct) mechanism or at least a complementary one exist? This is what Gilles Vandewalle and the other Belgian, Swiss and English authors demonstrated in their article which appeared in the journal PNAS in 2010.

They showed by means of experiment, that blue light does indeed affect the organisational workings of the brain during the processing of emotions. Compared with green light, to which the cones and rods are more sensitive, it increases the brain response to emotional auditory stimuli in the ‘voice area’, a region located in the temporal cortex which decodes vocal information and its emotional containt, and in the hippocampus, a structure that underlies certain aspects of our emotional reactions and memory. Furthermore, during the processing of emotions, blue light selectively strengthens the functional connectivity between the ‘voice area’, the amygdala and the hypothalamus. The amygdala is a very important structure for emotional responses and mood regulation while the hypothalamus is involved not only in the emotional response, but also in the regulation of biological rhythms and sleep. There is therefore one network which integrates emotion and light information.

Previously, several authors had discovered that blue light has a direct impact on certain regions of the brain involved in work memory and, more broadly, in cognition.

But how can the involvement of melanopsin-expressing ganglion cells in cognition, emotions, waking etc. be formally proved? While it has been demonstrated that in various processes in animals, particularly following genetic manipulations depriving newly-borns of cones and/or rods and/or melanopsin-expressing ganglion cells, the theories put forward for humans were only based on indirect arguments. In particular, the superiority of blue in relation to other colours to cause certain brain responses.