“It was quite complex, yet sufficiently simple for a trained individual to reach what is called a ceiling level, that is to say so that he can improve his performance any further. Indeed, significant changes in behaviour (variation in test results) would have interfered with the measurements we wanted to carry out”.

In this experiment, where brain activations were observed by FMRI technology and the results of which were published in March 2014 in the journal PNAS(3), the subjects were initially exposed to red light and then blue light, then after 70 minutes spent in darkness, were exposed to green light while they completed the 3-back test in the scanner. “The reasons the participants were plunged into darkness for more than one hour was to remove all influence of the initial light on the cones and rods, so that we could assess the impact that the light previously  later had on the melanopsin”, continues Gilles Vandewalle.

Photic memory

The researchers’ theory was that the color to which the participants had been initially exposed was going to influence the intensity of the brain responses involved in the cognitive task carried out under the light-test (green light). In other words, in reference to Howard Cooper’s theory, prior exposure to red light (maximum regeneration of melanopsin) should elicit the strongest response, in blue light the least intense response and in green light, an intermediate response. This is exactly what happened in the regions of the brain involved in the task: certain pre-frontal regions, the thalamus and amygdala. “Cooper’s theory works, says Gilles Vandewalle. “We have been able to demonstrate that the melanopsin-expressing ganglion cells play a role in the impact that light has on non-visual brain activity”.

The term ‘photic memory’ is used to describe this phenomenon of the persistence of an exposure to light on the response during the following light exposure. This is typical of certain photopigments that are to be found in invertebrates and plants, it is also one of the attributes of melanopsin.

In everyday life, the light that appears white to us is made up of different wavelengths. In other words, blue light, yellow light etc. Daylight is rich in blue light, that for which melanopsin has a particular affinity. On the other hand, the light emitted by bulb filaments favors wavelengths that are characteristic of yellow light. Recent work on the role of melanopsin-expressing ganglion cells argues in favor of the design of light systems where blue is dominant. This is one way, seemingly, of favoring cognitive performance and combatting mood problems. 

(3) Sarah L. Chellappa, Julien Q.M., Christelle Meyer, Évelyne Balteau, Christian Degueldre, André Luxen, Christophe Phillips, Howard M. Cooper, Gilles Vandewalle (2014), Photic memory for executive brain responses, Proceedings of the National Academy of Science in the USA (PNAS).