We are used to thinking about our eyes as organs for vision. They are light-sensitive organs that allow us to create images of our surroundings, helping us navigate our environment; cones and rods, the photoreceptors at the base of the retina, make this possible. If we think a little past this vision function, we may contemplate the importance of photoreception for the regulation of circadian rhythms and the sleep–wake cycles, a function mediated by another type of photoreceptor located in the retina, the intrinsically photosensitive retinal ganglion cells (ipRGCs), which express the photopigment melanopsin. However, very few of us will consider increasing cognitive function as one of our eyes' functions, let alone the role of melanopsin as a cognitive stimulant. This is probably because the evidence supporting the role of this photopigment in the facilitation of cognitive processes has been only indirect; that is, until now. A recent study published in the Proceedings of the National Academy of Sciences by Sarah Chellappa and her colleagues at the University of Liège in Belgium and the French Institute of Health and Medical Research in France provides support for the hypothesis that melanopsin is indeed involved in our capacity to remember things in the short term.
Melanopsin exists in two different states; a photosensitive or ‘active’ state and a photoinsensitive or ‘passive’ state. Long wave light (i.e. orange) triggers the transformation of melanopsin from the photoinsensitive to the photosensitive state, while shorter wavelengths (i.e. blue) do the opposite. This means that orange light increases the amount of ‘active’ melanopsin units in the retina while blue light increases the number of ‘inactive’ units. Chellappa and her colleagues used this information to try to understand whether melanopsin does indeed influence cognitive brain function. If so, exposure to orange as opposed to blue light would increase performance of cognitive tasks because a larger proportion of photosensitive melanopsin would be present in the retina.
To test the effects of different light wavelengths on cognitive function, the researchers exposed 16 participants to 10 min of blue or orange light. The volunteers were subsequently blindfolded for a period of 70 min and then asked to perform memory tasks while in a magnetic resonance imaging (MRI) scanner, which allowed the scientists to see what parts of the brain were stimulated while the participants performed the tests. The MRI scans showed that, compared with blue light, pre-exposure to orange light had a significantly higher impact in several regions of the prefrontal cortex and other regions of the brain crucial for the regulation of cognition, arousal and even emotional processes. Although more research is required, these results suggest that melanopsin is involved in the facilitation of cognitive processes and that the impact of light on cognition rises with orange light by increasing the ‘active’ state of the photopigment.
Our eyes do a lot more than seeing and helping us learn is one of these functions. With the help of orange light our eyes might help us to better retain information and perhaps, in the long term, improve our cognitive abilities. Perhaps next time we change the lighting for our office we should choose orange lights.
- © 2014. Published by The Company of Biologists Ltd