FEELING BLUE

As numerous Facebook statuses will attest, we've been out enjoying the first tantalising glimpses of the great British summer. Long, sunny days spent lazing in the park with a beer in hand are bound to have elicited a mood-boosting effect for most of us. 

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Bathing in high-intensity ambient light is well known for its positive effect on mood – light therapy is a pretty standard treatment for seasonal affective disorder and also has a positive (albeit modest) effect on non-seasonal depressive disorders, such as bipolar (1). Conversely, sitting indoors under the glare of standard electric light can be a sapping experience (attributed in part to the lower intensity of light at the blue end of the spectrum emitted by incandescent bulbs compared to sunlight).

Mood is generally a prolonged emotional state, something that manifests itself slowly and ebbs away equally slowly. Clearly the environment has time to play a role in this facet of our mental health. But what about our emotions - feelings that are much more transient and specific? Could they also be influenced by light quality?

The short answer appears to be yes. Gilles Vandewalle and his team of researchers at the University of Liege, Belgium used functional MRI to compare regional brain activation of a group of 17 20-somethings. The willing guinea pigs listened to nonsense words (removing any contextual priming of emotion) spoken by angry or neutral voices. Simultaneously, they were subjected to short bursts of blue or green ambient light (2).  

The team found that the degree of brain activation and connectivity between the voice area of the cortex, the hypothalamus (neurohormone release) and the amygdala (the emotional processing centre) were higher when the subjects were exposed to blue relative to green light.  In essence, the brain found the angry voices 'angrier' when it was also receiving the blue stimulus.  And this was transient – the heightened emotional response diminished once the blue light was removed.

But why were the researchers comparing just blue and green light – what about the rest of the visual spectrum?  The classical visual system, mediated by rods and cones (light-sensitive receptor cells) of the retina at the back of the eye, is maximally activated by green wavelengths of light (~530 nm). However back in 2007, a new type of photoreceptor was discovered in humans – melanopsin (3). This exhibits peak excitation under much shorter, blue wavelengths of light (~480 nm) and appears to form part of a parallel light-sensitive pathway.

Amazingly, cells expressing melanopsin enable blind people with damaged rods and cones to sense light, ensuring the non-image forming functions of visual processing – sleep patterns, hormone release and behaviour – remain intact. It was speculated that as well as being responsible for these large-scale physiological processes, this receptor might also play a role in our emotional response to light. Clearly Gilles' investigation appears to back this up.  And interestingly, this alternative system bypasses the V1-V4 cortical regions (responsible for basic visual processing, such as determining colour and shape) of the classical visual pathway and heads straight for the emotional processing hotspots.* 

So could our transient everyday exposures to blue light actually be messing around with our emotions?  Does a flashing blue light whizzing past on a police car or gazing at a blue-hued painting actual touch us at a deeper emotional level that if the colour was, say red?  Unlikely. What is clear from the research is that the light source needs to be immersive.  You need to be bathed in the light for it to have any effect as melanopsin is sparsely distributed across the retina, so a focussed beam of light is unlikely to excite enough receptor cells. It does however provoke some interesting questions about how we light our home and office spaces, and the effect this has on our personal interactions.

The idea of blue as an emotional stimulant may come as a surprise to some colour psychologists. Blue is often lauded for its calming, contemplative properties. But perhaps this is more to do with association. Work by Steve Palmer at the University of California has shown that we prefer colours that are associated with objects we like.  In the case of blue – sky, clean water etc. - would appear to be pretty universal triggers – reminding us of the relaxing nature of an expansive outdoor view (4).

Looking at the bigger picture, Gilles’ research and the work on melanopsin are great examples of how such a well documented part of human anatomy – the eye and the visual system – can still throw scientists a few curve balls.

*subsequent, unpublished studies have shown that melanopsin may also activate the classical pathway to a modest degree.

1. Tuunainen A, Kripke DF, Endo T. "Light therapy for non-seasonal depression". Cochrane Database of Systematic Reviews 2004, Issue 2. Art. No.: CD004050 (2): CD004050.

2. Vandewalle G et al. Spectral quality of light modulates emotional brain responses in humans. PNAS 2010 Nov 9;107(45):19549-54.

3. Van Gelder RN. Non-visual photoreception: sensing light without sight. Curr Biol. 2007 Dec 18;17(24):2122-8.

4. Palmer, S. & Schloss, K. An ecological valence theory of human color preference. PNAS 2010.