Smell changes how we see colour

No end of research shows that input from one sense can influence perceptions in another. Coffee drunk from a white mug tastes nearly twice as bitterly intense as coffee drunk from a clear mug, for example, and we are prone to perceiving an orange-coloured, cherry-flavoured drink as tasting of orange. 

Now, new work led by Ryan J. Ward at Liverpool John Moores University, published in Frontiers in Psychology, reveals a new cross-modal perception: odours can affect our perceptions of colours.

The team studied 24 people aged between 20 and 57 who self-reported to have no impairment in their sense of smell or colour vision. While each participant sat in a dimly lit room that had been purged of odours using an air purifier, the team used a diffuser to repeatedly pump in a series of five odours: caramel, cherry, coffee, lemon, and peppermint, plus water as an odourless control. At the same time, the participant was shown a colour patch on a screen. Using two sliders, they had to adjust the level of green, red, blue, and yellow in the patch until it resembled a neutral grey. 

When Ward and colleagues analysed the results, they found that when water had been released from the diffuser, the participants created colour patches that were indeed objectively grey. However, the 'greys' created while they smelled each of the odours were a little different. 

In earlier work, Ward and two of the co-authors of this new paper had found that people fairly consistently link these five particular odours to specific colours: caramel with dark brown-yellow; coffee with dark brown and red; cherry with pink, red and purple; peppermint with blue and green; and lemon with yellow, green and pink. The analysis of the new data revealed that, with the exception of peppermint, the odours slightly shifted the colour that the participants perceived to be grey towards the odour's matching associated colour. So, caramel led to a shift in grey towards a yellow-brown, cherry and coffee led to a shift towards red-brown and lemon led to a shift towards yellow-green. (Interestingly, peppermint led to a shift towards brown-red, which was not the colour association found in the earlier research.) 

It's not clear what exactly underpinned these shifts. A "controversial" explanation, to use the researchers' own description, stems from the concept of cross-modal harmony. If the participants' brains expected harmony in input from their senses of smell and of vision — that is, to see the colours typically associated with each of odours — this might have affected what they saw. 

The reported effects were small, and the study does have some limitations. One is that the team didn't gather ratings of the irritancy, intensity, or perceived pleasantness of each of the odours, which may have affected colour judgements. Also, although the participants were asked to try to identify each of odours, the group wasn't big enough for the researchers to explore statistically whether this had any impact on the nature of the 'greys' that were created. 

Still, they write, the results suggest "a small but systematic effect of the presence of odours on human colour perception." And given the abundance of findings in the field of cross-modal sensory perception, it does seem reasonable to think that specific odours may indeed influence the colours that we see. Further work is now needed to explore this in more detail, with bigger groups of participants. 

Read the paper in full: https://doi.org/10.3389/fpsyg.2023.1175703