Our brain cannot see, hear or feel inside our head. For that, it relies on sensory signals from the world around us.
However, processing all those signals at once is challenging for the brain. Therefore, to manage this barrage of information, it resorts to ‘crossmodal association,’ that is, linking two or more signals at the same time. For example, it processes smells and textures together or associates certain scents with specific food flavours.
That much was known. However, when researchers at Liverpool John Moores University, UK, studied this interplay of senses in greater detail, they found something new. They saw that smell can change the way we perceive colour. “Here we show that the presence of different odours influences how humans perceive colour,” said Dr Ryan Ward, lead author of the study, in a statement.
The study was published in Frontiers in Psychology on 6 October.
From smells to shades
In a previous study, Dr Ward had found that specific odours were associated with certain colours. For instance, caramel was associated with dark brown and yellow; coffee with dark brown and red; cherry with pink, red and purple; peppermint with green and blue; and lemon with yellow, green, and pink.
To understand the association between the perception of odour and colour, Dr Ward and his team conducted a study involving 24 adults aged 20 to 57. These participants had no visual impairment, loss of smell or colour blindness. They were made to sit in a room without external stimuli, and the room was cleared of pre-existing odours.
Later, using an ultrasonic diffuser, the researchers introduced one of the six different odours from caramel, cherry, coffee, lemon and peppermint into the room for five minutes. Odourless water was introduced as a control.
When each odour was introduced, the participants had to select colours on a screen and adjust a colour slider. Their goal was to achieve a neutral grey on the screen: they had to accomplish this by adjusting two sliders, one for changing the colour from yellow to blue and another for changing it from green to red.
The researchers repeated this procedure multiple times for each odour.
They saw that the participants consistently adjusted the colour sliders away from neutral grey. For example, when they smelt coffee, they perceived grey as a more reddish-brown colour. Similarly, the scent of caramel made them perceive a bluish colour as grey.
Odd one out
Surprisingly, the colour response to peppermint was different. When the participants smelt peppermint, their colour choice remained close to the actual grey. This indicated that the peppermint odour did not follow the expected pattern of colour distortion.
Dr Ward said these results show that the perception of grey tended towards its anticipated crossmodal correspondences for four out of five scents – lemon, caramel, cherry, and coffee. “This ‘overcompensation’ suggests that the role of crossmodal associations in processing sensory input is strong enough to influence how we perceive information from different senses, here between odours and colours,” he explained in the statement.
This research raises questions about the extent of crossmodal associations between odours and colours. However, Dr Ward emphasises the need for more research. “We need to know the degree to which odours influence colour perception. For example, is the effect shown here still present for less commonly encountered odours, or even for odours encountered for the first time?” he said.