How do blind people who’ve never seen colour, think about colour?

Think about the concepts of "red" and "justice" and you'll notice a key difference. If you're sighted, you'll associate "red" most strongly with the sensory experience, which relates to signals from cone cells in your eyes. "Justice", in contrast, doesn't have any associated sensory qualities – as an abstract concept, you'll think about its meaning, which you learnt via language, understanding it to be related to other abstract concepts like "fairness" or "accountability", perhaps. But what about blind people – how do they think about "red"? 

 A brain-imaging study of 12 people who had been blind from birth, and 14 sighted people, published recently in Nature Communications, shows that while for sighted people, sensory and abstract concepts like "red" and "justice" are represented in different brain regions, for blind people, they're represented in the same "abstract concept" region. 

"You could be talking to a blind person, and if you didn't know they were blind, you would never suspect that their experience of red is different from yours, because in fact they do know what red means," argues Alfonso Caramazza at Harvard University, US, senior author on the paper. "They know what it means in the same way that you know what justice means." That is, by hearing and reading about "red". 

This idea, that for blind people colour concepts are treated more like abstract concepts, was backed up by the new findings. Lead author Ella Striem-Amit at Harvard University, together with Caramazza and other colleagues, used fMRI to look at their participants' brain activity while they listened to words pertaining to different types of concept: concrete concepts that are familiar to both sighted and blind people, and which can be perceived in some way by both groups (like "cup"); visual concepts that are imperceptible only to blind people, such as "red" and "rainbow"; and abstract concepts without any sensory features, such as "freedom" and "justice". 

The results suggest that, for all of us, the medial anterior temporal lobe (ATL) is the brain area most responsible for the representation of concrete concepts with any kind of sensory perceptibility, while abstract concepts that are understood based on the meaning of the word, and which have no associated sensory information, are processed by the dorsolateral ATL. This led to a group difference for colour words, which were associated with increased activity in the anterior ATL in sighted participants, and increased activity in the dorsolateral ATL in the blind participants.  

Though blind people lack the sensory experience of colour, they can nonetheless – thanks to language – form rich and accurate colour concepts, Caramazza notes. Not only do they learn that it's a property of objects or scenes that's unlike other sensory properties that they experience, but they can learn about differences between colours. Earlier work has shown, for example, that they know that orange is more similar to yellow and red than to green or blue, Caramazza says. 

"Studies of colour knowledge in blind individuals have confirmed this aspect of their understanding of colour terms. However, our study has shown that this type of colour knowledge is represented in a region of the brain that is typically associated with knowledge of words that do not have sensory referents, like justice or virtue," he says. "This reflects the important role of this region in acquiring meaning through language."

Further reading

—Neural representation of visual concepts in people born blind

About the author

Emma Young (@EmmaELYoung) is Staff Writer at BPS Research Digest