Unique case of a stroke patient just changed our understanding of colour perception

How do we name and categorise colours? Scientists aren’t certain how these processes work in the brain, but a new case study of a stroke patient suggests that the neural processes of naming colours and categorising colours aren’t as interlinked as previously thought.

The patient – for privacy reasons named only by his initials, RDS – suffered a stroke that left a lesion in the left side of his brain. As a result, he was no longer able to attach labels like red, blue and green to the relevant colours.

But in tests carried out by researchers, RDS was still able to mostly match different shades of the same colour together, and tell the difference between different colours, even though he wasn’t able to put specific names to them.

“In our study, we had the unique opportunity to address the role of language in colour categorisation by testing a patient who couldn’t effectively name colours after a stroke,” says neurologist Paolo Bartolomeo, from the Salpêtrière Hospital in France.

“We perceive colours as continuous. There is no sharp boundary between, say, red and blue. And yet conceptually we group colours into categories associated with colour names.”

The idea that a name of a colour helps us categorise it is one that’s widely held in scientific circles, and backed up by research – that there’s some kind of ‘top down’ input happening, from the brain’s language system to the visual cortex.

As a result we can recognise different shades and variations of red as red, for example – cherry, crimson, scarlet and so on – even though every colour is just one point on a continuous spectrum of shades.

But the brain’s language system is thought to be on the left-hand side of the brain, and is disabled in the case of RDS – so perhaps colour naming and colour categorisation aren’t so closely linked after all. Perhaps categorising is a skill the whole brain can contribute to.

RDS has hung on to some colour naming abilities though – he’s still able to name white, black, and grey when they’re presented to him. That might mean greyscale identification is handled differently from other colours.

“We were surprised by his ability to consistently name so-called achromatic colours such as black, white, and grey, as opposed to his impaired naming of chromatic ones such as red, blue, and green,” says neuropsychologist Katarzyna Siuda-Krzywicka, from the Salpêtrière Hospital.

Of course, this is a case study of just a single patient – but cases such as this one can give neuroscientists useful insight into an area that’s still not fully understood. It goes beyond colours too, and could have relevance for other links between language and visual perception.

We’re still learning about the brain’s wonderful ability to make sense of the information coming in from our eyes. Previous studies have looked at how the brain’s visual processing systems can adapt to damage and imagine noise from silent movies.

Now RDS and his colour-categorising talents can be added to the pool of information that researchers can use as they try and figure out just how we recognise colours – and put names to them.

“The present evidence supports the view that colour categorisation can be independent from colour naming in the adult human brain,” conclude the researchers in their study.

The research has been published in Cell Reports.

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