The unusual side effect of a stroke has given French neurologists a rare opportunity to study the interaction between language and thought.

A male patient identified only as RDS discovered that while he could identify something as red, blue, green, or any other chromatic hue, he could not name the object’s colour.

This gets to the heart of an issue that neuroscientists and philosophers have long debated: do names shape the way we categorise what we perceive, or do they correspond to categories that arise from perception?

“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,” says Paolo Bartolomeo from the Brain and Spine Institute in Salpêtrière Hospital in Paris, the senior author of paper in the journal Cell Reports.

Many scientists believe categorising colours depends on top-down input from the language system to the visual cortex. Colour names are believed to be stored in the brain’s left hemisphere and to depend on language-related activity in the left side of the brain.

However, the new findings support recent neuroimaging studies suggesting that colour categorisation is distributed bilaterally in the human brain.

Before his stroke, RDS perceived and named colours normally. After the stroke, an MRI revealed a lesion in the left region of his brain, which apparently severed his memory of colour names from his visual perception of colours and his language system.

Yet he could still group most colours – even those he couldn’t name – into categories such as dark or light or as being a mixture of 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 first author Katarzyna Siuda-Krzywicka.

“This suggested that our language system may process black, white, and grey differently from chromatic colours. Such striking dissociations raise important questions about how different colour-related signals are segregated and integrated in the brain.”

To ensure RDS’s behaviour did not reflect abnormal brain organisation, the researchers compared the functioning of his unaffected brain areas to that of the same brain areas in healthy subjects and developed a non-verbal colour-categorisation test.

“Our result – that his colour categories were independent from language – could be generalised to healthy adults,” Bartolomeo says.