Does everyone see the same colour when they see red?

Is the red I see the same as the red you see?

At first, the question seems confusing. Colour is an inherent part of visual experience, as fundamental as gravity. So how could anyone see colour differently than you do?

To dispense with the seemingly silly question, you can point to different objects and ask, “What colour is that?” The initial consensus apparently settles the issue.

But then you might uncover troubling variability. A rug that some people call green, others call blue. A photo of a dress that some people call blue and black, others say is white and gold.

You are confronted with an unsettling possibility. Even if we agree on the label, maybe your experience of red is different from mine and – shudder – could it correspond to my experience of green? How would we know?

Neuroscientists, including us, have tackled this age-old puzzle and are starting to come up with some answers to these questions. One thing that is becoming clear is the reason individual differences in colour are so disconcerting in the first place.

Colours add meaning to what you see

Scientists often explain why people have a colour vision in cold, analytic terms: colour is for object recognition. And this is certainly true, but it is not the whole story.

The colour statistics of objects are not arbitrary. The parts of scenes that people choose to label (“ball,” “apple,” “tiger”) are not any random colour: they are more likely to be warm colours (oranges, yellows, reds), and less likely to be cool colours (blues, greens). This is true even for artificial objects that could have been made any colour.

These observations suggest that your brain can use colour to help recogniSe objects, and might explain universal colour naming patterns across languages.

But recognising objects is not the only, or maybe even the main, job of colour vision. In a recent study, neuroscientists Maryam Hasantash and Rosa Lafer-Sousa showed participants real-world stimuli illuminated by low-pressure-sodium lights – the energy-efficient yellow lighting you have likely encountered in a parking garage.

The yellow light prevents the eye’s retina from properly encoding colour. The researchers reasoned that if they temporarily knocked out this ability in their volunteers, the impairment might point to the normal function of colour information.

The volunteers could still recognise objects like strawberries and oranges bathed in the eerie yellow light, implying that colour is not critical for recognising objects. But the fruit looked unappetising.

Volunteers could also recognise faces – but they looked green and sick. Researchers think that is because your expectations about normal face colouring are violated. The green appearance is a kind of error signal telling you that something is wrong. This phenomenon is an example of how your knowledge can affect your perception. Sometimes what you know, or think you know, influences what you see.

This research builds up the idea that colour is not so critical for telling you what stuff is but rather about its likely meaning. Colour does not tell you about the kind of fruit, but rather whether a piece of fruit is probably tasty. And for faces, color is literally a vital sign that helps us identify emotions like anger and embarrassment, as well as sickness, as any parent knows.

It might be the colour’s importance for telling us about meaning, especially in social interactions, that makes variability in colour experiences between people so disconcerting.

Looking for objective, measurable colours

Another reason variability in colour experience is troubling has to do with the fact that we cannot easily measure colours.

Having an objective metric of experience gets us over the quandary of subjectivity. With shape, for instance, we can measure dimensions using a ruler. Disagreements about apparent size can be settled dispassionately.

With colour, we can measure proportions of different wavelengths across the rainbow. But these “spectral power distributions” do not by themselves tell us the colour, even though they are the physical basis for colour. A given distribution can appear in different colours depending on context and assumptions about materials and lighting, as #thedress proved.

Perhaps color is a “psychobiological” property that emerges from the brain’s response to light. If so, could an objective basis for colour be found not in the physics of the world but rather in the human brain’s response?

To compute colour, your brain engages an extensive network of circuits in the cerebral cortex that interpret the retinal signals, taking into account context and your expectations. Can we measure the colour of a stimulus by monitoring brain activity?

Your brain response to red is similar to mine

Our group used magnetoencephalography – MEG for short – to monitor the tiny magnetic fields created when nerve cells in the brain fire to communicate. We were able to classify the response to various colours using machine learning and then decode from brain activity the colors that participants saw.

So, yes, we can determine colour by measuring what happens in the brain. Our results show that each colour is associated with a distinct pattern of brain activity.

But are the patterns of brain response similar across people? This is a hard question to answer because one needs a way of perfectly matching the anatomy of one brain to another, which is really tough to do. For now, we can sidestep the technical challenge by asking a related question. Does my relationship between red and orange resemble your relationship between red and orange?

The experiment showed that two colours that are perceptually more similar, as assessed by how people label the colours, give rise to more similar patterns of brain activity. So your brain’s response to colour will be fairly similar when you look at something light green and something dark green but quite different when looking at something yellow versus something brown. What is more, these similarity relationships are preserved across people.

Physiological measurements are unlikely to ever resolve metaphysical questions such as “what is redness?” But the magnetoencephalography results nonetheless provide some reassurance that colour is a fact we can agree on.

Bevil R Conway is a Senior Investigator at the National Eye Institute, Section on Perception, Cognition and Action and Danny Garside is a Visiting Fellow in Sensation, Cognition & Action at National Institutes of Health.

This article first appeared on The Conversation.