If I would show someone a yellow object and ask them, "is this object yellow?" That person would say "yes".

But I could never know if my perception of the color yellow is the same as that other person's.
Because he or she could actually be seeing, what I know to be the color green.
But then tells me that its the color yellow because that has been taught to him or her from young age.

So how can you test if people are really seeing the same color?
(originally posted this question on physics.stackexchange but was advised to try it here) https://physics.stackexchange.com/questions/48731/how-can-you-test-what-color-different-people-perceive

  • $\begingroup$ This is a philosophical question, not biological. More exactly, it belongs to metaphysics. plato.stanford.edu/entries/qualia $\endgroup$
    – Anixx
    Jan 10 '13 at 19:08
  • $\begingroup$ I wonder why many people ask themselves "is another guys perception of color X same or different than mine" but nobody is wondering "is mine perception of 440Hz sound same or different than other guy perception of 440Hz sound". Why ? Why color perception seems fundamentally different than sound perception ? Behcause one can reproduce the sound one hears, back ? But then you can give him set of color pencils and he can reproduce the color he sees, back. If "reproduction" test works, then it CONFIRMS what person is REALLY SEEING, no ? $\endgroup$
    – Andrei
    Jan 10 '13 at 20:48

One way we can get evidence qualia are the same or very similar for different people is by reactions to it, beyond just the word.

For example, beyond the word "pain", we have other strong reactions to pain. So nobody suspects other people might experience pain as pleasure and vice versa. Obviously not! There are no obvious signs for colour qualia, so it makes sense to suspect some people experience red as blue and vice versa.

Still, there are weak reactions to colours, like sometimes certain colours are associated with certain emotions. But, this is very weak evidence and could easily mismeasured, or be bound up with culture rather than biology.

We could test it: take pairs of identical twins at birth (so that they haven't learned words for colours yet), re-wire optical nerves of one of each pair so that red and blue receptors were switched (and perform a placebo operation on the other one, so they don't know who is who), and see whether there is any statistically significant change in the attitudes to different colours or the like as they grow up.

The experiment has various technological and ethical problems :-) But it could be done in principle, so to me this shows the question isn't meaningless, just that it's hard to find the answer.


I have often wondered this myself and have never found a way this could be asserted.

In fact we know that not all people perceive color the same way: up to 5% of humans are colorblind and about 2-3% of the women are tetrachromates, they have an extra cone type in their eyes (usually somewhere near the red spectrum). So it would be logical to assume that those people perceive colors in a different way. We know this for a fact in colorblind people because we can create images which appear different to colorblind and non-colorblind people.

The evolutionary reason for the common prevalence of red-green color blindness might be because they are less affected by camouflage.


It's impossible to be sure how another individuals brain represents the world. For all we know, they way they sense depth and up and down might be completely different. The representation of colour could be completely different. Both individuals could explain it as images, but if it were possible to move another individuals representation into another individuals brain - it's likely the other individual would experience an abstract sensation that would be difficult to compare to anything else that individual has experienced.

It could well be like having a compass attached to your body, sending nerve signals to your brain. You would not understand how to interpret it unless your brain has plenty of time to associate the stream of signals of direction with other signals such as your balance, vision and audio.

The brain is a very complex organ, and it probably starts out like a tiny, one celled blank sheet of paper. That blank sheet of paper is receiving continous streams of electronic signals from the various sensory organs of the body. Over time, synapses between your neurons will bond with other synapses, due to both of them receiving signals simultaneously. For example, if you are listening to music, you will hear various frequencies. Usually there will be a drum beat, which you will also sense as vibrations in the floor and your stomach. The neurons handling those two different signals will then bond together, so that at a later time - when you sense bass signals in the floor, but not in your stomach and by your ears - your brain will still recognize that drum beat in the floor as music. Some people might even feel like they hear the music.

The nerves are then going from the sensor to the brain in various ways and paths, and those connections are different for everybody.

Your brain will visualize stuff your way, and other people will visualize stuff their way and I believe those representations of objects and things are completely different for everybody. The representations will be compatible among different people, in such a way that physical laws are obeyed - but that is because the brain alters representations according to previous experiences.

So, knowing that it is impossible to be sure, I really do believe that everybody represents sensations completely different. They will be handled in similar regions of the brain due to physical similarities of the body, but within those regions in the brain there will be huge differences in representations.

I believe that if a human, unknowingly had a compass operated into his body at birth and connected to an arbitrary nerve, then that human would experience a heightened sense of direction that it could not explain as an adult. If another human had an identical compass attached to a completely different part of his body and connected to perhaps the nerve going from one of the ears, then that human would experience the same sense of direction.

Disclaimer; I am a computer programmer with a keen interest in virtualized intelligence - I have no education in the area of biology. :-)