Is Your Blue My Blue?
Are the colours you see the same as the ones I see?
Future's resident psychologist has a moment of doubt and wonders if science can
help ease his worries.
Imagine the two of us, arm in arm, looking at a
sunset, where the horizon is fretted with golden fire and the deep blue night
encroaches from the opposite side of the sky. "What beautiful
colours", I say, and you agree.
And then, in the space of the following silence, I am
struck by a worry. I can point at the sky and say it is blue, and you will
concur. But are you really seeing
that blue the way I am seeing it? Perhaps you have just learnt to call what you
see "blue", but in actual experience you are seeing nothing like the
vivid, rich, blue I see. You are an imposter, calling my blue by the same name
as yours, but not really seeing it the way I do. Or, even worse, perhaps I am
the one seeing a pale imitation blue, while you
see a blue that is infinitely richer and more splendid than mine.
Now I admit that this worry lies in the realm of
philosophy, not neuroscience. You might even ask me why I am worrying about
this when we could be enjoying the glorious sunset. But when you think about
it, it is not clear that I could ever have direct access to what it is like to
be you, and you could never have direct access to what it is like to be me, or
someone else, or something else, such
as a bat. My worry seems more plausible when you consider colour blindness, which affects around 8% of men and half of one percent
of women. Many people
do not even realise they are colour blind. They live among the colour-seeing,
getting by on the fact that there is usually some other difference between
things of different colours that they can use to tell them apart, such as
differences in shade or texture.
How green is my valley?
Our colour vision starts with the sensors in the back
of the eye that turn light information into electrical signals in the brain ˇV
neuroscientists call them photoreceptors. We have a number of different kinds
of these, and most people have three different photoreceptors for coloured
light. These are sensitive to blues, greens and reds respectively, and the
information is combined to allow us to perceive the full range of colours. Most
colour blind men have a weakness in the photoreceptors for green, so they lose
a corresponding sensitivity to the shades of green that this variety helps to
distinguish.
At the other end of the scale, some people have a
particularly heightened sensitivity to colour. Scientists call these people
tetrachromats, meaning ˇ§four coloursˇ¨, after the four ˇV rather than three ˇV
colour photoreceptors they possess. Birds and reptiles are tetrachromatic, and
this is what allows them to see into the infrared and ultraviolet spectra.
Human tetrachromats cannot see beyond the normal visible light spectrum, but
instead have an extra photoreceptor that is most sensitive to colour in the
scale between red and green, making them more sensitive to all colours within
the normal human range. To these individuals, it is the rest of us who are
colour blind, as while most of us would be unable to easily distinguish an
exact shade of summer-grass-green from Spanish-lime-green, to a tetrachromat it
would seem obvious.
So yes, as we share this sunset, perhaps I am seeing
something you cannot see, or you are seeing something I cannot see. If our
colour vision is wired differently, the information going in could be more or
less the same between us. But as you tell me this, with the sun sinking slowly
below the horizon, you can sense that it has not really helped with my true
worry. I am worried ˇV and perhaps you are too ˇV that although we both have the
same machinery in our eyes and we are both able to see the green of the trees,
the red of the sun and the blue of the sky, that when I say "blue",
it creates an inner experience that differs from yours when you say
"blue".
Behind blue eyes
My worry about
your inner perception of the colour blue is a facet of the basic isolation that
is part of the human condition. Even if we think we can really know other
people, we cannot be certain of that knowledge. Historically, psychologists
have adopted a stance called behaviourism, which acts as if questions about
inner experience are irrelevant. This approach states that if you call my blue
"blue", and you can always tell it from red, and if we both know it
is the correct colour for the sky, my eyes and the Smurfs, then who cares what
the inner experience is?
There is a lot
of mileage in this perspective, but maybe there is also some wisdom in trying
to convince ourselves that the difference between our inner experiences is
real, and does matter ˇV and, in fact, that some difference is inevitable. We
use common words, and use them to refer to shared experiences, but nobody can
see the same sunset, merely because perception is a property of the person, not
of the sunset. Because there is something that it is like to be you, and your
ˇ§you-nessˇ¨ is unique, we are certainly seeing different things when we talk
about looking at something blue, if only because the act of seeing incorporates
feelings and memories, as well as the raw light information arriving at our
eyes.
In any case,
the sun has set and we walk away. You might be seeing a richer blue in the
sunset than me, but you will not have the same memories of the other sunsets I
have seen and the people I have watched them with. We could get our vision
tested and find out who was better at perceiving colours, but we would never
know what it was like to be the other person seeing a particular colour. As
long as we can both say that it is a beautiful sunset, we can agree and be
secure in the knowledge that I see my blue, and you see your blue, and although
we may not see the exact same thing, we have shared it. And that sharing is
itself unique to you and me, because no two other people in the world have the
same two minds.
Tom Stafford
14th February 2012
http://www.bbc.com/future/story/20120209-do-we-all-see-the-same-colours