Subscribe
Share
Share
Embed
Most of the purples you see have no wavelength behind them. Your brain invents them, to stand in for a green that should be there and isn't. Which is fun trivia—until you notice it does exactly the same with your reasons.
Most of the purples you see have no wavelength behind them. Your brain invents them, to stand in for a green that should be there and isn’t. Which is fun trivia—until you notice it does exactly the same with your reasons.
I describe photons as moving at different speeds. They don’t — all light travels at the same speed, and what differs is the wavelength (and so the frequency: how fast the wave oscillates, not how fast it travels). The transcript below says it correctly.
A brain scientist talking about (better) patterns of thought, of feeling, and of action. One pattern, one podcast—you see if it works for you. The btrmt. lectures, with Dr Dorian Minors. (btrmt.—said "betterment.")
the btrmt. lectures podcast. For the
article that inspired it, see
Purple doesn't exist.
Welcome to the Betterment Lectures. My name is Dr Dorian Minors,
and if there's one thing I've learnt as a brain scientist, it's
that there's no instruction manual for this device in our head.
But there are patterns. Patterns of thought, patterns of feeling
and patterns of action, because that's what brains do: they create
the patterns that gracefully handle the predictable shapes of
everyday life. So let me teach you about them. One pattern, one
podcast, and you see if it works for you.
Now, I want to do something a little more fun and speculative
today. As I've done more and more of these lectures, I've deferred
more and more of the process to AI, and
my last lecture
was uncomfortable to listen to, because you can hear me fighting
with the speech notes that the AI derived from the article the
lecture is based on. Very uncomfortable to listen to. So I'm going
to go back to what I was doing at the beginning of this, which is
just speaking off the article itself. And for that, I thought I'd
choose an easy one.
Reason is lazier than we like to think
There's one idea that's really come to colour an enormous
proportion of what I teach as an associate professor of behavioural
science here at the Royal Military Academy Sandhurst. It's the idea
that reason—deliberative reasoning—isn't the rational, logical,
evidence-driven process we like to think it is, but might actually
be more of a lazy process of rationalising our intuitions to
ourselves.
I'm not going to talk about that in detail today, but I'll link to
an excellent book on the subject, The Enigma of Reason by Mercier
and Sperber, as well as
an article or two of mine
where I do talk about it.
As it's coloured more and more of my teaching, I've started to
think about how it might be a reflection of some of the tendencies
of the brain that used to puzzle me when I was working as a brain
scientist at Cambridge. And I want to illustrate that today with a
pretty curious phenomenon, which is how the brain processes the
colour purple.
I think this might end up being a bit of a controversial take, but
at least it'll be fun. So let's talk about how the colour purple
doesn't really exist.
What we learn about colour in primary school
In primary school, we learn that colour is a property of what light
is reflected back at us. The light comes from the sun, it hits an
object, the object absorbs some proportion of the light and reflects
back some of it, and that's the colour we see. So a leaf absorbs
everything in the light spectrum that isn't green and reflects back
the greenness—from the chlorophyll, I think. I don't actually know
how that works, but I'm a brain scientist, not a leaf scientist, so
you'll forgive me. And as a consequence, we see a leaf as green,
because that's what's left of the light spectrum after the leaf has
absorbed whatever it absorbs.
So now, moving away from my ignorance about the interaction between
leaves and light, and more towards my domain of strength, the brain.
What's interesting about studying the brain is that you learn
colour actually isn't that. It's something more to do with how the
brain processes that information. Colour, for humans, comes about
because we have cells in the eye that are sensitive to different
wavelengths of light.
Three cells, and the whole spectrum
Light comes in a range of wavelengths. At one end you have the very
short wavelengths—the kind of radiation that comes from a nuclear
bomb—and at the other end the very long ones, the sort picked up
by radio antennas. In the middle of those two things is the visible
spectrum, which we call light.
The eyes of humans have three photoreceptors, each tuned to a
different part of that range. What differs between them is the rate
at which each one captures photons from the light arriving.
So we've got two things going on here. Light comes and hits our
leaf, and the leaf absorbs some of those photons but reflects back
the ones in the green part of the range. And the reason we see it as
green is that it comes into the eye and there's a photoreceptor
perfectly placed to capture light at that wavelength. We have a
green photoreceptor, essentially—the M-cone.
We have two other kinds. You have S-cones, which are sensitive to
shorter wavelengths—faster-oscillating waves—and we see those
as blue. And then we have L-cones, which are sensitive to longer
wavelengths, and we see those as red.
What's interesting is that obviously we can see more than just three
colours. We don't just see blue, green and red. We see yellow. And
we see purple, and we see pink.
The reason we see those things is that, kind of like mixing colours
in paint, the brain works out that if you're getting a little bit of
the M-cone, the green photoreceptor, and a little bit of the L-cone,
the red photoreceptor, then probably you're seeing something
reflecting light in between the two. And so we see that as
orange—a kind of mix between green and red.
So you don't have a cell in the eye that tells you what things are
reflecting yellow light. You have the brain processing a green cell
and a red cell and saying, oh, that's probably yellow.
And that's the entire visible light spectrum to us. These three
cells, these three photoreceptors. The entire colour spectrum is
your brain working out what colours it can see based on the activity
of three photoreceptors.
Where violet stops
What's really interesting is that the way this whole thing is set up
means sometimes the brain has to invent colours. Purple is an
example of this.
The shortest wavelengths of light we can see come through as this
sort of violet colour. And this is a really good way of illustrating
how the brain processes colour, because it's only got these three
photoreceptors and it's got to try to express to us what information
it has available. The information it has is the blue photoreceptor
responding weakly to something. So it wants to show us something,
but it knows the green photoreceptor isn't firing at all, so it
can't be anything longer-wavelength than blue. So it shows us this
sort of sub-blue colour, this violet.
Now, as the waves get longer, what we see becomes bluer and bluer,
because the brain's trying to tell us the blue cell is getting more
and more excited. Then eventually the green cell starts to kick in,
and the brain wants to tell us that, so we start to see cyan. It
changes from blue to something else. And as the waves get longer
still, we see this colour change to become more and more green,
because the brain is telling us the blue cell is firing less and
less but the green cell is firing more and more.
And the process continues. The waves get longer, and now you've got
the red cell kicking in, getting more and more excited. So the brain
says: the green cell is starting to fire less and less, but the red
cell is getting more and more excited. So you're going to see yellow
through orange and into red, which eventually becomes this sort of
muted colour as the red cell fires more and more weakly to the
increasing length of the light waves.
And then it stops being something we can perceive at all. On that
side it goes into infrared. And on the other side of the spectrum,
past violet, where the wavelengths are too short for our
photoreceptors to pick up, we get ultraviolet.
And that's the interesting point, because we shouldn't see any more
colours than violet. There's no wavelength of light that corresponds
to more purple than violet. We don't get more colours when we go
past violet—we get into ultraviolet, which is invisible to us. We
don't have a photoreceptor that responds to light at that
wavelength.
The colour the brain invents
So the question is: where do these other kinds of purple come from?
The answer is that it's a sort of invention of our brain, for
something that's confusing to this triadic system of photoreceptors.
What we're seeing when we see other kinds of purple is an invention
of the mind.
Things that reflect both short- and long-wavelength light are
usually also reflecting the middle, which our green photoreceptor
picks up. But some things reflect just the short and the long
without reflecting that middle, green wavelength. And the brain has
to solve a problem here. It knows there's something between blue and
red. It has a solution to that problem already—it has green, the
colour it shows you when the light is between blue and red. But in
this case the visual system isn't getting any of those green
signals, because whatever is reflecting the light to us isn't
reflecting green. It's only reflecting the fast and the slow.
So the brain has to do something. It can't show you green, because
that would be wrong—it's not getting any signals from the green
photoreceptors. So what it does is make up a brand new colour for us
to see. It makes up purple.
Purple is the brain telling us that this is something that isn't,
but should be, green. It's what we call a non-spectral colour. A
colour that doesn't exist on the light spectrum.
Half the colour wheel is a fiction
I really love this illustration, and not just because it's kind of
fun that the colour purple doesn't really exist. I love it because
it illustrates that the world we see reflects our purpose in being
in the world.
It's something I write about a lot here.
We don't care so much about the objective reality of the world, some
objective model of it. What we care about is what matters to us. And
at some point, things that reflect both very short and very long
wavelengths of light mattered to us. So our brain had to invent a
way of representing that.
And so a colour wheel is a beautiful illustration of that fact. Half
of it reflects an objective fact—something about the wavelength of
the light we're seeing. And half of that colour wheel is an
invention. Not reflecting some objective fact about the world, but
reflecting a human need. Something the brain needs to make up in
order to represent it to us.
The brain's job was never accuracy
What I'm hoping you're taking away from this is that the brain's job
isn't so much to reconstruct some objective fact about the world.
It's to convey information to you. To create some representation in
your mind of the world that's useful to you, that helps you navigate
it.
It's not really relevant whether purple exists or not. It's not
important to the brain that purple doesn't follow neatly along with
the rest of the visible light spectrum, because the brain isn't
interested in showing us something objective about the light
spectrum. The brain is interested in representing to us whatever we
needed to know about it. And sometimes we needed to know about
things reflecting both very short and very long wavelengths, but not
the middle. You can imagine something to do with foraging, for
example.
There are other non-spectral colours. Pink is one of them. You'll
actually notice that if you look at an image of the light spectrum,
pink doesn't appear at all—because we only get pink when all three
of our colour cells are active but the red photoreceptor is the most
active. The brain gets sort of confused by this and shows you
something that is kind of red, but also obviously not.
I tell you that because I think it's kind of fun that the prettiest
sunsets are basically a set of imaginary colours. Colours that are
an artefact of the way our body has learnt to respond to the world,
and that our brain has made up in response to that.
The split-brain patient and the snow shovel
And I guess that brings me back to the property of human reason.
There are a lot of examples in the empirical literature of people
creating nonsense reasons to explain phenomena they can't really
articulate, because those phenomena sit outside our common
experiences. Maybe the most famous examples come from split-brain
patients.
Split-brain patients are, as it sounds like from the name, patients
who've had their brains cut in half. Cut in half across what's
essentially connective tissue binding two functional halves of the
brain, the left hemisphere and the right. This is a drastic step,
but it's done for patients who have such bad seizures that cutting
the brain in half is preferable. Seizures are essentially an
electrical fault that spreads from one part of the brain across the
surface, and if you cut the brain in half it can only spread so
far—it can't spread to the other half.
What's really interesting about these patients is what happens to
their ability to describe what they're seeing. A bit of context
that's necessary to understand this: a lot of the architecture
responsible for helping you understand and produce speech sits on
the left side of the brain. It's one of the reasons you end up with
these myths about creative and logical brains—a lot of speech
production, or processing related to it, happens on the left side
for most people. Not all people.
I'm going to simplify this experiment a bit for a podcast, but I'll
find a link to some of these findings for you to follow up on.
Let's say you have one of these split-brain patients, understanding
that each eye plugs in to a different side of the brain, just like
each arm is controlled by a different side of the brain, and so on.
In the famous experiment—the seminal one—researchers flashed two
different images to the two eyes of the patient. In the eye
connected to the language side of the brain, they showed a chicken
claw. And to the other side of the brain, the one no longer
connected to all that language infrastructure, they showed a snowy
scene.
Then they asked the patient to use their hands to select associated
images that related to the things they could see. So the language
hand, remembering that this side of the brain saw a chicken claw,
selected a chicken. And the non-language hand, remembering that this
side of the brain saw a snowy scene, selected a snow shovel.
Then they asked the person to explain why they'd selected what they
selected. And the person said: well, I selected the chicken because
I saw a chicken claw. And obviously I selected the shovel so that I
could clean the chicken poop out of the chicken shed.
They said nothing about the snowy scene.
Again, that's a simplification of the experiment, but I'll try to
find a link for it. The point is that the faculty of reason
obviously had no access to what the non-language part of the brain
could see—and yet it had to come up with a reason for the
behaviour of the person. They'd picked a shovel. Why could they have
picked a shovel? Well, perhaps because you have to shovel out the
chicken coop.
A storyteller all the way down
And I think that bears some non-trivial resemblance to the way the
brain makes up colour. When it doesn't understand why we've engaged
in a certain kind of behaviour, it confabulates an explanation.
I guess what I'm wondering is: is the chicken shovel the same kind
of thing as the sunset? An explanation made by the brain to
represent stuff it doesn't really understand. And in these cases,
they're pretty harmless. But if they're happening in these cases,
when are they happening and they're harmful?
I think, going beyond that, the brain seems to be a storyteller down
to the lowest level of perception. And I guess that's something that
troubles me a little bit.
And on that happy note, I guess I'll leave it there.