0:12
Okay.
So, now let's talk about color, perceiving color.
And the key question that you're going to go away with
the answer to is does light have a color?
And the answer is no.
Light has no color.
Light has a wavelength.
We perceive a color.
Color is entirely a brain constructed concept.
Okay, so what I've done here is mapped out
the wavelengths of light that humans see and
and the colors that are perceive, typically perceived
when one views a particular wavelength of light.
Not always, but this is the typical perception for short wavelength,
we're talking violets and blues.
And then for long wavelength, we tend to see reds.
1:09
In between, we've got some we perceive green and yellow and orange.
So how do the cones accomplish what they do?
Well, there are three different types of cones and
one of them has has an absorption, or is responsive to light in this area.
So this is a short wavelength cone, it responds to short wavelengths of light.
And then there are two other cones one
of which has a peak about here, and
the other one just a little bit longer.
So this is called the medium wavelength, the medium, or
the M cone, and the long wavelength cone.
So how do we construct vision out of these three cones.
Well, the, we're going to move over to the board and what the,
what's amazing is the, the retina itself.
Before sending information on to the, the rest of the brain, the retina actually
makes three calculations using information from the three different cones.
And I'm illustrating that here.
Let's say that we saw this image.
This is a blue flower or
I perceive it as a blue flower with little red fruits hanging off of it.
Well, we have three channels.
And one channel, it mixes all this input from the medium and
the long wavelength, and we're going to call this the luminance channel.
This is our basic this is the basic form of what we're viewing.
2:57
And then there is a channel which allows us to see short wavelength,
and this is essentially a subtraction of input coming into the short wavelength
cone minus all of this input.
And what you see is that what's going to jump out at you is this blue flower.
3:20
Okay?
So that's the second channel.
And the third channel is a difference between these two very
similar cones, the medium and the long wavelength cones.
So, some, some type of difference, and that allows us to tell the difference
between something that we perceive is red and something that we perceive is green.
When might that be useful?
Well, that might be useful when we're looking for ripe fruit.
And it's thought that this is it, primates in general have the long wavelength cone.
The, they have these two medium and, and long wavelength cones which enable
primates to detect the difference between red and green, unripe and ripe fruit.
And this is thought to be a major
evolutionary advance that primates enjoyed.
Now, how did that happen?
Well, it turns out that the gene that codes for the medium and
the long wavelength cones.
The photoreceptor, the photo,
the, the opsin that absorbs these two different wavelength lights.
The genes for these two cones sit side by side on the X-chromosome.
4:39
As you may know, women have two X-chromosomes.
Men have one X-chromosome.
So men have half the chance of actually getting these these genes.
So amongst men, color blindness is very common because,
and very common, what I mean by very common is on the order of 5% of men,
5 to 10% of men might have some type of color blindness.
And so, how does that work?
Well, let's say that this is the M cone, and this is the L cone.
This is wavelength.
This is the wave length that they and this is the Y axis here is the response.
So the M cones respond to something,
to wavelengths that are slightly shorter than the L cones.
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Now, there are four different types of color blindness.
One is when the, there's no L.
And one is where there's no M.
And the other one is where the L shifts over towards the M, or
the M shifts over to the L.
So let's just take the example of the long shifting over to the M.
So if, in this type of person,
what they would have is two cones.
But their response preferences are so close together that
no longer is that person able to tell what we common,
what somebody like me who has intact color vision de,
perceives as red and green.
6:29
So the most common version of color blindness in men, is actually this one.
The M cone shifting over towards the L responsiveness.
So are, is this a big problem?
It's not a big problem, but it's it, it,
it, there are instances where a lack of color vision is problematic.
And there are instances where societies have
have turned a blind eye to some fairly obvious neurobiology.
And one of those instances is the fact that an individual with the most common
type of color blindness cannot tell a red light from a green light at night.
Because at night, unlike during the day, there's no positional information.
Here in the United States the the red and
the green light have different positions, but you can only see
the position when you're approaching an intersection during the daytime.
At night all you see from a distance is this shining light.
The red and the green are going to look the same and
a color blind person can't can't tell the difference.
So that, that's an example.
It's not a, it's not a, maybe it's not one of the world's most intractable problems,
but it's also a place where it, it would seem common sensical,
if one knew neurobiology to to construct the system differently.
Peggy MasonProfessor
