Learners will be introduced to the problems that vision faces, using perception as a guide. The course will consider how what we see is generated by the visual system, what the central problem for vision is, and what visual perception indicates about how the brain works. The evidence will be drawn from neuroscience, psychology, the history of vision science and what philosophy has contributed. Although the discussions will be informed by visual system anatomy and physiology, the focus is on perception. We see the physical world in a strange way, and goal is to understand why.
How the Retina Initiates Color Vision (part 1)
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Do curso por Duke University
Visual Perception and the Brain
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Na lição
Seeing Lightness, Darkness and Color
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- Dale PurvesM.D.
Duke Institute for Brain Sciences
In this next lesson, let's consider how the retina initiates vision.
What's the basis for saying that color is something that the nervous system,
the brain, makes up, the visual brain makes up.
And not a property, per se,
of the objects that we are looking at and perceiving as blue, red, whatever.
So the person who came about a century after Newton
was another great natural philosopher named Thomas Young.
He was not, again, like Newton, just interested in vision, but
he was interested in a variety of physical phenomena.
But he certainly had the idea that again,
made a signal advance in thinking about color vision.
And what he recognized is that, and it's the basis for
the complexities that we're now going to talk about in color vision.
What he realized is that to see color you need to make a comparison,
you need to have elements.
And of course, he didn't know what these elements were in the retina.
That can make a comparison between short, medium and long wavelength light.
If you don't have a mechanism of making this comparison,
well then you are just seeing illuminance, right?
If you have only one receptor type, and
Young didn't know about receptors, but he had the general idea.
If you have only one receptor type,
you can only detect the overall intensity of light.
If you want to have an ability to see different energies across the spectrum,
you have to have multiple photo detectors that can be compared one to the other.
And we'll talk about this in a minute, but Young was the first to realize this.
And he, as I said, didn't know about photoreceptors.
He certainly didn't know what we know now about photoreceptors.
But he did have the idea that there must be at least,
based on the phenomenology of colors that we see, at least three kinds of particles,
as he called them, in the receptor apparatus.
Here are the three different spectral sensitivities
of the three different cone types.
The short, middle, and long wavelength
cones and, as I said before, these are due to the different cone opsins.
The different protein photo pigments that absorb photons and
elicit the initiation of seeing color in the first place.
They have different spectral sensitivities and
they're different from the rods which you see here.
And remember as I said before that rods are specialized for seeing in dim light.
The cones are specialized for seeing in normal light,
sunlight, outdoor light, indoor light, but relatively high levels of light.
And the cones are operative and the rods,
as you remember, become inoperative in those photopic conditions.
As opposed to the scotopic conditions,
which are dim lights in which rods are primarily active.
These different spectral sensitivities were described by
George Wald In the 1960s.
So centuries after Newton and Young first made their
inroads into understanding how it is that color vision works.
But you'll be aware that these three spectral sensitivity curves overlap.
Now that makes things a little bit more complicated, as we'll go into.
But basically the comparison of short,
middle, and long wavelength light coming to the retina.
The difference between the spectral intensities,
the spectral energies that are coming from any seeing to the retina.
Are being reported to you by the differential activity of these three
cone types.
To make a little bit more sense out of this,
let's look again at a picture I showed you before.
Which is looking at the retina through an ophthalmoscope but through
a microscopic ophthalmoscope that can actually reveal the density of receptors.
And you remember what we said before was that [COUGH] in the fovea,
in the central region of vision.
Where we direct our gaze when we want to see something in color,
with great sensitivity.
We point our eyes in that direction and see color well because
the fovea Is entirely made up of cones.
As you move away from the fovea,
the rods increase in number as we went through before.
And as you move into peripheral vision, the cones are sparse.
You could again easily show this to yourself
by taking a colored picture of some kind that has a colored checkerboard or
the equivalent in any picture that you might look at.
And ask yourself when you focus on a particular spot in the scene.
What's the color of the objects or surfaces that you see a little bit away,
a few degrees or many degrees away from the point that you're focused on?
The answer is, you have a lot of trouble seeing what the colors are, reporting what
the colors are once you move a little bit away from central vision.
Again making the point that you need cones to see color vision.
And you need these three different pigment types in the cones to allow you to make
the comparisons that are needed to give us our subjective sensitivity to color.
Now we want to talk about how we or
any individual reports color light or can report the colors of light that we see.
This a psychophysical measurement and you remember before,
I said that there are a variety of ways of making psychophysical measurements.
But one way that's very useful in thinking about color vision and
talking about it is a comparative wave.
So this diagram here makes a number of points and it's a bipartite screen.
On one half of the screen are directed
in an adjustable way long, medium, and short wavelength lights.
So this is the test side of the screen,
so to speak, the side that you adjust to set the stimulus that you want.
And this is the reporting side of the screen.
So you present, or the experimenter presents,
a color on this side of the screen.
And asks the subject to match a surface
by adjusting these three amounts of colored light.
And for almost all colors, it's possible for any surface that's shown
to an observer to match the two sides of the screen.
Based on the amount of long, medium and short wavelength light.
So this makes a number of points, the first of which is that yes,
what we said or what I said a minute ago is really so.
That depending on the amount of long, medium and
short wavelength light you can create any color that you like.
If you maximize the long wavelength you're going to see something that's towards
the red end if you maximize the short wavelength you're going to see something
that's toward the blue or violent end of the spectrum.
But as I said before, this is not just a simple matter of short wavelength,
blue long wavelength red.
As you know from the spectral sensitivities I showed you a minute ago.
This depends on the energies that are coming from any scene from
any surface that's presented to a subject.
It depends on those energies in a very precise way.
So this is a good way of demonstrating, number one, that it really is the mixture
of different light energies that can match, or that
is seen as the basis of the colors that we see.
And can subjectively report in a test like this which referred to as
colorimetry and I'll write that down.
This is just the word that's
applied to this kind of psychophysical testing.
And it also makes the point, and
I want to emphasize this because I think a lot of you are aware of it, but some not.
That color mixing, mixing different amounts of spectral light is very,
very different from mixing paints.
So this is [COUGH] an additive process you
add different amounts of long, medium and short wavelength light.
When you mix colored paints,
you mix red paint, blue paint and get some purplish looking thing.
Or mix green and red paint and
get some muddy looking color that is hard to describe.
You are doing a subtractive process, you're taking away wavelengths,
not adding them.
So I just want to make the point that it's really
an opposite concept of color mixing.
Mixing color paints is very different than mixing lights with different wavelengths.
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