Now let's go on in the process of better understanding

how the retina initiates the perception of colors.

Now that we understand that there are three different cone types and

that they are basically integrating their sensitivities and

their responses to the light that's being introduced onto the retina.

Thomas Young, whom I've mentioned before, set the stage for

understanding spectral sensitivities.

And in the 19th century it was Helmholtz, who we've talked about before,

and James Clerk Maxwell of the Maxwell field equations in physics.

Again, two absolute geniuses in physics,

who further extended the Young Theory into what is often

referred to as the Young Maxwell-Helmholtz Theory of

color vision which is called color trichromacy.

The idea that you have three different receptors, trichromacy refers to that.

And Ewald Hering who was, again,

a first-order thinker in the 19th century,

a contemporary of Helmholtz, saw a little bit

beyond the trichromacy theory that Helmholtz and

James Clerk Maxwell had advocated, and

is referred to in that way today.

So what he recognized was that, in some sense,

the colors we see are more complicated and

in a very interesting and important way than

just the idea that you have three pigments in the retina and the sensitivities of

those pigments give rise to what you see in the way that we've been talking about.

And he recognized that when you see red and

green or blue and yellow, you

are seeing color opposites that somehow, in a subjective sense,

blue and yellow are opposites perceptually.

That blue and yellow are perceptual opposites.

And that are green and red are perceptual opposites.

And this opponency is associated with Hering's name and

he and Helmholtz battled over this in a very interesting and more or

less civil way, but they were definitely on opposite sides of the argument here.

Hering argued that color was much more complicated than trichromacy,

that it depended on somehow having color opposites that was part and

parcel of the way we see light.

And Helmholtz eventually sort of went along

with this idea, but had to be dragged kicking and

screaming into the view that trichromacy was

not the total explanation of color vision.

So color opponency, the basis for seeing blue and

yellow as color opposites and red and

green is color opposites, has been amply verified

by electrophysiology in the mid 20th century.

And what you see here is basically the outcome of that kind of experiment.

So, let's take a closer look at this part of the diagram.

And here you see the kind of details that substantiate, with electrophysiology,

what Hering described just based on subjective awareness of color and

what he thought must be the case based on the added phenomenon of seeing red and

green as color opposites perceptually and blue and yellow is color opposites.

So why should that be?

So remember what we talk about last time, that the receptive fields of neurons,

be they retinal ganglion cells, be they cells in the lateral geniculate nucleus,

or the visual cortex, have a center surround arrangement.

They're excited by light that falls on the center, and

inhibited by light that falls on the surrounds.

So now what happens if the light that falls on the center of the surround

are not just considered in terms of overall intensities,

as they are here, we'll talk about that in a second.

But when the spectral input to the eye is a combination of lower, medium, and

short wavelength light that's picked up by cones instead of rods in dim light or

all the cones acting together in seeing black and white.

So what happens, and the reason for opponency,

is that if you have red light and green light falling on cone receptors,

you're going to activate long wavelengths and middle wavelengths.

And what electrophysiology has shown

is that you have cells that are color opponent cells.

That is, their surrounds are sensitive to one wavelength light, and

their centers are sensitive to green light that is middle wavelength light.

And when you stimulate these opponents,

then you're going to get a perception of color that depends on

the amount of red and green light, short and middle wavelengths

that are falling on that receptor type, on that opponent receptor type.

So, if there is green light falling on the center and

little red light in the stimulus, you're going to see green.

And vice versa, if there's a lot of red light and

little green light you're going to see red.

But the result is the outcome of these color opponent cells and how we

the amount of long wavelength and medium wavelength cone excitation that exists for

whatever stimulus is that is falling on the retina.

It's exactly the same thing with blue and yellow light.

You have long wavelength, and middle wavelength, and

now added short wavelength light, and a cell like this with a center

surround opposing field that's blue in the surround and

yellow in the center is going to give you the sense of blue, yellow opponency.

And those cells exist, and that's the reason,

in modern electrophysiological parlance, for

the perceptual phenomenon that Hering described that was an add on,

basically, to the trichromacy theory that James Clerk Maxwell and

Helmholtz extended from Young's initial observations.

How the Retina Initiates Color Vision (part 2)

Visual Perception and the Brain

Meet the Instructors