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.
