 I actually haven't heard you speak before about the dictum that if we don't know where we're coming from then we don't know where we're going, but that seems to be a theme that we've talked about from Sarcophagi and today, and I'm going to go back about 40 million years, so hopefully that's long enough. So I wanted to talk today, I'm Rachel Patel by the way, I think I've met most of you guys before. I'm also one of the first-year residents, about something that we learned in passing in medical school but not very much, and I thought was really fascinating. So I wanted to bring a little bit of a historical perspective here, and that's how we've come to see the color vision that we do today. So humans are trichromats, but actually that is fairly unusual in the animal kingdom, and chromacy refers to the number of primary colors, which combinations the traction of the three of them results in the visible spectrum that we see, and in a lot of animals including wolves, cats, dogs, ones that we're familiar with tend to be dichromats, have two different types of proteins that create their realm of color vision. There are actually a lot of sea mammals like walruses and seals that are monochromatic. Humans and are related primates, some of them including gorillas are trichromats, and then there are birds and fish, which tend to have tetrachromacy and have visibility into the UV spectrum of light. And so the chemical basis of this is that we have these opsin proteins that we know about and that the number of types of opsin that we have usually determines the type of chromacy, the degree of chromacy that we have. So these are seven transmembrane domain proteins. They fall under the G-protein couple receptor category, and as we know they bind to the chromophore retinol, so when 11-cis retinol becomes activated by light and this becomes the all-trans retinol, starts the visual cycle, that's how we get to the vision that we have. And this complex of the chromophore and the opsin is located in the outer segments of the photoreceptor in this cartoon on the left hand side. And so we have these three different visual pigments in our cones. There is one pigment that has a maximal absorption wave of around 120 nanometers corresponding to the blue that we see. And this, because this corresponds to the shortest wavelength of light that we see, that's known as the S pigment, SWS, but I'm going to just go with S for now. There are also another pigment that absorbs closer to the 530 nanometer wavelength corresponding to green, and which is referred to as M for medium pigment. And then there's the longer ones that correspond to the red spectrum of light in the 569 nanometer range. And as humans, we also have a different type of opsin that is in our rods with an absorption wavelength maximum somewhere in between, but because we don't get a lot of color vision from dim light conditions, that doesn't really contribute to the degree of chromacy that we have. And so, genetically, when we're talking about how we get these pigments, we know that we have the S pigment gene, the one corresponding to blue light that's located on chromosome 7. And then there are the longer the M and L wavelength pigments that are located on the X chromosome. And this is not surprising when we found this out genetically several years ago, because we know that red-green color blindness tends to be X-linked and found in males. And while the difference between the S pigment gene and, or I should say the protein, and that of the M and L pigment proteins is quite different, the M and L ones are really similar. They're about 96% similar. And there's only a difference in just a few amino acids that changes their spectral sensitivity. In fact, there are just three of them that can account for almost all of the difference in the spectral sensitivity between the two of them. And because they're located right next to each other on the X chromosome, this phrase is the, or this originally raised the possibility that this probably evolved as a duplication mistake that then became passed down. And one could see how becoming trichromatic from dichromatic would maybe confer an evolutionary advantage. The classic postulation is that you could differentiate like ripe fruit from, you know, unripe fruit on a tree, but then there have been other studies that have looked at it maybe playing a role in mate selection or in predator detection, so we don't really know. However, just a simple duplication doesn't really explain everything that happened. And that's because 40 million years ago, we had different ways of getting to trichromacy. So there are around the time that the continents were splitting, we had primates that became separated by the continental barriers and took different paths. So the catering, and I have never actually said that we're out loud in front of people, so I'm saying that wrong, I apologize, the population that has descendants including humans, chimps, gorillas started off in Africa and southern Asia, and this population is referred to in the literature as Old World Primates. And then there's the platyremes, which was located more in South America. And their descendants included squirrel monkeys, marmosets, and these are referred to as New World Primates. And so they have evolved a little bit differently. In terms of Old World trichromats, including humans, as we talked about they have males and females are both generally trichromatic. They have one S pigment gene, and then they have two longer wavelength, usually M and L, pigments on the X chromosome becoming trichromatic. Whereas in the New World it's a little bit different. All the males and about a third of the females are dichromatic, the remainder of the females are trichromatic. And so to get to this, they found out that they have, again, the one S pigment short wavelength pigment, and then they have a longer wavelength pigment, just one of them on the X chromosome. But there are three alleles that can be located at this locus. There's one that codes for a protein that in its absorption wavelength spectrum is similar to the human M pigment, one that's similar to the human L pigment, and one that's kind of in between. So to put this all together 40 million years ago in both the Old World and New World males and females have this S pigment gene on the non sex chromosome, kind of got that down. In the Old World both males and females will have two different wavelength proteins on the X chromosome, they become trichromatic. In the New World the males, for example, have one gene, and it could be any one of these three alleles, and so they're dichromatic, but the females have two copies of two alleles, and if they are different, then they become trichromatic. If they happen to be the same on their two X chromosomes, then they kind of got genetically shafted and they're dichromatic as well. But it turns out that just having these pigments isn't enough to interpret the signal as color. So color interpretation relies, in part, on comparing the signals from different cones to each other and relaying that pathway to the brain. So to function optimally a cone should just express a single pigment, rather than lots of them, and then it should be surrounded by neighboring cones that express different pigments, and in fact this is actually what happens. So a cone cell generally expresses a single pigment, and there are nearby ones that express a different pigment, and then there are different types of ganglion cells. For example, midget, or what I learned this morning was also called parpocellular ganglion cells, that convey the comparison between M and L cones, and relay a blue, a red-green axis. And then there are these bistratified ganglion cells that can compare signals from the S cones and off signals from the M and L cones, and they relay a more of a blue-yellow axis. So I'm going to get back to that in a second, but I just wanted to focus on for a moment how we get to cones expressing a single pigment. And so there's an element of randomness in this development. So in determining whether or not a cone is going to express an S or an M or L pigment, there is a transcription factor during fetal development that will turn on or off the S pigment. If the S pigment is turned on, it then suppresses the M and L expression. And then in terms of the new world and old world, there's a little bit of difference in how we get to M versus L pigments. In the new world, so not in humans, X in activation plays the large portion of that. So of course, if you only have one pigment gene on your X chromosome and X in activation turns off one of them, the cone is going to express the other one, whereas nearby cells might have chosen the other chromosome to be turned off, and therefore you get this mosaic. In the old world, yes, there is X in activation, but you also still have two pigment genes on that cone. So it turns out that there is this control region nearby that will then interact with either the M or the L sites to determine which one becomes the one that the cone is going to express. And this seems to be, although we don't know exactly, a little bit of a random process as well. So just to get back to X in activation just really quickly, it occurs early in development, really early, 5 or 10 cell stages when it starts. And then a lot of the most, for the most part, all the daughter cells are going to express a similar chromosome that it turns off. And so one would expect that if that was the case you would have these large amounts of clumping of similar cones that are expressing the M or the L pigments. But that's actually not really what we see. So there have been studies recently that have used adaptive optics. I thought this was pretty cool to essentially bleach out photoreceptors of a certain wavelength absorption and then image the remaining ones so they can get these artificially colored sequences of an individual cone expressing which opsin that they have. So of course the red ones are expressing the red wavelength, etc. And they imaged a couple, a lot of men and then one female. The men had some variable degrees of clumping. Those are those first two there. I didn't put all of them on here. And then this last one is a female carrier of the protan defect. So she has one of the types of red-green color blindness carrier. And she actually didn't have a lot of clumping. One would expect that X in activation would play a larger role in her based on how it determines L versus M cones. But in fact she doesn't really have that. So there was the suggestion that in the migration of cones to form the phobia that it kind of intermixes the cones and overcomes this clumping effect that might be expected to happen by X in activation. So to get back to how we interpret color there are these different types of ganglion cells that convey the red-green versus the blue-yellow axis. And then they go through a similar but not exactly the same pathway through the brain. So the red-green aphorins synapse in the parvocellular layers of the lateral geniculate nucleus. And then they go to the deeper parts of the cortex. Whereas the blue-yellow aphorins synapse in the intercalated layers of the lateral geniculate nucleus and end up in a more superficial region of the cortical layers. And so this is important because when since there's more to developing color vision than just having a new option that's suddenly there, we need to understand how evolution would suddenly favor this development of trichromacy. And so one idea is that this red-green axis when it developed didn't just develop a new pathway to the brain but it actually took advantage of an existing pathway. And that's the pathway of spatial resolution. So the determination of like the borders of something that you're looking at and then how far away it is from you. And in doing like the spatial resolution pathway compares the input from a cone to that of its neighbors. And so one could see how since it uses the same pathway the red-green axis could have taken advantage of that. And potentially the first person, first primate who developed this new option might have been able to use it and actually have trichromatic vision. Of course we can't really prove that. So this has all been very non-clinical. So I just wanted to bring up a couple of clinical applications or ways that this manifests in society today. So because there are just a few amino acid changes that can determine how the spectral sensitivity between the like L&M gene pigments is has a different wavelength of absorption. There is some natural variability in the human population. And so there are these anomalous trichromats in the human population. And these have two different types of genes. But their wavelengths might overlap a little bit more, might be closer together. And so therefore although they are technically trichromatic, they don't have quite the same spectrum of absorption that someone who has the normal M&L pigments. There also are the possibility of, and there is evidence for this being the case, human females to have more than trichromatic vision. Because if they have the same natural variability, and they've got essentially four genes on their X chromosome or their sets of X chromosomes that could be expressed in the retina, that you can have more than trichromatically, such as tetrachromatic vision. And they've done studies where they've taken these females who are genetically proved to have the possibility of having tetrachromatic vision. And one study that I was looking at, they imaged, they tested 24 females who had that possibility, but only one of them actually functionally tetrachromatic. So the possibility of this is potentially more than the actual applications of it. And then finally, and this is probably the most important, is that there are several ways for humans to become dichromatic. Of course, the absence of one of these genes can determine it, can become dichromatic. And so we have these terms like protein defect missing the L pigment, and duetan missing the M pigment, and tritan, which is really unusual, missing the S pigment. And simple crossover misalignments can result in these defects. But there, in fact, when we have a lot of men who have the duetan defect, that is, they're missing the M pigment, they actually have this, and I'm not, I don't have the pointer, but they have this configuration all the way over on the right hand side. And that is, they have an intact M pigment, an intact L pigment, and then a hybrid gene in between. And the reason we were a little bit, we don't know exactly why this is the case, but it certainly can come from this misalignment of overlapping proteins when the proteins look really similar, or I should say, when the genes look really similar, they can overlap a lot, and cause this type of misalignment. But it's been shown in the vast majority of cases that only the first, even when they have three different genes like this, only the first two are expressed, and therefore they don't express M pigment and still become dichromatic. And just to bring up one remaining mystery, even in old world primate populations, we as humans are a little bit unusual in that we have a higher rate of dichromacy in human males, 2.2 to 3%, 2 to 4% of human males compared to super low rates in macaque monkeys, for example. And so there's been this theory that there's been some relaxation in the natural pressure to maintain trichromacy in humans, but we don't know why or if that is the case. So I will conclude just with this picture of a mantis shrimp, which in the popular world has been gotten a lot of fame for having 12 different types of pigment receptors, and so people have been saying, oh look, it must have this extraordinary color vision of the coral reef that it inhabits, but in fact they've actually tested this thing, and it has this very poor wavelength differentiation. It requires about 12 to 15 nanometers of difference in the types of wavelengths that absorbs to actually discriminate between colors. So it doesn't quite have the color vision that we thought, whereas humans just require like 2 to 3. So all is not just in the pigment absence. That is all that I have. Any questions that I can take? Yes? So great presentation on a fascinating subject. I went to a lecture that Robert Mark gave and some of you people may not remember Robert, but he was a brilliant physiologist, anatomist, and retina, and he talked about this color issue, and he talked about evolutionary biology that as a hunter-gatherer that if you lose the red-green differentiation the single biggest problem that affected is the inability to track blood, because you can't see the blood against the green. And that would have been critical in regards to the ability to actually track your prey. And that would definitely decrease your ability to survive. That's just not an issue. But that was interesting. He said the other one he pointed out is we like to think of these as distinct groups, but because our midget bipolar have such a big impact in what we interpret, that there's really quite a continuum of all of this. And so what he did is he had a series of slides and then he asked us, what color do you think that is? How many say it's brown? And how many think it's blue? It was amazing how this among the majority had to been there obviously with trichromats how different we were in regards to these different areas. And so he pointed out that there is an amazing diversity. And then he talked about some of these tetrachromats who've been looked at in detail. And some of them have an amazing ability to see both into the infrared and into the ultraviolet and have a richness of color. And so when people talk about some colors and other issues that we think we're talking the same thing, we may not be. And that there's really an unusual richness and a subjectivity that we don't know because we only see it as we perceive it. And others don't. But anyway, it's a fascinating subject. It was one of the most incredible hour and a half lectures I'd ever heard talking about all of this. And then some of the animal species you mentioned, there's some that have up to seven and see way into the ultraviolet infrared and have a color differentiation that we can't even imagine of the clarity and the importance of what they can see in association with that. So it's an important subject we don't talk about very often. The thing that I didn't talk about was I thought this was super important to the talk. There was an article that I read about reindeer and how because they live in the climates which have a lot of UV radiation and UV light and reflection off the snow, they have developed the ability to see UV light because you can track like urine on the snow and things that you would otherwise would miss like foliage that's underneath the snow. But it's not actually because they have a different option that absorbs that but more because their lens unlike the human lens doesn't filter out UV light. And so if there's just so much UV light it overwhelms the ability to do it like it allows even though the wave light isn't peeked to there your nearby ultraviolet or your violet pigments can absorb it and you can use it to your advantage. And then let's not forget the amazing loss of color that happens from the natural changes of our lenses moving to the cataract phase. Patients generally are amazed when their colors are restored but I've had occasional patient define suddenly seeing the clarity of some of these colors like disturbing. It's rare that every once in a while you'll find people who will do that and not like that. I think it's very strange. I know it's hard for me to see the difference between a navy blue and a black anymore. I really have to look hard and get a lot of light before I can see that. I'm not sure I would say any more. Dr. Bernstein. Thanks for taking on a very difficult topic and doing a very good job. Can you talk a little bit about where we are in your therapy for acromotapsia? No, even for red-green. There are companies working on it. Yes they are. I did not read about it. I read about acromotapsia a little bit but not very much so no I can't really speak to that. But if you would like to. They have tried it in monkeys and they've seen some positive results and the question is do you want to do gene therapy on 3% of you know it's probably the most common genetic effect to the eye. That there is and that huge population but they're otherwise normal-wise. So that actually makes me wonder because a lot of these men. One in 40 males, that makes it not uncommon right? One in 40 male. One possibility would be that a lot of these men actually do have a copy of the opsin but they just can't express it. So if we would be able to change the locus control region that is expressing how many different types of pigment genes that would be a way of a little bit less emasically potentially or less destructively trying to do gene therapy there but I don't know. It is an active area. Yes. Very good.