 Hello and welcome to physiology open. See these pictures and tell whether these colors are same or different? By the way do colors even exist in real world? How do we perceive colors? Do all people perceive colors similarly? In this video we will solve some of these mysteries related to perception of color. Across the entire range of visible electromagnetic spectrum that is from 400 to 700 nanometer wavelength we perceive a range of different colors. When the photons from this range of electromagnetic radiation stimulates cones and the subsequent signal reaches the brain. As such there are no colors in the physical world. We have three different types of cones based on the wavelength which stimulates them. S cones are a short wavelength cone also known as blue cones then M or middle wavelength cones also known as green cones and L or long wavelength cones also known as red cones. So these cones respond to photons from a range of different wavelength of electromagnetic spectrum. This is possible because the G protein coupled receptor opsin in their photopigment differs in each of these cones. This makes them respond to photons from different wavelength. But how do we perceive so many varied colors by only three cones? Well there are certain theories which discuss this. Important ones being young Helmholtz trichromatic theory and opponent process theory. First let's see young Helmholtz theory. Have you heard about RGB model which stands for red green blue model? This model is used for color generation digitally. In this model we give different numbers to R, G and B and it produces the perception of different colors. Well this RGB model uses this trichromatic theory of color vision. Fundamentally it's the proportion of stimulation of different cones which leads to perception of different colors. This is better understood by looking at a spectral sensitivity graph of different cones. This graph shows the relative responses of cones to photons from different wavelength. Here x-axis shows the wavelength of light in nanometers while the y-axis shows the relative responses of the cones. See here the S cones respond to shorter wavelength with peak response at 445 nanometers of wavelength but there are lesser responses at other wavelengths also. Similar is the case with M and L cones. Peak response of M cones is at 535 nanometers while that of L cones is 570 nanometers but you see there is quite an overlap between the spectral sensitivity graph of different cones. That means photons from a particular wavelength will elicit varied responses from different cones. So suppose a photon from 500 nanometer wavelength will excite S and L cones almost equally while M cones will be stimulated more. So there is a pattern of response which we can write like this. S is to M is to L cones responses say here it is 30 year and it is 60 year so it is 30 is to 60 is to 30 where the numbers represent the relative responses from the different cones. So each wavelength is eliciting a pattern of responses from cones. So because of this pattern of responses we can differentiate a lot of colors just by the presence of three cones. This is known as trichromatic theory of color vision. Try for three and chromatic is color so this is for cones and the people who have three different cones are known as trichromats. So now did you understand how RGB model works in computers? Basically different numbers means they will stimulate S, M and L cones similar to a photon from a particular wavelength in nature and voila we perceive the color in a screen. But just look at this picture and tell are these colors same? Well for my perception they appear similar. However you will be surprised to know that they are produced from different combinations of RGB. First one with this composition and the second one with this conclusion. Sometimes different proportions can produce same perception of color. Similar thing happens with wavelength compositions. So since this combination of wavelengths elicits same pattern of responses we see these are same color. Now imagine a condition in which somebody has four cones or are tetrachromats yes there are people with four cones. Can they appreciate more or less colors? Well as you can guess by the theory if you have understood that the wavelength which appear as same colors for trichromats will actually produce different pattern of cone stimulation in tetrachromats. So they can actually differentiate more colors than trichromats and by the same logic people with lesser number of cones can differentiate lesser number of colors as happens in defects of color vision. But look at these pictures taken in broad daylight and then in evening light. Do they appear same? Broadly you can say they are same colors but they appear of a different shade right? So basically color perception also depends on background illumination. Now see since cones have a higher threshold of light for stimulation the color perception is only possible in presence of light. On the other hand we hardly perceive any color in dark since in dark only rods are stimulated. But why does different levels of illumination or for that matter different types of illumination say you are seeing any object in broad daylight or maybe in room light how it affects perception of color as is already seen in these pictures. Well you see the background levels of illumination has different spectrum of light for example when you are seeing object in broad daylight a particular set of wavelength is stimulating the cones while the objects reflect another set of wavelength. But the cones are stimulated by both isn't it? So depending on the combination generated by both the color is perceived. That's why the background illumination is a very important factor for color perception. Okay now let's revisit the picture you saw in the beginning. Now have a look at these two pictures and tell what's the color in the center in figure A and figure B. Well in figure A it appears brown while in figure B it is gray. Now what if I tell you that actually they are same colors. So I'll just remove this outer ring for you to just show you that I'm not lying. What do you think now? Yes they are same colors. Well this happens the perception of color also depends on surrounding colors. This concept is difficult to explain by young Helmholtz trichromatic theory. Since if it's only trichromatic theory same color should elicit same pattern of responses from cones is it? And hence same color should be perceived similarly despite the surrounding color but it's not happening. That means there might be some other explanation isn't it? So this is explained by color opponent theory. Color opponent theory says that the signals from cones are processed further also. Fundamentally there are three different types of calculations which occur because signals from cones are impending on another cell also. Basically the calculations are occurring at the level of the ganglion cell and this cell output leads to color perception. We'll take one example to understand this. See this picture of green in the center surrounded by red color. What this picture will do is in exactly the center of case it will stimulate M cones and the surrounding area will stimulate the surrounding L cones. Now these signals impinge on another cell but this cell is stimulated by this center of case cones but inhibited by the surrounding cones. So it's on or excited with M cones and off or inhibited by surrounding L cones. So basically with this picture there is excitation also and inhibition also. So the response from this cell is basically the subtraction of these two effects. Similarly there are some cells which are excited by red center and inhibited or off by surrounding green but you can understand that this will also elicit a subtraction calculation. So this calculation produces a signal where signal from M cones is subtracted from L cones. This is known as a red green channel. Similarly there are two other calculations. One is blue yellow channel and the other is luminance channel. Just seeing this picture of yellow color surrounded by blue yellow at the center of case stimulates both M and L cones while blue stimulates surrounding S cones. The signal from M and L cones stimulates downstream cells while from the surrounding S cones inhibits them. So the response from this cell is different and just like red green channel there will be cells which are on blue center and off with yellow surround. Then as already stated there is one more channel where the signal from M, L and S cones is added. This is the black white channel or luminance channel. This tells us how much bright the color is. So we saw two theories of color vision young helmholtz trichromatic theory and color opponent theory in which we talked about three processing channels that is red green channel, blue yellow channel and luminance channel. Well thanks for watching the video. If you liked it do like and share the video and subscribe to the channel Physiology Open. Thank you.