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8/8/2019 DIY Calculator __ Color Vision_ One of Nature's Wonders

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Color Vision: One of Nature's Wonders

Evolution has dictated that we are visually orientated. The human brain and visual cortex are capable ofprocessing huge amounts of visual data and can quickly and efficiently recognize and extract useful informationfrom this data. In fact, studies have shown that we receive approximately 80% of our external information invisual form. Generally speaking, however, most of us tend to take our visual capabilities for granted, especiallywhen it comes to color vision...

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The Electromagnetic SpectrumThe Visible SpectrumThe Discovery of InfraredThe Discovery of UltravioletPrimary ColorsMixing Light vs. Mixing PaintTurning Things Upside DownHow Color Vision WorksColor Blindness

The Evolution of Color VisionTetrachromats, Pentachromats, Etc.Genetically Modifying Human Vision?An Amazing ExperimentLeft-to-Right and Top-to-BottomSeeing Sounds and Tasting ColorsInteresting Nuggets of TriviaVision and Visual Illusion Websites

The Electromagnetic SpectrumActually, the way in which all of this works (vision in general and color vision in particular) can be a littleconfusing at first, so sit up, pay attention, and place your brain into its turbo-charged mode. What we refer toas "light" is simply the narrow portion of the electromagnetic spectrum that our eyes can see (detect andprocess), ranging from violet at one end to red at the other, and passing through blue, green, yellow, andorange on the way (at one time, indigo was recognized as a distinct spectral color, but this is typically nolonger the case.):

DIY Calculator :: Color Vision: One of Nature's Wonders http://www.diycalculator.com/sp-cvision.s

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Of course, when we use terms like "red", "yellow", "green", and "blue", these are just labels that we havecollectively agreed to associate with specific sub-bands of the spectrum. Just outside the visible spectrumabove violet is ultraviolet, the component of the sun's rays that gives us a suntan (and skin cancer). Similarly,just below red is infrared, which we perceive as heat.

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The Discovery of the Visible SpectrumStrange as it may seem when one is first introduced to the idea, white light is a mixture of all of the colors inthe visible spectrum. This fact was first discovered around 1665-1666 by the English mathematician andphysicist Sir Isaac Newton (1642-1727), who passed a beam of sunlight through a glass prism to find that itseparated into what he called "a spectrum of colors":

In reality, even before Newton's famous experiments, a number of other people were using prisms whichwere fairly new at that time to experiment with light. Actually, when you come to think about it, it's more thanpossible that some caveman tens of thousands of years ago observed sunlight passing through a block ofnatural glass and reappearing as a rainbow of colors.

In Newton's time, however, most folks thought that it was their prisms that were coloring the light. Newton's

experiments took things much further. First, he used a prism to separate white sunlight into his spectrum ofcolors as shown above. Next, he used a piece of card with a slit in it to block all of the colors except one say green and then he passed this individual color through a second prism. Newton's thought was that if itwas the prism that was coloring the light, then the green light entering the second prism should come out adifferent color. The fact that it came out the same color indicated to Newton that it wasn't the prism that wascoloring the light.

Newton then took the spectrum coming out of his first prism and fed it into an "upside down" prism. Thiscaused the individual colors to recombine back into white light. By these experiments, Newton was the first toprove that white light was made up from all of the colors in the visible spectrum and that his first prism wassimply separating these colors out.

As a point of interest, Newton originally declared that there were eleven colors in the visible spectrum. Later,he toned this down to seven in order to make his spectrum fit with contemporary Western ideas about musicalharmony (specifically, that there were seven notes/tones in an octave). This is why the spectrum wasoriginally said to include indigo, but more recently is defined as comprising only red, orange, yellow, green,

blue, and violet.

Last but not least, we're used to seeing the same effect as Newton's experiment in the form of a rainbow,which is caused by sunlight passing through droplets of water, each of which acts like a tiny prism. In fact, asfar back as the 13th century, the famous Franciscan friar and English philosopher Roger Bacon (1212-1294)suggested that rainbows were caused by the reflection and refraction of sunlight through raindrops, but atthat time he had no way to prove that this was indeed the case.


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The Discovery of Infrared

Friedrich Wilhelm Herschel (1738-1822) was born in Hanover, Germany. In 1757 he moved to England,where he became known as the famous astronomer and musician Sir Frederick William Herschel.

One thing for which Herschel is particularly well known occurred in 1781, when he discovered the seventh

planet from the sun Uranus. As an aside, the eighth planet Neptune was discovered in 1846, while theninth Pluto was discovered in 1930. From that time, every high school student has been taught that oursolar system has nine planets. Ever since its discovery, however, referring to Pluto as a planet has beensomething of a pain to many astronomers. Apart from anything else, Pluto has a very eccentric orbit, whichmeans that some of the time it comes closer to the sun than Neptune.

Just to increase the fun and frivolity, in 2004, astronomers discovered what some regarded at that time asbeing the tenth planet Sedna which takes 10,500 years to orbit the sun (as compared to Pluto, whichcompletes the trip in only249 years). But 18 months later, in 2005, astronomers discovered what becamecommonly called Xena (see also the note below with regard to changing Xena's name), which falls betweenthe orbits of Pluto and Sedna, and which takes 560 years to orbit the sun. This would make Xena the tenthplanet and Sedna the eleventh planet.

The real problem was that believe it or not until the middle of 2006, there actually was no rigorousdefinition that everyone agreed on as to what was (and was not) a planet. The discovery of Xena and Sednabrought things to a head, because astronomers now expect to find large numbers of similar objects. Thus, aswas reported in the Washington Post, in August 2006, the International Astronomical Union stripped poorold Pluto of its planetary status, reclassified the little scamp, and placed in a new category called "dwarfplanets" (these are similar to what had been referred to as "minor planets" in the past).

Furthermore, as perThis Report on MSNBC, on Wednesday 13th September 2006, the InternationalAstronomical Union officially changed Xena's designation to Eris (aptly named after the Greek goddess ofchaos and strife).

But we digress... In 1800, Herschel started to wonder if the different colors in the spectrum had differenttemperatures associated with them (you have to admire someone like that, because this sort of thoughtsimply wouldn't occur to the majority of us). Anyway, he used a thermometer to measure the temperatures ofthe different colors, and he observed that the temperature rose from violet (with the lowest value) throughblue, green, yellow, and orange, until it reached its peak in the red portion of the spectrum.

Now here's the really clever part Herschel next moved the thermometer just outside the red portion of the

spectrum in an area that to the human eye contained no light at all. To his surprise, he discovered thatwhat he came to regard as being "invisible rays" in this area had the highest temperature of all. Following aseries of experiments in which he proved that these invisible rays behaved just like visible light (in that theycould be reflected, refracted, and so forth), Herschel christened his discovery infrared rays (where the prefixinfra comes from the Latin, meaning "below").

In addition to leading us to an understanding of heat, Herschel's discovery was also important because it wasthe first time anyone had demonstrated that there were forms of radiation ("light" in his terms) that humanscouldn't see.

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The Discovery of Ultraviolet

Although his was a short life of only 33 years, Johann Wilhelm Ritter (1776-1810) certainly made the most ofit. Born in Samitz, Silesia, which is now part of Poland, he studied science and medicine at the University ofJena.

While at the University, Ritter performed numerous experiments with light and later electricity. Afterhearing about William Herschel's discovery of infrared light beyond the red end of the visible portion of thespectrum, Ritter decided to see if he could discover his own "invisible rays" beyond the violet end of thespectrum.


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Ritter knew that silver chloride turned black when exposed to light. Thus it was that, in 1801, in the same waythat Herschel had used a thermometer to measure the temperature of the different colors, Ritter decided touse silver chloride to see if it reacted at a different rate to the different colors.

First, he placed a quantity of the chemical in the path of the red portion of the spectrum and observed thatany change was relatively slow. Next, he tried orange, followed by yellow, green, blue, and violet, observingthat each new batch of silver chloride grew darker faster as he progressed through the spectrum.

Finally, Ritter placed a quantity of the chemical just outside the violet portion of the spectrum in an area that

to the human eye contained no light at all. To his delight, he discovered that invisible rays in this area hadthe greatest effect on the silver chloride. This new type of radiation eventually came to be known as ultravioletlight(where the prefix ultra comes from the Latin, meaning "beyond").

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Primary Colors

In the case of color televisions and computer screens, each picture element (pixel) is formed from a group ofred, green, and blue (RGB) dots (see also our paper on The Origin of the Computer Console). If all three ofthese dots are active (lit up) at the same time, from a distance we'll perceive the group as a whole as beingwhite. (If we looked really closely we'd still see each dot as having its own individual color.) If we stimulate justthe red and green dots we'll see yellow; combining the green and blue dots will give us cyan (a greenish,lightish blue); while mixing the red and blue dots will result in magenta (the color magenta, which is a sort ofpurple, was named after the dye with the same moniker; in turn, this dye was named after the battle ofMagenta, which occurred in Italy in 1859, the year in which the dye was discovered). Furthermore, mixingdifferent proportions of the three light sources will result in a gamutof colors, where the word "gamut" means"a complete range or extent".

Now, this may seem counter-intuitive at first, because it doesn't seem to work the way we recall being taughtat school, which was that mixing yellow and blue paints together would give us green, mixing all of thecolored paints together would result in black (not white as discussed above), and so on. The reason for this isthat mixing light is additive, while mixing paints or pigments is subtractive:

The appellationprimary colors refers to a small collection of colors that can be combined to form a range ofadditional colors. In the case of light, the primary colors we typically use are red, green, and blue. Sincebringing in new color components "adds" to the final color, these are known as the additive primaries. Bycomparison, when it comes to paints or pigments, the primary colors used by printers are cyan, magenta, andyellow (CMY). In this case (for the reasons discussed in the following topic), bringing in new colorcomponents "subtracts" from the final color, so these are known as the subtractive primaries. Actually,


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forming black by mixing cyan, magenta, and yellow inks together is expensive and typically results in a"muddy" form of black, so printers typically augment these primary colors with the use of black ink. The resultis referred to as CMYK, where the 'K' stands for "blacK" (we don't use 'B' to represent "black" because thiscould be mistakenly assumed to refer to "blue").

Now, it may be that you have accepted all of the above without a quiver of doubt. On the other hand, you maybe staring at this page with a furrowed frown on your forehead saying to yourself: "Just a minute, that's notwhat my old art teacher Professor Cuthbert Dribble taught me at elementary school. When it came topaints, he said that the three primary colors were red, yellow, and blue (RYB); that mixing red and yellow

gave orange; combining red and blue gave purple; and blending yellow and blue gave green. So, can youexplain this conundrum?"

Well of course I can! Look into my eyes ... have I ever lied to you before? The simplest explanation is thatteachers can tell you anything they like at elementary school and you'll believe them. A slightly more complexanswer is that the concept of red, yellow, and blue as primary colors predates our modern scientificunderstanding of color theory. However, although both of these arguments are true in their own way, the factthat you are reading this paper marks you as a person of high discernment, sharp wit, and keen intellect whodemands nothing less than the most fulsome of explanations for your reading pleasure, so here goes...

In reality, you can pretty much pick any three (or more) colors and call them "primary" colors, and this will betrue on the basis that they are your primary colors. Mixing two of your primary colors together will result in asecondarycolor; mixing one of your primary colors with one of your secondary colors will result in a tertiarycolor, and so forth.

One example of a non-standard collection of primary colors was an early color photographic process known

asAutochrome, which was invented circa 1903-1904 in France by the Lumire brothers, Auguste and Louis.This process typically used orange, green, and violet as its primary colors.

In 1666, as part of his experiments with prisms, Sir Isaac Newton developed a circular diagram of colors thatis now commonly referred to as a "color wheel". For one reason or another, theorists of that time decided thatred, yellow, and blue were the best primary colors for pigments, and even though we now know that red,yellow, and blue primaries cannot be used to mix all of the other colors they have survived in color theoryand art education to the present day.

Purely for the sake of completeness, let's consider a color wheel based on red, yellow, and blue as it'sprimary triad as shown below:


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Using our three primary colors as a starting point, we can generate three secondary hues: mixing red andyellow gives orange, yellow and blue gives green, and blue and red gives purple. Similarly, mixing the primarycolors with their adjacent secondary colors results in six tertiary hues: red-orange, yellow-orange, yellow-

green, blue-green, blue-purple, and red-purple.

There are lots of different theories regarding the way in which different colors can be used in conjunction witheach other so as to produce a pleasing effect to the eye (that is, so that it looks good to humans). Forexample, complementary colors are any two colors that are directly opposite each other on the color wheeland provide maximum contrast, such as red and green, red-orange and blue-green, and so forth. Bycomparison, analogous colors are any three colors that are side-by-side on the color wheel, such as yellow,yellow-green, and green.


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The problem is that the above diagrams notwithstanding red, yellow, and blue are not well-spaced arounda perceptually-uniform color wheel that embraces the entire spectrum of colors. This means that using red,yellow, and blue as primaries yields a relatively small gamut, and it is impossible to mix them so as to achievea wide range of colorful greens, cyans, and magentas. This is the reason why modern color photography andthree-color printing processes employ cyan, magenta, and yellow as primaries, because these offer a muchwider gamut of colors.

At this juncture, we should perhaps briefly mention terms like shade, tint, and hue. The problem is that all ofthese words have several different meanings depending on whom you are talking to. For our purposes here,we may say that hue is the quality of a color that allows us to assign it a name like "greenish-blue" or"reddish-orange". More formally, one might say that the hue is the dominant wavelength of a particular color that is, the "color of a color".

Meanwhile, shade may be described as "the degree of darkness within a hue" and tintmay be considered tobe "the degree of lightness within a hue". In the case of painting, for example, artists have long used the word"shade" in the context of mixing a color with black, so a shade is a color which has been made darker in thisway. By comparison, artists use the word "tint" to refer to the mixing of a color with white, so a tint is a colorwhich has been made lighter in this way.

As a further point of interest, it is common to refer to red, yellow, green, blue, white, and black as being thepsychological primaries, because we subjectively and instinctively believe that these are the basis for all ofthe other colors.

Before we move on, a reader of an earlier version of this paper retired electronics engineer Dwight W.Grimes emailed me to say that he'd been pondering my original "Additive and subtractive colorcombinations" diagram shown at the beginning of this topic. After considering the additive and subtractivecolor combinations in the context of Venn Diagrams (one of the logical tools used by electronics and

computing engineers), Dwight suggested that a slightly more intuitive representation might be as shownbelow:


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Dwight's idea is that, in the case of light, the surrounding "world" (in the form of an empty theater/stage/roomwith the lights turned off, for example) should be black, then we addred, green, and blue light by activatingappropriately colored spotlights; the combination of all of these light sources results in white light. Bycomparison, in the case of paint, the surrounding "world" (in the form of a large piece of paper, for example)should be white, then we subtractcolors by applying cyan, magenta, and yellow pigments to the paper; thecombination of all of these pigments results in black. By Jove, I think Dwight is right (I'm a poet and I neverknew-it). I will use this new representation in the future. (If you want to know more about the origin of VennDiagrams, please feel free to peruse and ponder ourLogic Diagrams and Machines paper.)

Last but not least, I recently (as I pen these words) ran across yet another representation of the mixing ofprimary colors that rather took my fancy. As you can see in the following illustration, this is similar to Dwight'sproposal, except that this new version is presented as a gradual merge between the various primary colors(I'm sorry I couldnt arrange for the primary colors in this new illustration to be in the same relative locationsas for my earlier diagrams, but this is the way I found them):

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Mixing Light versus Mixing Paint

So why does mixing light work one way while mixing paint works another? Gosh, I was hoping you wouldn'task me that one. Well, here's a question right back at you what colors come to mind when you hear thewords "tomato," "grass," and "sky"? You almost certainly responded with red, green, and blue, respectively,but why? The main reason is that when you were young, your mother told you that "Tomatoes are red, grassis green, and the sky is blue,"and you certainly had no cause to doubt her word.

However, the terms "red," "green," and "blue" are just labels that we have collectively chosen to assign tocertain portions of the visible spectrum. If our mothers had told us that "Tomatoes are blue, grass is red, andthe sky is green,"then wed all quite happily use those labels instead.

What we can say is that, using an instrument called a spectrometer, we can divide the visible part of thespectrum into different bands of frequencies, and we've collectively agreed to call certain of these bands"red," "green," and "blue." Of course everyone's eyes are slightly different, so there's no guarantee that yourcompanions are seeing exactly the same colors that you are. Also, as we shall see, our brains filter andmodify the information coming from our eyes, so a number of people looking at the same scene will almostcertainly perceive the colors forming that scene in slightly different ways.

Here's another question for you: "Why is grass green?" In fact we might go so far as to ask: "Is grass reallygreen at all?"Surprisingly, this isn't as stupid a question as it might seem, because from one point of view wemight say that grass is a mixture of red and blue; that is, anything and everything except green! The reasonwe say this is that, when we look at something like grass, what we actually see are the colors it didn'tabsorb.For example, consider what happens when we shine white light on patches of different colored paint:

The red paint absorbs the green and blue light, but it reflects the red light, which is what we end up seeing.

Similarly, the green paint absorbs the red and blue light and reflects the green, while the blue paint absorbsthe red and green and reflects the blue. The white paint reflects all of the colors and the black paint absorbsthem all, which means that black is really an absence of any color. Thus, returning to our original questionabout the color of grass: we could say that grass is green because that's the color that it reflects for us to see,or we could say that grass is both blue and red because those are the colors it absorbs.

This explains why mixing paints is different from mixing light. If we start off with two tins of paint say cyanand yellow and shine white light at them, then each of the paints absorbs some of the colors from the whitelight and reflects others. If we now mix the two paints together, they each continue to absorb the same colorsthat they did before, so we end up seeing whichever colors neither of them absorbed, which is green in this


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case. This is why we say mixing paints is subtractive, because the more paints we mix together, the greaterthe number of colors the combination subtracts from the white light.

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Turning Things Upside Down

Believe it or not, there is a point to all of this (well, most of it ... well, at least some of it), although we won'tfind out what that point is until later in this paper. But before we move on, it is perhaps appropriate to note thatalthough the concept of colors is reasonably simple (being merely sub-bands in the visible spectrum), colorvision is amazingly complex.

The human visual system, is composed of our eyes, brain, and nervous system (actually, the eyes and brainare part ofthe nervous system, but I tend to think of them as separate entities). Our visual system hasevolved to such a sophisticated level that for a long time we didn't even begin to comprehend the problemsthat nature had been compelled to overcome. In fact, it was only when we (the human race) came toconstruct the first television cameras and television sets and discovered they didn't work as expected thatwe began to realize there was a problem in the first place.

First of all, it is commonly accepted (athough not necessarily correct, as is noted in the sidebar below) that wehave five senses: touch, taste, smell, hearing, and sight. Of these senses, sight accounts for approximately80% of the information we receive, so our brains are particularly well-adapted at processing this informationand making assumptions based on it.

For example, if you give someone yellow jello, they will automatically assume that it will taste of lemon;

similarly for green jello and lime and red jello and strawberry. (What the Americans call "jello" would bereferred to as "jelly" in England. By comparison, what the Americans call "jelly" would translate to "jam" in themother-tongue, and don't even get me started on what the Americans refer to as "preserves".) Thisassociation is so strong that if you give people yellow jello with a strawberry flavor, they often continue tobelieve it tastes of lemon. One theory to explain this is that our brains give more "weight" to what our eyes aretelling us compared to what our taste buds are trying to say; another hypothesis is that this has more to dowith repeated learning and pairing of particular colors with particular tastes.


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When we use our eyes to look at something, the data is pre-processed by an area of the brain called thevisual cortex, followed by the rest of the brain, which tries to make sense of what we're seeing (this issomething of a simplification see also the How Color Vision Works topic below). The brain's ability toprocess visual information is nothing short of phenomenal. For example, in a famous experiment that was firstperformed in 1896, a psychologist at the University of California in Berkeley George Malcolm Stratton(1865-1957) donned special glasses which made everything appear to be upside down. Amazingly, after afew days of disorientation, his brain began to automatically correct for the weird signals coming in and causedobjects to appear to be the right way up again.

Similarly, when he removed the special glasses, things initially appeared to be upside down because his brainwas locked into the new way of doing things. Once again, within a short period of time his brain adapted andthings returned to normal. (Actually, the way in which the lenses in our eyes function means that the images

we see are inverted by the time they strike the retina at the back of the eye, so our brains start off by havingto process "upside-down" data as illustrated in the figure below see also the Left-to-Right and Top-to-Bottom topic later in this paper).


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How Color Vision Works

The previous topic exemplifies the brain's processing capabilities, but it doesn't begin to illustrate how well wehandle vision in general and color vision in particular. As a starting point, we should probably remindourselves as to the main constituents of the human eye:

First, light from the outside world passes through the cornea, which acts like a clear, transparent, protective"window". Just inside the cornea we find the iris, which gives our eyes their distinctive color. When we say"she has blue eyes,"for example, we are talking about the color of that person's irises. The hole in the middleof the iris is called thepupil, which determines how much light is passed into the body of the eye. The iriscauses the pupil to shrink in bright light and to enlarge in dim light.

Next, the lens is used to focus the light on the back of the eye, which is covered by a layer called the retina.The retina often used to be compared to the film in a conventional camera, but it is actually more akin to the

sensor element in a modern digital camera. Amongst other things, the retina contains special photoreceptornerve cells that convert rays (photons) of light into corresponding electrical signals. After some processing inthe eye itself, these signals are passed along the optic nerve into the visual cortex region of the brain(actually, this is something of a simplification if you want a little more detail, check out the Left-to-Right andTop-to-Bottom topic later in this paper).

As an aside, the reason our pupils appear to be black is that generally speaking light goes into our eyesbut it doesnt come back out. One exception to this rule is when someone takes photograph of you with thecamera's flash turned on and you end up with so-called "red eye" that is, your eyes appear to be red in thepicture. In this case, the light from the flash is reflected back off the blood-rich retina on the rear of your eyeand returns out of the pupil as red light.

Now, most high school biology textbooks would tell us that the retina in the human eye features three differenttypes of color photoreceptors; some are tuned to respond to red light, some to green, and some to blue (ifyou're a physicist or a biologist or any other "ist", please read the disclaimer in the following paragraphs

before you start jumping up and down, ranting and raving and rending your garb with regard to ourterminology). Based on this assumption, related engineering text books would tell us that this is why we usered, green, and blue dots on our television screens and computer monitors to generate all of the colors,because this directly maps onto the way in which our eyes work. Sad to relate, all of these text books areincorrect.

Before we proceed further, it's time for some "weasel words." Originating in the late 1800s, this is a term thatmay be defined as: "equivocal words used to deprive a statement of its force or to evade a directcommitment,"or"words that make one's views equivocal, misleading, or confusing,"or my personal favorite "the art of saying what you dont mean."This turn of phrase may have been sparked by the weasel's habit


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of sucking the contents out of a bird's egg such that only the shell remains. (Lest we be unfair to the poor oldweasel, we should also remember the saying: "Eagles may soar high in the sky, but weasels rarely getsucked into jet engines at 20,000 feet!").

But, as usual, we're wandering off into the weeds. In a moment we're going to introduce the concept of colorphotoreceptor cells called cone cells. The point is that when we say things like "blue cones"or"cones thatrespond to blue light,"we really mean "photoreceptor cells that are tuned to respond to the range offrequencies in the electromagnetic spectrum that we perceive as being blue". However, although thescientists amongst us would prefer this more precise terminology, it's a lot easier to refer to things like "blue

cones," so if we occasionally slip, you'll know what we mean.

Moving on, the reason most references talk about red, green, and blue receptors in the human eye datesback to around 1801-1802, when the English physician Thomas Young (1773-1829) performed a series ofexperiments and proposed his "trichromatic theory."

Young's hypothesis originated in observations by artists, clothing manufacturers, and so forth, whorecognized that if you had three different pigments you could mix them to form any other color. Prior to Young,people had suggested that there were three different types of light, so Young's recognition that the "three"was due to human anatomy and physiology rather than the physics of light was a major conceptualbreakthrough. Young's hypothesis, which was refined around 50 years later by the German scientist Hermannvon Helmholtz (1821-1894), proposed that the human eye constructed its sense of color using only threereceptors for red, green, and blue light. Based on this theory, humans are known as trichromats.

It was originally assumed that the electrical signals from the different types of color receptor cells were fed viathe optic nerve directly into the visual cortex portion of the brain, which used these signals to determine

different colors. However, we now know that the truth is a little more complex. We'll start by saying it is truethat the human boasts three types of color receptors called cone cells (so-named because they have asomewhat conical appearance when viewed under a microscope). There are about 6 million of these cells ineach eye. Each type of cone cell covers a range of frequencies, but is primarily sensitive to a particularportion of the spectrum. As it happens, one kind of cone cell is primarily sensitive to bluish-violetlight, but theother two are most sensitive to greens; one peaks at a bluish-green and the other peaks at a yellowish-greenas shown in the illustration below. (You can discover more about this by visiting the Wikipedia entry on ConeCells and also in the The Evolution of Color Vision topic in this paper.)

As there are around an order of magnitude fewer bluish-violet cone cells than the other two types and asthe other two types are both sensitive to greens this explains why the human eye is particularly sensitive tovariations in the green portion of the spectrum. (For the more pedantic amongst us, the actual ratio of bluish-


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violet to bluish-green to yellowish-green cone cells is about 1:10:20.)

Young's trichromatic theory was extremely successful and became generally accepted wisdom for almost 175years. However, as opposed to our perceiving different colors by directly accessing the signals beinggenerated by our cone cells, we now know that our color perception is based on something called theopponent process. This alternate theory was first proposed by the German physiologist and psychologist KarlEwald Hering (1834-1918), but the opponent process didn't gain a wide following in the scientific communityuntil the 1970s.

The idea behind the opponent process is that although their sensitivities peak at different frequencies there is a large amount of overlap with regard to the wavelengths of light to which the three types of conecells respond, so our visual systems are designed to detect differences between the responses of thedifferent cones. In order to achieve this, the retina boasts large numbers of "comparator" cells each of thesecells compares the signals being generated by a number of different cone cells, and it is the signals comingout of the comparator cells that provide color information to the brain The end result is that we perceive thecolor yellow when our yellowish-green cones are stimulated slightlymore than our bluish-green cones, forexample; similarly, we perceive the color red when our yellowish-green cones are stimulated significantlymore than our bluish-green cones.

Actually, it's worth taking a little time to make sure that we more fully understand the way in which this works.Observe how the response curves for the blue-green and yellow-green receptors in the illustration abovestrongly overlap each other. Now, remember that the author drew these images in Microsoft Visio and healso created the spectrums at the bottom using Adobe Photoshop, so these are just approximations.Having said this, if you look toward the right of the curve associated with the yellow-green receptors where it'sover the yellow area of the spectrum, you'll see that at this frequency these receptors are being stimulated

more than are the blue-green receptors, and thus we end up perceiving this portion of the spectrum as beingyellow.

Now, move a little to the left into the green portion of the spectrum. Once again, the yellow-green receptorsare being stimulated more than the blue-green receptors but the trick here is that both types of receptorsare being stimulated more than they were for the yellow portion of the spectrum, so we perceive this as beinggreen.

This is where things start to get a little tricky, because you could just say: "But that's just a matter of intensity!"Sad to relate, this is where I (the author) pass beyond the scope of my knowledge. However, I think it ties intothe An Amazing Experiment topic later in this paper, which describes how the brain weights all of the colorsit sees against all of the other colors.

All of this serves to explain why, when the day draws towards dusk, we loose the ability to see red first. This isbecause even in reasonably bright light we are only obtaining a relatively small amount of stimulation inthat part of the spectrum; thus, as the overall intensity of the ambient light starts to fall, this stimulation ceasesfirst. It also explains why in the middle of the night (assuming a full moon) we can still see hints of green,because the main spectral component of moonlight is relatively close to the most sensitive portion of theresponse curve for our blue-green receptors.

Moving on ... in addition to cones, the human eye also has a fourth (dim light) type of receptor called rods,which are so-named because of their shape when viewed under a microscope. These cells, which are muchmore sensitive than cone cells, come into play in low-light conditions like dusk and throughout the night. Rodcells, which outnumber their cone cell companions by a factor of around 20-to-1, have their peak sensitivityaround 498 nanometers (nm).


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The fact that rod cells are so much more sensitive to dim light than are cone cells, and also the fact that thereare so many rod cells, explains why our sense of color drops as the level of ambient light falls. In bright light,our peak sensitivity is to the bluish-green and yellowish green cones, which we use to perceive colors likegreen, yellow, and red. Under these bright light conditions, our rod cells are being completely over-stimulatedand are not providing any useful information whatsoever. As the day heads toward dusk and the ambient lightdims, however, our peak sensitivity switches to bluish-violet and bluish-green, which is why we still see blueand green hues after the other colors have faded away. This effect, which is known as the Purkinje Shift, isnamed after the Czech physiologist Jan Evangelista Purkinje (1787-1869). Finally, when the ambient lightbecomes very faint, our cone cells effectively shut down and only our rod cells remain functional to supply us

with our night vision capabilities.At this point, it may be worth noting that some references state that the peak sensitivity of rod cells is close tothe main spectral component of moonlight. Based on this, one might hypothesize that rod cells first evolved innocturnal animals. Alternatively, one might propose that the evolution of rod cells facilitated certain animalsbecoming nocturnal. In fact, irrespective as to whether or not rod cells first evolved in nocturnal animals, theirpeak absorption is notparticularly close to the main spectral component of moonlight, which actually occursat anywhere between 548 and 575 nanometers depending on your source of data (I've shown the lowerbound in the following illustration see also the discussions on the The Evolution of Color Vision later inthis paper).


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Before we leap into the topic on the evolution of color vision with gusto and abandon (and that topic is areally, REALLY interesting one), let's briefly summarize what we've learned thus far. First, the typical humaneye has three different types of cone photoreceptors that require bright light, that let us perceive differentcolors, and that provide what is known asphotopic vision. Second, we have rod photoreceptors that areextremely sensitive in low levels of light, that cannot distinguish different colors, and that provide what isknown as scotopic vision.


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As one final a point of interest, some animals have only two types of cone receptors, so these animals arereferred to as being dichromats. One example of this that is close to home would be "man's best friend" thedog which has some cones that are primarily sensitive to blue light and others that are primarily sensitive toyellow light.

It's a bit difficult to conceive what this might look like, but I just ran outside and asked a friend to take a pictureof me wearing a red Hawaiian shirt standing between a blue car and a yellow car with a green field behind meas shown in the image below. I then modified the picture to simulate the effects of having only blue and yellowcones like a dog as shown in the lower picture below.


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So, how did humans come to be trichromats while dogs ended up being dichromats? Well, as we shall soondiscover (following a brief discussion on Color Blindness), the evolution of color vision is a tale with so manyexciting twists and turns that it will make your head spin.

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Color Blindness

Before we proceed, this is probably a good time to note that several forms of color blindness which meansthe inability to distinguish certain colors or hues are caused by deficiencies in one or more of the cone


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receptors. Color blindness (also known as "Dyschromatopsia") may also be referred to as "Daltonism,"so-named after the English physicist John Dalton (1766-1844) who was one of the first to describe thiscondition, and who was himself affected (in addition to the purple and blue portions of the spectrum, he couldperceive only one other color yellow).

Most of us remember the tests the eye doctor gave us at high school involving cards containing imagesformed from large numbers of different sized circles of different colors. The idea was to determine if you coulddistinguish a number formed from circles with one selection of colors presented against a background ofcircles with another selection of colors. Some websites presenting examples of this sort of test are the

Ishihara Test for Color Blindness, the Color Blindness Self-Test, and Mike Bennett's Color Vision Testpages.

Also, for your interest, there is a really amazing website on Color Vision that allows you to mix different textand background colors and then see how they would look to folks with different types of color blindness.)

But wait, there's more, because the Vischeck website presents a tool called Vischeckthat simulatescolorblind vision and another tool called Daltonize that corrects images for colorblind viewers.

And yet another very clever tool is Visolve from the folks at Ryobi System Solutions. This is specialsoftware that takes colors on a computer display that cannot be discriminated by people with various forms ofcolor blindness and transforms them into colors that can be discriminated. In addition to a variety oftransformations and filters, you can also instruct the software to apply different hatching patterns to differentcolors. This really is very clever technology and you should take a moment to check it out.

Last, but certainly not least, on May 21, 2007, an article on the Science Daily website discussed how gene

therapy was used to Restore Cone Cells in blind mice. As reported in this article, scientists have used aharmless virus to deliver corrective genes to mice with a genetic impairment that robs them of vision. Thisdiscovery shows that it is possible to target and rescue cone cells the most important cells for visualsharpness and color vision in people. In the future, it may be possible to deliver gene therapies targeting avariety of visual problems such as color blindness and degenerative diseases (see also the GeneticallyModifying Human Vision topic later in this paper).

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The Evolution of Color Vision

Before we start this topic, it's worth remembering the famous quote by Sir Isaac Newton in a letter to RobertHooke circa 1675-1676, in which he modestly said (in Latin): "Pigmaei gigantum humeris impositi plusquam

ipsi gigantes vident,"which translates as " If I have seen further, it is by standing on the shoulders of giants."

This comes from the idea that: "A dwarf standing on the shoulders of a giant may see farther than a gianthimself."For those who are interested, the first usage of"on the shoulders of giants"(Latin: "nanos gigantiumhumeris insidentes") is attributed to the French Neo-Platonist philosopher, scholar, and administrator Bernardof Chartes (Bernardus Carnotensis) around 1130 (check out This Wikipedia Entry for more details on thistopic).

But I'm wandering off into the weeds again. The point is that, in this case of this topic, I'm balancedprecariously on the shoulders of research scientist Mickey P. Rowe of the Neuroscience Research Instituteand Department of Psychology at the University of California, Santa Barbara, California.

Mickey is at the forefront of current understanding with regard to color vision in general and the evolution ofcolor vision in particular, especially with regard to mammals and more specifically primates. His work isbased on our ever-increasing understanding of the paleontological record and the application of new toolsand techniques in molecular biology.

My first introduction to this "man amongst men" was when I read an article authored by Mickey and hiscolleague Professor Gerald H. Jacobs. This little scamp, which was entitled Evolution of Vertebrate ColorVision, was published in the Journal of Optometrists Association in Australia (and some folks would say that Idont know how to party down and have a good time go figure!).

What an article I am a bear of little brain I didnt understand a word of it that's how good it was! Thus,following an exchange of emails, I chatted to Mickey on the telephone and he was kind enough to walk methrough things step-by-step. You wouldnt believe how convoluted this all is, so it's important to understandthat the following summation is my greatly simplified version of the tale Mickey wove for me (any errors are


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mine own).

One final qualifier before we leap into the fray is that, for the purposes of this paper, we're primarily interestedin following the evolutionary path as it pertains to color vision from the dim-and-distant past to humans.There are many other paths for other creatures such as insects that we dont have the time to discusshere (having said this, later in this paper you will run across occasional notes with regard to the visualsystems of a variety of creatures such as mantis shrimp, butterflies, fish, and birds).

OK, just to provide a sense of the time scale with which we're working, it's now generally accepted (check out

the US Geological Survey website, for example) that the earth formed around 4.6 billion years ago give ortake 100 million years or so, possibly on a Wednesday morning at about 9:00 am, but probably not (note thatwe're using the American interpretation of "billion" equating to "one thousand million"). The earliest forms oflife possibly based on self-reproducing RNA molecules are thought to have originated somewhere around4,000 million years ago (the Wikipedia entry for the Timeline of Evolution provides a useful starting point forthis sort of thing). Proto-cell-type organisms may have arisen as early as 3,900 million years ago, and the firstreal single cell-like organisms began to appear on the scene sometime between 3,500 and 2,800 millionyears ago (many folks think this was probably a good deal closer to the older age). These were followed bythe first multi-cell organisms, which entered the stage sometime between 1,500 and 600 million years ago(recent findings are increasingly pushing us toward the earlier time).


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Next, vertebrates (animals with backbones and/or or spinal columns) started to evolve somewhere between530 and 510 million years ago during the "Cambrian Explosion" portion of the Cambrian Period. (A goodstarting point for information on these creatures is the Vetrebrates section of the University of California,


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Berkeley website. The first tetrapod(an animal that has four limbs, along with hips and shoulders and fingersand toes) crawled out of the Earths oceans some time between 375 and 350 million years ago (this eventprobably occurred relatively soon after the "walking fish" called Tiktaalik roseae took the stage, whichhappened 375 million years ago in the late Devonian Period.)

Observe that the vertical scale in the above illustration is logarithmic. This provides a method forrepresenting a large span of time while maintaining resolution at the more recent end of the scale.Had we used a linearscale in which 1 mm was used to represent a million years, for example, thenour chart would have been 5 meters long. This wouldnt have worked because I had only a little

over 7 inches to play with (stop smirking, you know what I mean).

Even though they arent particularly relevant to our story, it would be remiss of us to omit the creatures wecommonly think of as dinosaurs, which were a group of vertebrates that appeared during the Mesozoic Era(when I say "...we commonly think of as..."I mean that we're talking about non-avian dinosaurs). These littlerapscallions had a good run from late in the Triassic Period (about 225 million years ago) until the end of theCretaceous Period (about 65 million years ago), at which point they exited the stage.

Meanwhile, the first mammals (which were small shrew-like animals) evolved in the Late Triassic and EarlyJurassic Periods, some 208 million years ago (the term "mammal" refers to the group of vertebrates havingmammary glands, which females of the species use to produce milk to nourish their young).

The termprimate refers to the group of mammalian vertebrates that contains all of the species related tolemurs, monkeys, and apes (where "apes" includes humans). Until relatively recently, it used to be thoughtthat the evolution of the primates started in the early part of the Eocene Epoch (this epoch began around 55million years ago and lasted for around 20 million years). However, even though it was only the size of a

modern mouse, Purgatorius is arguably a primate or at least a proto-primate and this little scamp livedduring the early Paleocene Epoch, so it's probably more accurate to say that primates began to evolvearound 60 to 65 million years ago.

The first primates of the human genus (that is, to which the honorific "Homo" was applied) were Homo habilis;these were users of stone tools who took their turn on the stage from around 2.2 million years ago to 1.6million years ago. (The term hominidused to be popular to describe all of the creatures in the human linesince it diverged from that of the chimpanzees, but the scientific community now favors the term hominin forthis purpose. If you have any questions relating to how we evolved, a good place to start is the PBS websiteon the Origins of Human Evolution).

Neanderthal man was on the scene from around 250,000 years ago until 30,000 years ago. (It used to bethought that Neanderthals were only on the scene from around 150,000 to 35,000 years ago, but ongoingdiscoveries keep on pushing the boundaries out in both directions.) Meanwhile, the generally accepted datefor the arrival of anatomically modern humans is around 100,000 years ago. In this case, however,discoveries by ProfessorFrank Brown, Dean of the College of Mines and Earth Sciences at the University ofUtah, suggest that this could have occurred much earlier perhaps even as early as 195,000 years ago. Lastbut not least (for the purpose of these discussions), the first appearance of the Cro-Magnon culture occurredaround 40,000 years ago. This leaves us with the current peak in human evolution, which would be me (andyou, I suppose).

So, how does the evolution of color vision map onto the above? Ah, ha! That's the million dollar question, isntit? Until relatively recently, many folks worked under the incorrect assumption that the path from the originallife forms to humans was largely one of monotonic improvement. In the case of color vision, for example,many folks assumed that the evolutionary path started with black-and-white vision and progressed first todichromatic color vision and then to trichromatic color vision. However, more recent developments in ourunderstanding of the paleontological record coupled with new tools and techniques in molecular biology have revealed that the picture (if youll forgive the pun) is far more complex.

Let's take things one step at a time. First, even as a "thought experiment," it would seem unlikely that rodcells preceded the earliest cone cells. This is because rod cells are so much more sensitive to light than are

cone cells, which makes it logical that at least one type of cone cell evolved first. In fact, several lines ofevidence now point to the fact that rod cells are derived from cone cells. So when did the first cone cellevolve? Well, the ancestor of all animals with bilateral symmetry may have evolved anywhere from 550million to 1,000 million years ago, so this is the age range during which photoreceptors first evolved.

Furthermore, photoreceptors may have evolved twice or even more times, although this was probably fromthe same precursor cell population. Either that, or there was a very early split into two very different typesafter the emergence of the first photoreceptor. Note that we aren't talking about rods and cones here; insteadwe're referring to the difference between ciliated photoreceptors (like the ones we use for vision) andrhabdomeric photoreceptors (like the ones arthropods use for vision).


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Be this is it may, at some stage along the path say 800 million years ago on a Wednesday afternoonfollowing a small lunch some multi-cell organisms managed to develop photoreceptors that gave them theability to detect and respond to some form of light. Purely for the sake of discussion, let's begin by assumingthat these first photoreceptors were cones that were primarily sensitive to ultraviolet (UV) light.

Observe that the spectrum as perceived by these creatures would have been monochromatic, which in thiscontext means "having or appearing to have only one color."The point is that they would have had theability to perceive only differences in the intensity of the band of wavelengths to which these cones were

sensitive. This is why we've represented their "perceived" spectrum as being gradations of black-and-white.Also, the fact that these creatures had only one type of cone means that they would be classified asmonochromats. (Note that the 360 nanometer peak sensitivity associated with these cone is an educatedguess based on the capabilities of existing life forms.)

It's important to remember that the idea that the first cones were primarily sensitive to ultraviolet light is purelyconjectural (an alternative scenario is presented a little later in this topic). So why would ultraviolet light makea good candidate? Well, ultraviolet radiation is more energetic than what we now consider to be the visibleportion of the spectrum, which would make it "easier" for a biological system to evolve to detect it.

Another possibility is that the first photoreceptors were used fornegative phototaxis (where "phototaxis" refersto the influence of light on the movements of primitive organisms). Ultraviolet light is harmful (this is whatgives us skin cancer). Early in the earth's history, the ozone layer didn't protect us like it does now. Initially thismay not have presented too much of a problem, because the first animals probably lived in water deepenough to protect them from harmful radiation. As animals began to come closer to the surface, however,they faced new challenges, including slow death caused by overexposure to ultraviolet radiation. Thus, theability to detect ultraviolet and move away from it would have provided an evolutionary advantage.

Before we proceed, this is probably a good time to briefly consider the way in which cones are actuallyformed and the way in which they perform their magic. One way to think about this is to visualize an"antenna" formed from a molecule ofretinal, which is a derivative of vitamin A (our bodies produce vitamin Afrom the beta carotene found in many of the foods we eat, including of course carrots). The role of theretinal molecule is to convert incoming light rays (photons) into corresponding electrical signals that can beprocessed by other structures in the eye and ultimately by the brain. Each cone is formed from a largenumber of these "antennas" (say 100 million or more).


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Each retinal molecule is surrounded by an associated "pigment" molecule. The purpose of this pigmentmolecule which is actually an incarnation of the protein iodopsin is to "tune" the sensitivity of the cone to aparticular band of frequencies. The pigment molecules for the ultraviolet cones (introduced above) and theblue, yellow, orange-red, blue-green, and yellow-green cones (discussed below) are all formed from different"flavors" of iodopsin only a few of the amino acids located near the site where the iodopsin binds to theretinal molecule are varied in each of the proteins. Collectively, these pigment molecules are referred to asthe opsins.

Now, remember that the idea that UV cones came first is purely speculative. In fact, here's another

hypothesis that, given the data, is equally plausible. One of the things we consider to be "noise" in vision isthe thermal isomerization of pigments (where isomerization is what normally happens when a photopigmentabsorbs a photon of light). This can also occur when the pigment gets jostled or absorbs a verylong-wavelength photon. There are more of these long-wavelength photons and more jostling at highertemperatures, so photoreceptor signals are noisier at higher temperatures. But one cell's trash is anothercell's treasure. Thus, it may very well be that the first photoreceptors were not visual at all. It could be that thefirst photoreceptor was a "temperature sensor." Thermal isomerizations are more likely in pigments with peakabsorptions at longer wavelengths, so the first photoreceptor may have been an orange-red cone with a peaksensitivity of say 625 nanometers as opposed to an ultraviolet cone with a peak sensitivity of 360nanometers. (The 625 nanometer peak sensitivity associated with the orange-red cone is an educated guessbased on existing life forms.)

So the UV cone may have been first, or the orange-red cone, or even one of the blue, green, or yellow conesdiscussed below. Any of these scenarios are consistent with existing data. The point is that having even arudimentary form of vision obviously conveyed a tremendous evolutionary advantage, such as the ability tosneak up on your visionless contemporaries, tap them on the metaphorical shoulder, and shout "Boo!" (Ofcourse, you could simply eat them if you weren't feeling in a party mood.)

On the downside, having only only one type of cone cell means that you're limited to seeing only a smallportion of the electromagnetic spectrum. If you can extend your visual capabilities to encompass additionalportions of the spectrum, this will obviously convey an even greater evolutionary advantage. Thus, during thecourse of the next several hundred million years (sometime before 450 million years ago), our ancestorsevolved four different types of cone pigments. (When we use the term "ancestors" in this context, we arereferring to the creatures that were to evolve into vertebrates, dinosaurs, mammals, primates and ultimately


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humans.) How good is this date? Well, we know that diversification of ciliary photoreceptors into fourspectral cone types occurred some time before the emergence of the most recent common ancestor ofparakeets and goldfish, and this is generally taken to be around 450 million years ago.

Creatures with only two types of cones are called dichromats; those with three types of cones are calledtrichromats; and those with four types of cones are called tetrachromats. Observe that we've illustrated thespectrum as perceived by these early creatures in two different ways. One representation shows multiplebands of black-and-white intensity. The reason for this is that, at the time the second type of cone cellevolved, it seems likely that the signals being output from both types of cone cells were fed directly to thecreature's nervous system and/or brain. That is, there is a strong possibility that the "comparator" cells wenow use to compare the relative outputs from different cone cells had not yet evolved in those early years.Similarly, it's more than possible that the "comparator" cells were still not present when the third and fourthcone cells evolved.

Having said this, it may be that these monochromatic representations paint a somewhat bleaker picture thanwas actually the case (again, you'll have to forgive the pun). This is because even without the presence ofspecial "comparator" cells each type of cone cell would respond to different intensities in its own portion ofthe spectrum. For this reason, perhaps we should visualize the spectrum perceived by these creatures asbeing more like the "alternate possibility" portion of the preceding illustration. And, of course, there is always

the possibility that these creatures hadevolved the special "comparator" cells used to compare the relativeoutputs from different cone cells, in which case they might have perceived the spectrum in a similar mannerto the way we do now.

As clever as they are, one problem with cone cells is that they function only in bright light. For this reason, rodcells appeared at some stage in the game. As we discussed in the previous topic, rod cells are much moresensitive than cone cells and they give their owners the ability to see at night (assuming some level ofmoonlight and/or starlight). We can visualize rod cells as having large numbers of "antennas" (say 100million) formed from the same retinal molecule we find in cones. In this case, however, the retinal is


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surrounded by a pigment molecule formed from the protein rhodopsin.

We aren't sure exactly when rod cells appeared on the scene, but ourrods (the ones that eventually ended

up in human eyes) probably evolved after the split between jawless and jawed vertebrates; let's saysomewhere around 450 to 500 million years ago just to give round numbers. As we mentioned in the previoustopic, some references state that the peak sensitivity of rod cells is close to the main spectral component ofmoonlight. Based on this, some folks hypothesize that rod cells first evolved in nocturnal animals. However,we also noted that in fact the peak absorption of rod cells (498 nm) is notparticularly close to the mainspectral component of moonlight, which actually occurs at anywhere between 548nm and 575nm dependingon your source of data.

In reality, we really don't know why rod cells peak where they do. In a classic paper published in the QuarterlyReview o f Biologyback in 1990, author Tim Goldsmith goes through several possible explanations for theposition of the peak of vertebrate rod pigments (virtually all such pigments have a peak near 500nm interrestrial animals). The bottom line is that Goldsmith found none of the common explanations including anyrelationship to the main spectral component of moonlight to be plausible. (If you wish to peruse this paperyourself and be warned that it's not an easy read then you'll have to subscribe to the scholarly archiveknown as JSTOR.)

Now, after pondering the previous illustration for a while, you are probably saying to yourself: "Just a moment;as rod cells primarily respond to the wavelengths we now regard as being in the cyan-green portion of thespectrum, why would they not perceive some form of color, and therefore why would we not class thesecreatures as being pentachromats?"Well, there are a number of answers to this as follows:

All of our previous cone-and-rod response curve illustrations have sported the words "Normalizedresponse/sensitivity"on the vertical axis. In simple terms, this means that we've artificially drawn thecurves such that they have the same maximum height. As we noted earlier, however, rod cells areMUCH more sensitive than cone cells. The next illustration which is NOT to scale is intended to

1.


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provide a "feel" for this difference in sensitivity. The bottom line here is that rods and cones simplydont play together in the same lighting conditions. Cones require bright light to function, but rods aresaturated in a bright light environment and arent in a position to generate any useful data. Bycomparison, in the dim lighting conditions when rods some into their own, cones shut down andprovide little or no useful information.In the case of modern animals (and we are probably safe in assuming that this was also the case withthe creatures of yesteryear), the only information used from rod cells is intensity, which is passedthrough the eye's luminosity channels; that is, signals from rod cells do not make any contribution to

the eye's color channels.

2.

The termspentachromat, tetrachromat, trichromat, and dichromatare generally associated with havingfive, four, three, or two types of cone cells, respectively. Similarly, creatures like Owl Monkeys thathave only one type of cone cell (along with their rod cells) are known as monochromats (as the onlynocturnal monkeys, Owl Monkeys are also known as "Night Monkeys"). Furthermore, in the case ofmodern creatures like the skate (small cousins of the giant rays that have roamed the earth's oceansfor around 400 million years) that have only rod cells and no cone cells and in the case of humanswhose cone cells dont function the term rod monochromatis used to distinguish this type ofmonochromat from creatures sporting only a single type of cone cell.

3.

Wow, four types of color cone cells androd cells, things were starting to look pretty good back then (onceagain, you'll have to forgive this turn of phrase I can't help myself). Sad to relate, however, sometimebetween 310 and 125 million years ago our ancestors lost first one and then two of these pigments. We dontknow exactly when or why, although one possibility is because these creatures became nocturnal.

So, how did we arrive at the dates noted in the preceeding paragraph. Well, The first loss had to occur after


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the divergence between mammals and reptiles, which is generally taken to have occurred somewherebetween 288 and 338 million years (we averaged and rounded this out to 310 million years ago). Next, allliving mammals are divided into three groups: monotremes (those who lay eggs),placentals (those who givebirth to live and more mature young), and marsupials (those who give birth to live, less mature young thatthey subsequently nurse in pouches). The point is that we also know that the first cone loss had to haveoccurred before the split between placentals and marsupials, which is thought to have been some time in therange of 130 to 175 million years ago.

The earliest known placental mammal is Eomaia scansoria, while the most primitive and oldest known

relative of all marsupial mammals is Sinodelphys szalayi. Both of these creatures lived around 125 millionyears ago during the early Cretaceous period. The second cone loss most likely occurred shortly after themarsupial/placental split, which occurred prior to the emergence of the most recent common ancestor of allplacentals. The most recent common ancestor of all placental mammals appears to have had only two conepigments. Creatures that still use this system such as dogs are known as dichromats.

Also, at some stage along the way, creatures evolved the "comparator" cells in their eyes that allowed them tocompare the output signals from different cone cells and to perceive the results as being a range of differentcolors. This means that modern dichromats like dogs probably enjoy a far richer visual experience than didtheir antediluvian counterparts.

Don't worry, we're almost home. Sometime between 45 and 30 million years ago, the primates that were toevolve into humans "split" their yellow cones into two new types: blue-green and yellow-green. Actually, thesituation with regard to primates is complicated because the majority of New World monkeys are so unusual.

But sticking with the lineage to us, it's almost certainly true that the duplication event that gave us back a thirdphotopigment occurred shortly after the split between New and Old world monkeys. That would have beensome time afterEosimias, which was an early primate that lived about 40-45 million years ago in China.

Furthermore, analysis of the skull bones of a bunch of primates suggests that the cone split occurred sometime around the appearance ofAegyptopithecus zeuxis. (Such analysis involves comparing fossil skulls ofancient creatures with those of modern animals whose visual ability we know and understand, and using anysimilarities or differences as the basis to hypothesize different evolutionary scenarios.) Also known as theDawn Ape, Aegyptopithecus was a small, tree-dwelling, fruit-eating animal that lived some 35 to 33 million


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years ago in the early part of the Oligocene epoch. So, taking all of this into account, 34 million years ago isprobably a good estimate for the time of the cone split.

Last but not least, we also have the "comparator" cells that allow us to use these cones to perceive the entirevisual spectrum. This leaves us in our current situation in which normal humans have three types of conesand are therefore known as trichromats.

And there you have it. As you can see, getting to our present state of color vision has been something of aroller coaster ride, but the final results are rather spectacular, arent they? Having said this, different creatures

have evolved their visual systems in different ways, and as we'll discover in the next topic onTetrachromats, Pentachromats, Etc. some of these developments put us to shame. (See also theInteresting Nuggets of Trivia topic for some additional discussions on creatures with rods but no cones,creatures with cones but no rods, and... the mind boggles!)

So let's pull all of the above into a timeline diagram that combines our original representation of evolution ingeneral with the evolution of color vision:


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Eon, Epoch, Era, Period, and Time (in the geologic sense) actually mean and how do they relate to eachother? Well, in the context of geology, an eon is defined as the largest division of geologic time, comprisingtwo or more eras; an era is defined as a major division of geologic time composed of a number of periods; aperiodis the basic unit of geologic time, during which some standard rock system is formed (a periodcomprises two or more epochs and is included with other periods in an era); and an epoch is defined as asub-division of a geologic period during which a geologic series is formed. Finally, the term age is used torefer to some span of time that is shorter than an epoch and that is distinguished by some special feature,such as the Ice Age.

The problem is that the actual definitions of the names and times associated with these various geologicterms are somewhat fluid, because geologists are constantly making new discoveries that cause them toreassess and "tweak" things. As an example of what we mean, consider the following illustration, whichreflects the way in which various reference sources (books, websites, etc.) would have presented things untilrelatively recently:


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Isn't the above a pretty diagram? (It should be, it took me long enough to draw!) As fate would have it,however, geologists have recently gone through a fairly thorough revision of the time scale. Personally, I wasa little concerned about the Paleocene Epoch (as I'm sure you will understand), but it looks like this has not


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been revised away (at least, not yet). Some of the more significant changes are as follows:

There is no longer a Tertiary Period. This used to stretch from 65 to 1.8 million years ago andencompass the Paleocene, Eocene, Oligocene, Miocene, and Pliocene Epochs. (See also point 3below).

1.

People are still wrestling with the subject of the Quaternary Period. This used to stretch from 1.8million years ago to today and encompass the Pleistocene and Holocene Epochs, where the Holoceneis the name given to the last 10,000 years or so; that is, since the end of the last major glacial event, or

"Ice Age", to the present day. (See also point 3 below).

It should be noted that people who study relatively recently deceased things really like the idea of theQuaternary, so there are a couple of different recommendations on how to keep using it as a term(only one of which keeps it as a "period"). If you have more fortitude than I, you can wade through theRecommendations by the Quaternary Task Group, which operates under the auspices of theInternational Commission on Stratigraphy (ICS) of the International Union of Geological Sciences(IUGS) and also under the auspices of the International Union for Quaternary Research (INQUA). Ofthe two recommendations that the committee members backed, the one that does notconsider theQuaternary to be a period appears to be winning.

2.

An acceptable version of current consensus (the "latest-and-greatest" as it were) that integratescurrently available stratigraphic and geochronologic information is known as the Geologic Time Scale2004 (GTS2004). Boiled down, the current state-of-play is as follows:

The old Tertiary Periodhas been renamed the Paleogene Period, which is truncated atthe end of the Oligocene Epoch (that is, the old Tertiary Periodused to encompass thePaleocene, Eocene, Oligocene, Miocene, and Pliocene Epochs, but the new PaleogenePeriodencompasses only the Paleocene, Eocene, and Oligocene Epochs).The old Quaternary Periodhas been remaned the Neogene Period, and this nowextends to encompass the Miocene and Pliocene Epochs (that is, the old QuaternaryPeriodused to encompass only the Pleistocene and Holocene Epochs, while theNeogene Period has been extended to also encompass the Miocene and PlioceneEpochs).The Quaternaryis now regarded as being a sub-period of the Neogene Period.The Holocene is now regarded as being a sub-epoch of the Pleistocene Epoch.

A lot of the dates associated with the various Periods and Epochs have been "tweaked".In the following illustration I've mostly rounded things to the nearest million years. (Notethat work is already underway on the next revision of the Geologic Time Scale as I penthese words.)

3.


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More Than Three Color Receptors: Tetrachromats, Pentachromats, Etc.

Just when you thought things were complicated enough, some animals have the ability to detect infrared. Forexample, rattlesnakes have infrared detectors in a hole or pit in front of each eye (this is why they are calledpit vipers).

Furthermore, some birds and bees have four different types of color receptors in their eyes, so they areknown as tetrachromats (bees in particular can see much further into the ultraviolet than humans). But wait,there's more, because some butterflies have five different types of color receptors, so they are known as

pentachromats.

Actually, things become even more amazing, because I recently read an article (I can't remember where Ishould have made a note curse me for a fool!), that reported the discovery of a species of fish (Cichlidsfrom the East African Rift Lakes) whose genes can code for seven different types of color receptor (cone)pigments. Having said this, only three types of color receptor pigments are primarily expressed ("turned on")in any particular fish. So why bother? Well, if you take a group of these fish that are living at one side of a lakein certain lighting conditions, they will be born with a specific set of three color receptor pigments in their"turned on" state. Meanwhile, if you take another group of fish (remember that these are the same species)living in a different area of the lake with different lighting conditions, then members ofthis group will be bornwith a different set of color receptor pigments in their "turned on" state. The theory (as I recall it) is that asthese fish move around or as the environment in the area where they live change over time they canquickly evolve such that the next generation will be better equipped to handle the new environmentalconditions.

But wait, because there's yet more! Known as "sea locusts" by the ancient Assyrians, mantis shrimp are little

tricksters, because they arent actually shrimp or mantids, it's just that they bear a physical resemblance toboth sea-living shrimp and the land-living praying mantis. (The term "mantid" refers to a group of around1,800 carnivorous insects one of a few types of insect that can rotate their heads.) But we digress. Theselittle rapscallions which can grow as big as 30 cm and can live for 20 years or more are said to have themost complex eye known in the animal kingdom. The intricate details of their visual systems (three differentregions in each eye, independent motion of each eye, trinocular vision, and so forth) are too many and variedto go into here. Suffice it to say that scientists have discovered some species of mantis shrimp with 16different types of photo-receptors: 8 for light in (what we regard as being) the visible portion of the spectrum,4 for ultraviolet light, and 4 for analyzing polarized light. In fact, it is said that in the ultraviolet alone, theselittle rapscallions have the same capability that humans have in normal light.

But the really amazing point to note here (perhaps one of the most unexpected discoveries toward the end ofthe twentieth century with regard to color vision), is that an extremelysmall percentage of female humans aretetrachromats because they have four different types of cone cells in their eyes. Now, each type of cone candetect about 100 gradations of color, so the combination of three different cone types allows a typical person

to distinguish 100

100

100 = 1 million different hues. A true tetrachromat has a fourth type of cone whosepeak sensitivity falls half way between those of the standard blue-green and yellow-green cones.Theoretically, this means that these folks may be able to distinguish as many as 100 100 100 100 = 100million different hues!

So what is it like to be a tetrachromat? That's a tricky one, because neither we nor they have the words todescribe this sort of thing (how would you describe color to someone who was colorblind?). What we do knowis that tetrachromats can make more color distinctions between shades that appear to be identical to themajority of us, For example, there's an interesting article on a lady called Susan Hogan who is an interiordecorator. Susan can look at three samples of beige paint that appear identical to her clients, but she candetect a gold undertone in one, a hint of green in another, and a smidgen of gray in the third. As anotherexample, Susan can look at a river and distinguish relative depth and the amounts of silt in different areas ofthe water based on subtle differences in shading that the rest of us simply don't see at all.

So, it's probably safe to say that tetrachromats have a much richer visual experience in the real world than dothe rest of us. However, there is a downside, because the images presented on display devices like television

sets and computer screens which are formed by mixing the three additive primary colors do not appear asrealistic as they do to the rest of us (similarly for images in print, which are essentially formed by mixing thethree subtractive primaries).

Now, this is a little tricky, but I wanted to give a hint of how an image that looks "photo-realistic" to us mightappear like to a tetrachromat. Consider the pictures below in which I'm wearing a red Hawaiian shirt standingbetween a blue car and a yellow car in the foreground with a green field and light blue sky in the background.In the upper version I've used all of the colors available to me; by comparison, the lower image employs acut-down color palette, resulting in everything looking "flatter" and "chunkier".


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With regard to the previous topic, many of us will ponder what it might be like to be tetrachromat with fourtypes of color cones. But why stop there? What if we had five different types of color receptors like butterflies,or maybe even the sixteen photo-receptors sported by the mantis shrimp?

Of course, there are two major problems associated with all of this. The first consideration is whether or not itwould be technically feasible to generically modify ourselves such that our eyes contained more types ofphoto-receptors. And, even if this were possible, would our brains be able to adapt to process the additionalinformation coming from the new receptors in a meaningful way?

Well, both of these questions appear to have been answered by researchers at John Hopkins and theUniversity of California at Santa Barbara in a study published in the journal Science on March 23, 2007 (youcan access different articles pertaining to this announcement Here, Here, and Here). (See also thediscussions on using gene therapy to restore cone cells in the Color Blindness topic earlier in this paper.)

As we discussed in The Evolution of Color Vision topic, sometime before 450 million years ago, thecreatures that were to evolve into vertebrates, dinosaurs, mammals, and primates had evolved four differenttypes of cone pigments. Then, sometime between 310 and 125 million years ago, our ancient ancestors lostfirst one and then two of these pigments. The

diy calculator __ color vision_ one of nature's wonders

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