• Vincent Campese PhD

Art of Perception: Photopigments and the striking manipulation of nature, art, and beyond


Whether they understood it or not, artists have been using the basic principles of visual perception to persuade our primary sense (vision) to play along and perceive depth on a flat, two-dimensional canvas for a very long time; but, what we ultimately see depends on an interaction between the art and the anatomy of the visual system.

Renowned Gestalt psychologists of the early 20th century spent lots of time trying to decode the rules of perception. Their mantra can be summed up by a quote from Kurt Koffka (1886 - 1941): “The whole is something else than the sum of its parts.” The creators of Op art — a style of visual art using optical illusions that emerged in the 1960s — such as Victor Vasarely (1908 - 1997) and Bridget Riley, creatively incorporated this concept into their work. Much of the illusion in visual art can be revealed by understanding how basic Gestalt principles are applied.

For example, in visual art, depth can be created by interposition, that is, by placing one object in front of another in a scene; a sense of belonging called grouping can be generated by arranging items in a contiguous manner. Similarly, pattern completion or closure — called prägnanz by the gestalts — is a tendency to fill in the missing gaps in an incomplete image, like when we see an open circle, or a line of text with missing words. In fact, our drive to complete the picture is so strong, we don’t even see a missing piece of the world where our retinal cells project out from the eye and into the brain.

You can find the blind spot, as it’s called, with a simple demonstration. If you cover one eye, and focus the other on a point drawn on a piece of paper, you can move this paper to just the right distance from your eye, so that a target image on the opposite side of the paper from your fixation point disappears. The world will still be there, but the target image will not be a part of it any longer. As is evident by this demonstration, the visual system can be manipulated into seeing much more than what simply meets the eye when elements of perception are harnessed effectively.

In classic works such as the Necker Cube and Rubin’s Vase illusion, boundaries and form appear to pop back and forth. Even illusory motion can be perceived given a simple flicker of two separated light points in the Phi Phenomenon. This particular illusion is interesting because it has also been shown to produce illusory color perception. While vibrancy illusions are achieved by placing opposing colors adjacently, the colored version of the Phi phenomenon is more striking.

In the simple version, using only white light, two light emitting diodes — also known as LEDs — are positioned at a fixed distance, with one LED briefly illuminating and then the other. By calibrating the amount of time between the two LEDs being turned on/off, people watching will report seeing a dot of light traveling back and forth.

In the colored version of the Phi phenomenon, each LED is a different color; let’s say a red one and a blue one. In this case, illusory motion is still evident, but so is the illusion of a color transition happening along the way. While we have known for some time that the visual system processes different elements like color, motion, and form in separate pathways, it is a powerful demonstration when such a simple illusion can trick multiple elements of perception.

Each sensory system has a special external stimulus that activates its sensory cells, like sound waves for audition and light waves or photons for vision. When we look at something, light is focused onto the retina by the lens of the outer eye, and photoreceptors — specialized cells that detect light and line the retina on the back wall of the eyeball like tiles in a Roman mosaic — are activated by the light that’s coming in. While we have a class of non-colored photoreceptors called rods, we also have three types of color receptor cells called cones.

Now, rods are dominant under low light conditions; so when you turn the lights off in a room, and the colored objects become contours in shades of grey, you’re using your night vision thanks to the rods that are running the show. But, when light is plentiful, your cones can tease apart the different frequencies, or hues, to create what we experience as color vision. They can do this because each cone class contains a unique photopigment that effectively tunes a given receptor class in to a specific photon frequency along the visual spectrum.

For example, rods (think night vision) contain a photopigment called rhodopsin, which is comprised of an opsin called scotopsin bound to a molecule called retinal. Think of it like this: The opsins in photopigments are like their tuning keys — they make the photoreceptors sensitive to what we understand as different colors. When light hits rhodopsin, it is bleached and the retinal detaches from the scotopsin. This causes a neural signal to be sent from the retina to the brain indicating that light has been detected.

Cones (think color vision) work the same way, but based on the specific opsins they contain are classed as short, medium, or long wavelength receptors; sometimes they are called Blue, Green and Red receptors, because they are tuned to wavelengths we typically perceived as blue, green and red colors, respectively. But, this isn’t the case for everybody; some people have a genetic condition that results in a deficient or entirely absent cone class, otherwise known as colorblindness.

If long-wavelength (Red) cones are missing, the condition called protanopia results in red-green confusion — meaning that people have a tough time telling those two colors apart, hence the moniker red-green blindness. However, these individuals also have problems distinguishing between green and blue, so that name can be misleading. When the medium-wavelength (Green) cones are absent, the condition is called dueteranopia, and while the outcome is similar as with protanopia with regards to color discriminations, there are some nuances that distinguish the two.

For example, deuteranopes do not experience the same dimming that protanopes do when viewing a bright red, which to them, may appear black (which, by definition, is the absence of color). This is because deuteranopes are missing the medium-wavelength (Green) cones, but the intact short (Blue) and long wavelength (Red) receptors can pick up the slack to signal general illumination — just not the color difference. When the long-wavelength receptors (Red) are missing in protanopia, the light is just not detected at certain frequencies (hues) and dimness is the result.

While these conditions are seen in people entirely lacking these receptors, mutated versions of these conditions also exist where the tuning of the receptors are off; in other words, the mutated cones are sensitive to frequencies (hues) other than red and green. These conditions are called Protanomaly and Deuteranomaly — although some function of these receptors is preserved due to the presence of mutated opsins, mostly, the problems of color blindness persist.

These kinds of problems with color vision can impact how individuals perceive art that uses color to persuade the viewer. Trying to find a hidden image in a sea of color blotches can be quite frustrating for some of us indeed. This is the idea behind the Ishihara test, which fuses art with color perception diagnosis.

Since the genes for the red and green cones are on the X-chromosome, males experience these problems at a much higher rate than females. And yes, there are also tritanopes too, people missing the short wavelength (Blue) receptor, but this is an extremely rare and gender nonspecific affliction. While these conditions show how fundamental photopigment is for human color vision, it’s not the whole story.

Receptors are combined along the pathway to the brain in a way that sets up opponent pairs of colors, such that red and green oppose each other, as do blue and yellow. Try as you might, you will have difficulty imagining a greenish red or a yellowy-blue. You can’t, because these systems actively suppress each other when the other is present. While this can sound confusing, a very powerful demonstration of negative afterimages can help one understand it a bit better.

Color vision is heavily dependent on cones in the retina, but colorblindness can come from problems upstream too, and not just from the retina. For example, when the mechanism in the brain that puts the color signals together is compromised, it results in complete color blindness — a condition called cerebral achromatopsia, which people have described as drab. An analog to this disorder is blindness to motion, a condition called akinetopsia that makes those affected see motion in a very selective way; while other visual processes seem normal, watching something move, like a ball being thrown, is like flipping through a flipbook with missing pages.

Photopigments of the eye determine the way people experience color vision.

Whether or not the anatomical and perceptual knowledge gathered by people like the Gestalts actively guides Op artists, like Victor Vasarely and Bridget Riley, in their work, the results, nevertheless, are a striking manipulation of nature. You would think that tricking the eyes to perceive depth, duplicity and motion, along with color illusions would require an intimate understanding of perceptual psychology. But then again, the rules that govern the visual sense may just be more intuitive to some.

Note* Dr. Vincent Campese is an Assistant Professor of Psychology and Neuroscience at the University of Evansville.

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