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Download Chapter One: Neurological Bases for Visual Communication
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Chapter One: Neurological Bases for Visual Communication In this chapter we will consider some basic constraints on the processes of seeing and perceiving that are imposed by our visual system. Visual communicators need to be aware of these basic neurobiological principles in order to design effective graphics. In addition, if your audience might include visually challenged individuals, your ability to anticipate their particular needs will prove crucial to the reception of your document. An overview of the visual system The visual system does not just include the eyes; it also comprises the nerves that conduct signals from the eyes to the brain as well as multiple brain areas that process visual signals and information. As you can see in the Figure 1 below, signals from the light receptor cells in the eyes are carried via the optic nerve to the back of the brain, where they are first received by areas in the occipital lobe. After basic processing of information such as contrast and movement, the information is then available to higher-level areas in the brain that recognize familiar faces, appreciate a pleasing display of colorful flying kites, or perceive a threat in a sudden movement in the dark. Figure 1: Human visual system showing how information from visual fields is passed to optical cortext and processed. From http://homepage.psy.utexas.edu/homepage/class/Psy301/Salinas/sec2/Brain/13.GIF The eye Notice in Figure 1 that by the time the visual information gets to the back of the brain, it’s on the opposite side and is upside down. This transformation in part is due to what happens to light in the eye. Light rays penetrate the lens and pass through the vitreous humor in a straight line to be received by cells that line the satellite-dish-shaped retina in the back of the eye. A light ray coming in from above us will strike toward the bottom of the retina’s receptive dish. Likewise, a light ray coming in from below us will strike high on the retina. This causes whatever we see to be transferred to the back of the retina upside down (Figure 2): Figure 2: Inverted image on the retina. From http://www.yorku.ca/eye/invert.htm The retina itself is covered with two types of receptive cells: rods, which perceive light and dark; and cones, which perceive color (Figure 3). The cells are not distributed evenly. You have more cones near the center or fovea of your retina. The parabolic shape of the retina causes light and images to be most intense at the fovea, and cones operate best here because they need light to see color and because they can distinguish extremely fine variations in color that give you important information about the shape and orientation of the object you’re focusing on. Rods, on the other hand, are disabled by bright, direct light. This is why your eyes have to adjust to a darkened movie theater before you can begin to perceive the dim contrasts between people’s heads and the backs of seats. Because they are so sensitive to contrast, they populate the periphery of your retina. This is why you can detect dim stars at night better if you don’t look right at them. Figure 3: Electron micrograph of rods and cones in the retina Peter Kaiser, from http://www.abc.net.au/science/news/stories/s200509.htm The fovea itself is very small, meaning that you can only truly focus on a small area of your visual field at once. Look at the center of Figure 4 (you won’t be able to read the very smallest letters due to the poor quality of this image; to see the real thing, go to http://www.anisn.it/scuola/strumenti/visione/images/fovea.1.jpg). From this position, you can clearly read the letters around the outside of the “circle,” but those letters have to be exactly that size for you to be able to read them. If you shift your focus to the outside of the circle, you won’t be able to read the letters even two rows in: Figure 4: Fovea demonstration diagram The optic nerve If you refer to Figure 1 again, you’ll see that the nerves connecting the eyes with the visual cortex take some interesting twists and turns. The first thing to notice is that the left side of both eyeballs are fed by the same nerve; same with the right side of both eyeballs. This strange arrangement is crucial to stereo vision, i.e., your ability to see in three dimensions. Signals from the left visual field (everything to the left of center of each eyeball) are combined with the signal from the right visual field (everything to the right of center of each eyeball) at the optic chiasmus, the crossing of the two “strands” of the optic nerve (Figure 5). As you have probably guessed, there is a lot of overlap in these two signals; i.e. each eye sees most of what the other eye sees unless you have a really enormous nose. Your brain mixes the signal from each eye to produce what seems like a seamless, three-dimensional picture from the overlapping areas of your visual field. Figure 5: Combining signals from visual fields into a stereo image. From http://homepage.psy.utexas.edu/homepage/class/Psy332/Salinas/Origins/visualsystem.gif Stereograms take advantage of this “image mixing” to produce an artistic effect. Stereograms use dots to create two copies of an image that are offset from each other the same distance that your visual fields are offset from each other due to the space between your eyes. To make a stereoimage from this seeming random mess, you must unfocus your eyes so that each is pointing roughly straight ahead instead of toward a particular point on the stereogram. Then, your brain mixes the view from each eye, and the pre-inscribed image “pops” out (Figure 6). Figure 6: “Peace” stereogram From www.lessons4living.com/ peace1.htm You’ve probably deduced that you don’t have depth perception in any part of your visual field that’s not replicated by the other eye. To test the limits of your 3D vision, close one eye at a time to see where your stereovision stops and your monovision begins. The visual cortices The “mixed” signals from the optic chiasmus travel all the way to the back of your brain first, to a region known as the primary visual cortex, or V1. This low-level vision area is responsible for recognizing orientation of lines, contrast, and some motion. You are not consciously aware of what your brain is doing at this level. In fact, many of the initial experiments that determined the function of this area of the brain were done on anesthetized cats. From these experiments researchers learned that V1 is populated by pinwheel-shaped configurations of neurons, each of which automatically fires if and only if the visual signal it receives contains a line of a certain orientation. Figure 7 is a “map” of a very small slice of V1 showing how individual “orientation” neurons are nested in with each other. Figure 7: A color-coded map of orientation-detection neurons in the primary visual cortex. The neurons that fire for vertical lines in the visual signal are coded purple, etc. From http://www.cf.ac.uk/biosi/research/neuroscience/staff/sengpiel.html After these low-level processes are complete, this information is passed along with the visual signal to other areas associated with the primary visual cortex for additional processing. Figure 8 is an iconic map of the areas of the brain responsible for individual processes. The important generalizations to note are as follows: the areas of your brain that are responsible for motor control are toward the top, in the parietal lobe, so visual information heading that direction helps you orient and coordinate yourself; the temporal lobe, below the large fold in your brain called the lateral sulcus, handles identification processes, so visual information heads here to be matched up with your knowledge of the world. Figure 8: Visual cortices and associated brain areas. Courtesy David Heeger. System anomalies and deficits Our discussion of the visual system so far has been premised upon the idea of the “average” individual, a mythical person who combines all the features that are typical of human vision across the population. But in reality, many of our brains have anatomical or neurochemical differences that cause us to see differently than the average person. It is important that we understand some of these common variations when we prepare visual documents for broad audiences. Color blindness Probably the most commonly encountered variation is what is known as color blindness. Color blindness is not really a “blindness” but a difference in the ways colors are perceived, usually a weakening in the perception of one or two colors or a “shift” of these colors toward other colors in the visible spectrum. This condition can be caused by a range of things from a genetic predisposition to a different mixing of rods and cones in the eye to damage to the area of the temporal lobe where colors are perceived and identified. Color blindness occurs in men more than women; some estimates hold that one in 12 men have some kind of anomaly in their color perception. The most common type of color blindness is red-green deficiency. Depending on the specific type of this deficiency that a person has, he will either have trouble distinguishing red from orange and green, or he will have trouble distinguishing anything in the low part of the visible spectrum from red to green. Figure 9 is a typical test for red-green deficiency. The full tests plus a great deal more information on visual anomalies and color is available at http://webexhibits.org/causesofcolor/index.html. Figure 9: Part of the Ishihara series of red-green deficiency tests From http://webexhibits.org/causesofcolor/2D.html Eye disorders and damage Some of us are born with other anomalies in our eyes. Differences in the shape of our lens or the strength of the muscles that focus it can lead to myopia (near-sightedness) or hyperopia (farsightedness). An imperfect curve to the back of the eye can cause a stigmatism, which is most noticeable when perceiving perfectly circular objects like the outside contour of Figure 8 above; people with stigmatisms will see a slight ellipse instead of a circle. Non-congenital factors can also impinge on normal vision. As people age, they often develop crystals in the vitreous humor of the eye called cataracts that cause their vision to gradually grow misty. Diseases such as glaucoma and macular degeneration can cause partial or total loss of vision. Tunnel vision is a particularly interesting degenerative condition in which the visual field narrows over time; this condition apparently has psychological bases as well as physical causes such as glaucoma and retinitis pigmentosa, in which rods and cones on the periphery of the retina die off. Of course, the most severe visual deficit is complete loss or absence of vision due to genetic factors, environmental factors, or trauma. Lessons from the neurobiology of vision for the visual communicator While the visual system is fascinating, and entire textbooks could be and have been devoted to its description, for our purposes in this handbook, we need to pause and ask: How can visual communicators apply this information to their work of designing graphic documents for audiences? There are several key take-away points from this quick tour of the neurobiology of vision: Overall, about one in 10 people has some form of visual anomaly, so unless you’re designing for an extremely small, well-known audience (almost never), you need to take visual differences into account. Don’t just rely on one feature (color, shape, or contrast) to communicate important information in your graphics. You can and should assist viewers with visual differences by using multiple cues. When designing web documents, make use of the Alt-tag function for graphics. Viewers with visual deficits often have software that will audibly read text on webpages, including Alt-tags that describe the content of the photos and technical graphics. People can only focus on a spot the size of a nickel (at arm’s length) at any one instant. If you want something at the periphery of a document to catch the viewer’s attention, make it very large and high-contrast. Consider the environment in which your documents will be viewed and adjust contrast, brightness, and size accordingly to make them as easy to view as possible. Remember that color does not come across well in low light.