Download Chapter One: Neurological Bases for Visual Communication

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.