Discovering the Third Dimension

Pre-renaissance artists had trouble with depicting depth in their paintings. Can you find some examples of this? How did their paintings fail to depict depth? What aspects of a painting give it depth?

Das Paradiesgärtlein, dating from the early fifteenth century, is an example of a two-dimensional painting that lacks the illusory power of creating a third dimension that is achieved more successfully in later paintings. What does this colorful, creative work lack that its successors would gain? The answers can be found in monocular cues – that is, visual depth cues that do not depend on the binocular power of stereopsis. They include:

1. Occlusion – better known as overlapping. The painting shown uses this technique well to create a sense of depth: flowers arch in front of the garden wall, a lady holds a book before her, a man hides behind a tree trunk, etc.

2. Size – in the painting, this depth clue is all about relativity and scale. Objects that are normally larger or smaller than other objects must retain that proportional size. Furthermore, variations in size due to depth or distance must also be included. Thus, a man must be shorter than a tree unless he is portrayed as much closer to the viewer than the tree, for example. In the painting, the latter of these size-rules is not followed as well. The flowers in the background, against the garden wall, are almost as tall as the women in the foreground.

3. Perspective – objects lessen in perceived size and rise in perceived horizontal location as they recede into the distance. As already discussed, the size scale is a bit off in this painting; objects don’t really lessen in size in the background, and may even be irregularly large in the first place. The rising-rule is followed a bit better: the garden wall, which is the furthest object in the picture, is near the top of the canvas, while the people are scattered through various latitudes. However, the overall placement is a bit extreme, and the extremeness is accentuated by the objects’ oddly uniform size.

4. Shading – shadows signify the location of a light source and show where light is lacking, thus conveying the three-dimensional shape of objects. There is some light shading in the painting, such as in the ladies’ skirts, but not as much as we would perceive in the natural world.

Overall, the painting relies on overlap, an exaggerated rise toward the horizon, a bit of shading and the brightly colored, clear-cut figures of the objects to convey the idea of separate images that recede in depth. However, while this painting is lacking in some ways, it is much more realistic than its predecessors and would be followed soon by surprisingly real achievements.

Das Paradiesgärtlein (The Paradise Garden), c. 1410

Final Blog Entry

It sounds a bit crazy, and maybe unsympathetic, but I always enjoy hearing about perception-related disorders. They reflect evidence and embodiment of what we have learned, and they may also just be strange, opening up our eyes to new perspectives, to different ways of living life and functioning in this world.

The most fascinating disorder that I read about this semester was Anton’s syndrome. Blake describes Anton’s syndrome as “a neurological condition in which a cortically blind person denies his or her blindness.” In one of the quirks of the human brain’s structure, the cortical area for visual processing is separate from the cortical area for visual awareness. If both are damaged at once, Anton’s can result.

Apparently the only way for someone with the syndrome to realize and accept their blindness is through physical trial-and-error: they describe what they are “seeing” with confidence (which is inaccurate), but eventually have enough trouble navigating to evoke doubt.

On the site ScienceIQ.com, David Gamon discusses some of the peculiarities of the condition: “the really weird thing about it is that they’re not lying… they really are convinced that they can see.” Conscious awareness and reasoning are completely disconnected from actual physical perception.

Or, perhaps, are these people still actively seeing, just seeing in a different way? We have discussed color-blindness, and how people who are color-blind still see the world, just not as the majority of humanity sees it. And we have discussed how some animals have only one or two cone pigments, and some have eyes more laterally placed on their heads. Is Anton’s just another mode of sight? Or another deficit of the visual system, like blind spots?

I’d have to say no. The examples described are biological differences derived from differences in receptors; Anton’s is caused by neurological damage. And deficits are evolved so that the brain has adapted to them, and can manage to work around them, whereas Anton’s is a complete and sudden loss of accurate visual information. People with Anton’s aren’t just perceiving the world differently – say, through a different filter; the neurological signals they are processing are misfires.

To an English major like myself, I see something almost comically literate about such a condition: the disconnect between reality and perception, between the internal and external, etc. It seems like someone should write a story, or at least a TV episode, about it. In the end, though, I’d have to say I’m grateful that it’s one more disorder that I don’t have to actually deal with myself. Functional perception is a useful thing.

Color-Math

What is the difference between adding and subtracting colors? What does that even mean?

Sometime around kindergarten, we learn that there are three primary colors – red, yellow, and blue – and that mixing red and yellow paint makes orange paint, red and blue makes purple, and yellow and blue makes green. Sometime later – sixth grade science, perhaps – we learn that light has different primary colors: red, blue, and green. But color is made of light, right? How can these two rules be reconciled?

There is a difference in primary colors and their resulting mixtures because of what the forms of color do. Paint is a material that changes the hue of color that is reflected from the paper. For example, white paper reflects all colors; putting yellow paint on the paper means that only yellow light is reflected. When the yellow is washed over with blue, the only light that is reflected is the light in the color spectrum that borders on both yellow and blue: green is left. And when green is washed over with red, only brown or black is reflected. This narrowing, decreasing range of color hues reflected from the painting – starting at white and progressing to black – is an example of subtractive color mixture.

Imagine the opposite scenario. You start with a dark space; no light shines into it. It is black. Next, a green light is switched on. The space is lit with a green hue. Then a red light is turned on. The space becomes yellow: more wavelengths of light are available. Finally, a blue light is turned on. The space is filled with white light, because it contains all of the wavelengths of light visible to the human eye. This process of light addition – starting at black and progressing to white – is an example of additive color mixture.

Therefore, in subtractive color mixture, more and more light is absorbed when colors are mixed. In additive color mixture, more and more light is reflected or produced when colors are mixed.

Nature v. Nurture in the Visual System

How does the nature versus nurture debate come into play in the development of a normal visual system?

One example of the nature versus nurture debate arises in the phenomenon called the “oblique effect.” The oblique effect refers to the visual system’s preference of horizontal and vertical lines to oblique lines. On page 129 of Blake, one can see a diagram of the unequal distribution of cortical cells to various preferred orientations.

The nature versus nurture question in this case is whether this cell distribution arises naturally from our genetic code, or whether the distribution is an adaptation to the environment we are exposed to during development. Research has shown that our environments do have a natural bias toward the visually horizontal and vertical. However, the development of this cell distribution occurs very early in our overall development – within our first few months, in fact.

According to Blake, the generally accepted view of nature versus nurture in this case is similar to the overall view of nature versus nurture which most scientists hold: both genetic and environmental factors influence our development, and so neither can be separated from the other or ignored as irrelevant.

Orienteering for the Visual System

Why are gratings such popular stimuli in studying visual perception?

Gratings are popular stimuli because they can be used to explore one of the key characteristics of the visual system: orientation preference. Both retinal ganglion cells and visual cortical neurons may show topographical preference for specific orientations of stimuli. Orientation preference is founded on the ability of receptive fields to detect lines and edges; when you have a line or an edge, you can have a slant (a certain angle of orientation). And the detection of lines and edges are based on the grouping of ON- and OFF-center ganglion cells. Overall, gratings test a fundamental ability of the visual system that is based on a few other fundamental abilities of the visual system.

Furthermore, there are some interesting characteristics of orientation preference that have been discovered and evoke further questions. For example, discovery of the oblique effect led to the realization that humans “see” vertical and horizontal lines better than oblique lines. This begged the institutional question of nature vs. nurture: does the oblique effect arise from environmental conditioning, genetic predisposition, or both? Probably both. Further exploration of visual orientation will hopefully lead to further questions, discoveries, and innovations.

Cortical Magnification

What is cortical magnification and is it a good or bad thing?

Cortical magnification is, according to Blake, the mapping of the retina onto the visual cortex so that the representation of the fovea is exaggerated or magnified.

The fovea is the area of the retina where the focus of our eyes is projected. If we are looking at a string of words on a computer screen, the words will appear on the fovea, while our surroundings appear on other, less-precise areas of the retina. So when the images of the fovea are magnified, the image we are looking at is magnified – with the trade-off images in the periphery.

Therefore, there are benefits and disadvantages to cortical magnification. It means that the focal point of our vision is that much clearer – but also that items not in the focal point are that much fuzzier. Perhaps if we were still in a prehistoric world, cortical magnification would be a bad thing – we would see less of our surroundings and thus be less able to react quickly to changes in our environment. However, in our modern industrialized world, our environment seems to have adapted itself to us – our vision is employed in tasks that require this specific focus: reading, watching television, driving.

It’s impossible to really make a call on this one, having never experienced a perceptual world without cortical magnification; but I’ll assume that as it has survived natural selection and benefits us in the modern age, that it is really a good thing.

More About Eyes

What is an OFF-center ganglion cell receptive field? Why is it organized they way it is?

• OFF-center: area of retinal cells that respond to a decrease in light perception; surrounded by ON-responsive cells that respond to an increase in light perception

• ganglion cell: retinal neurons that receive and summarize sensory information from photoreceptors

• receptive field: area of environment perceived by ganglion cells (not photoreceptors)

So that OFF-center + ganglion cell + receptive field = area of retinal neurons that responds most to a decrease in light from its perceived environment.

An OFF-center field is organized to recognize differences in perceived light, and thus identify edges that can signify depth, shadows, borders, etc. This ability is enhanced by its surrounding of ON-center cells that respond to increases in light. Furthermore, the receptive fields may be organized in patterns such as bars, and thus be able to detect differences more effectively.

Contact Lenses

How do contact lenses really work?

Contact lenses aid the focusing power of the eye’s natural crystalline lens. In order for the brain to visually perceive its surroundings, light must be focused onto the retina cells, located at the back of the eye. The lens focuses light on the retina.

Ideally, the focal point of the lens will fall precisely on the retina (emmetropia). However, the crystalline lens may overshoot or undershoot the focal point, leading to a diagnosis of myopia/nearsightedness (when the lens is too curved and the focal point is too shallow) or hyperopia/farsightedness (when the lens is too flat and the focal point is too deep). Of course, the lens is not a concrete shape; it can become slightly more or less convex depending on the point of focus (accommodation). However, some lenses still cannot focus as well as others.

This is where contact lenses come in. If the person has a diagnosis of myopia, the contact lens will be slightly less convex than the crystalline lens, so that the summed effect of the multiple lenses is to move the focal point deeper, to retina. If hyperopic, the person will use slightly more convex lenses so that the focal point is shallower.

Perceptual Safety-Nets

We use interaural cues to locate acoustic stimuli, but this information isn’t always so clear. How might one disambiguate information presented within these “cones of confusion”? And why are we not always confused about a sound’s location?

As Blake points out, there are a few simple ways that we naturally resolve a sound’s location when the “cones of confusion” created by IID and ITD interfere. The first is head movement: moving your ears out of the cones of confusion and re-orienting them in respect to the sound source will allow IID, ITD, the pinnas, and other mechanisms to work.

There are other subconscious cues not involving the ears that allow us to locate sounds. One is context. One of the areas where IID and ITD become confused is distinguishing sounds from directly behind and directly ahead. If you sleep with your face toward your alarm clock, for example, your ears won’t be able to tell you if the alarm is ringing from in front or behind your head; but familiarity allows your brain to compensate and tell you that the alarm is, as usual, in front of your face.

Another cue is another sensory system: vision. Vision allows us to see our environment and possible sound sources, and match the sources with specific objects. Vision can even correct mislocalization of sounds by the ears, as sound waves may bounce off surfaces before reaching the ear while light from objects goes directly to our eyes. However, the strong influence of vision over hearing can sometimes confuse perception, as shown by the McGurk Effect.

Effective Communication: Cell Phones or AIM?

Helen Keller said, “Blindness cuts me off from things; deafness cuts me off from people.” What do you think about this statement?

In the introduction to his chapter on the ear and auditory system, Blake writes that “Hearing provides the basis for many forms of social communication, particularly speech. Without the ability to hear voices, an individual must struggle heroically to remain in touch with the flow of social discourse” (353). And as noted above, Helen Keller felt that hearing loss was more socially detrimental than vision loss. Why is the sound of speech key to social interaction, and why might it be more so than vision?

First of all, conversation has countless nuances that would disappear once translated into purely visual messages. Word choice would become more limited, inflection and tone and volume would disappear, only one voice could be “heard” at a time. Laughter would seem completely absurd. Anything said by someone out of one’s line of sight would be lost.

One could argue that sight can substitute for hearing. Sign language and visual messages can be used to communicate, and visual cues such as facial expression can provide context for a statement. However, imagine talking to someone in a completely flat tone – over AOL for example. Even when one gives in to the use of emoticons, it is often still hard to tell exactly what the other person feels, especially when those feelings are more complex. A telephone conversation is much easier to understand, where sight of the person is lost and communication depends solely on hearing each other’s voices.

The one example that I can think of in which visual communication expresses sentiment accurately is through stories or books. However, most of us aren’t prize novelists, and (I imagine) find it easier to connect to a person over the telephone than through a written letter.

Therefore, while deaf people may come to use vision resourcefully to communicate with others, I understand the social significance of hearing and sympathize with Keller’s loss of her own.