Johns Hopkins Magazine - September 1996 Issue

Vision: In Short

A central key to balance

"When we stand still, we're always moving," says Kathleen Turano, associate professor of ophthalmology at Hopkins's Wilmer Eye Institute. A contradiction? Not exactly. If you stand with your feet planted firmly on the ground, as ramrod straight as a West Point cadet, you are, by most interpretations, standing still. However, technically, as a sophisticated apparatus known as a force platform would show, you are, almost imperceptibly, swaying.

So, given our innate oscillatory state, what keeps us from falling down? One theory says that the central visual system plays a large role. In a series of studies, Turano has found evidence to support that theory.

Three systems--the visual, somatosensory (pertaining to touch), and vestibular (the movement of the head, as determined by an inner ear mechanism)--provide sensory information that allows us to maintain our balance, notes Turano. Recent research indicates that people can lack reliable feedback from one system and still maintain their balance. Imagine standing on a plush carpet or a slippery floor (in which your somatosensory feedback is hindered). You can still maintain your posture because you use cues from the vestibular and visual systems. "But if you lose two of the three systems, posture stability is compromised," says Turano. A blind person has great difficulty walking on a slippery surface, for example. But Turano was interested in learning how great a role central vision plays in helping the body maintain its balance. (Central vision corresponds to the central 20 of the visual field. A thumbnail held out at arm's length is roughly 1 1/2.) So using the force platform, she measured postural sway under two test conditions: in 19 patients with central vision loss due to macular degeneration; and in 20 people with normal vision.

The platform measures force by means of four pressure transducers in its base. Volunteers place their feet within a pair of footprint outlines on the platform, and stand as still as possible, with their arms at their sides. The pressure transducers measure the difference in force applied between one transducer and the next. The greater the differential, the more a volunteer is swaying.

In one "lights off" test, the volunteers stood in the dark for 20 seconds. In the second "lights on" test, they stared at a pattern of dots on a screen straight in front of them for 20 seconds. The tests were repeated three times under each condition.

Turano hypothesized that volunteers would sway less when the lights were on because their central vision would supplement the vestibular and somatosensory senses in helping to maintain balance.

And she indeed found that for volunteers with normal vision, having access to visual information (the lights-on condition) reduced sway 41 percent. In contrast, for volunteers with central vision loss, visual information reduced sway 29 percent. When central vision is impaired, as it is in macular degeneration, the ability to use vision to maintain a steady posture is compromised, Turano concludes.

The central visual system detects the "small oscillatory movements" of the body, explains Turano. Even though we do not notice we are moving, "the image on our retina moves. As soon as we detect these movements, we can compensate for them. So we don't fall down."

She reports the findings in the July Investigative Ophthalmology & Visual Science.
--Melissa Hendricks

The edge between beautiful and horrific

Artist Dedree Drees (MA '79) is fascinated by the computer's capacity for manipulating color, pattern, and imagery. An instructor at Catonsville Community College in Baltimore, Drees takes digitized photographic images and uses a Macintosh to create intricate compositions like the one below, titled "Bug Pals."

"Bug Pals" began with a color photograph of caladium leaves, a photo that Drees shot in Baltimore's Druid Park Arboretum. She was interested first in the patterns on the leaves: "The patterns had both spots and bifurcations that suggested branching form-- pattern as fragmentation toward infinity. Sort of a Hindu idea about a center where everything's all together and then it fragments."

To start, she selected a small section of the image and used a software package to create a "tile" that she then could replicate over and over, like the tiles in a mosaic. Next she played with the colors, and got a pale pink, which she says reminded her of "veins under skin." Drees wanted more contrast, so she took a section of the image and used a program called Kai's Power Tools to manipulate the visual elements according to a mathematical formula, in this case the Mandelbrot fractal set. The result was the dark area in the upper right quadrant. Another fractal pattern, called the Julia set, generated the series of wavy lines that look like ripples.

In the dark area formed by the Mandelbrot, she manipulated color to achieve an illusory effect of transparency. "You get the condition of transparency by finding the psychological middle mixture between two colors," Drees says. It's the equivalent, she explains, of sequencing, say, solid areas of green, gray, and red on a card. What's physically there is a series of three solid colors; but the brain sees instead green and red meeting in the middle, with a swath of transparent gray overlaying the center. In "Bug Pals," the eye looks at the darker area in the upper right and the visual brain perceives that a transparent color has been laid over the underlying pattern, which is not, in fact, the case.

Drees also likes to play with how people react to certain forms. "To some extent, I wanted the long, liney forms in this image to be leg-like, and the round ones to be like bug heads, and for the heavy form on the top to have an ominous character, so that the image could be jewel-like and delightful, or scary and threatening at the same time. I like that edge between whether it's beautiful or horrific."
--Dale Keiger

A digital magnifying glass

Many visually impaired people would never sit down to read the morning paper without their magnifying glass. Likewise, a software program invented by Hopkins undergraduate Steve Crutchfield, 20, is becoming an indispensable tool for a growing number of visually impaired computer users. The program, which magnifies computer screen images, is so successful that Corel Corporation, a major software company (in Ottawa), recently retained Crutchfield to adapt the program for inclusion in one of its own software packages; Johns Hopkins installed it on Macintosh computers in the Krieger Hall Computer Lab; and the Virtual Assistance Technology Center, a group that promotes technology for the handicapped, is plugging it on its World Wide Web site.

The program, Zoom Lens, allows users to magnify the entire computer screen, or any portion of the screen. The unenlarged image remains in the background. Although there are other computer screen magnification programs, they replace the entire screen with a magnified image.

A senior double-majoring in electrical and computer engineering and computer science, Crutchfield did not have the visually- impaired in mind when he created Zoom Lens. Rather, he thought magnification would help software developers like himself (he has been creating computer programs for a decade). "I created it because it was useful when I was developing software to look at small parts of the screen," he says.

Crutchfield made Zoom Lens available through the Internet on several freeware libraries, which allow users to download software for free. The rave reviews started coming in, but overwhelmingly they were from visually impaired people. One letter was from a man who said Zoom Lens enabled him to continue working while he awaited cataract surgery.

Crutchfield then modified the program to make it more useful for people with visual disabilities. For example, he gave the user more control over the scale of magnification. He has copyrighted the software, but says he will not charge for use of Zoom Lens.

Zoom Lens can be downloaded from the Macintosh software libraries at Compuserve and America Online, or obtained over the Internet through Stanford University; the address is sumex-aim@stanford.edu.
--MH

Cascade "snapshots"

Vision begins when pigments in the rod and cone cells of the retina absorb light. But it takes a long cascade of steps to move from photon to visual message. Using bacteria, assistant professor of biological chemistry Sriram Subramaniam is taking "snapshots" of the first step in the cascade: a change in the shape of a rod-cell pigment called rhodopsin.

Bacteria do not see, but they use rhodopsin to translate light into chemical energy. Each photon of light allows for one proton to cross the bacterial membrane. In humans, rhodopsin activates molecules called G proteins, starting the cascade that leads to visual image, explains Subramaniam.

Subramaniam took his "snapshots" by first crystallizing samples of bacterial rhodopsin, then shining a light on the crystals and rapidly freezing the samples in liquid nitrogen at various times following the exposure to light. Each frozen sample reveals rhodopsin at a different millisecond in time following illumination. Subramaniam then examined the crystals using electron microscopy.

All rhodopsins are a bundle of seven helices, which surround a light-absorbing molecule called retinal, explains Subramaniam. In bacteria, one of the helices is kinked. In response to light, the kinked helix opens outward, much the way a hinge opens up, he reports. "We now have a good idea of how the pigment opens up," says Subramaniam. His ongoing task is learning how it closes.
--MH

Cortical seats that are fundamental to art

Neurobiologists have discovered, in only the last 20 years or so, areas of the brain that handle highly specialized visual processing. Semir Zeki, a neurobiologist from University College, London, believes that attributes of the visual scene that have special cortical seats in the visual brain are also fundamental to art: "Among these, one can mention color and form, motion and depth, and faces. All these faculties have a separate location in the brain and all of them have primacy in art."

Zeki, who recently delivered the David Bodian Lecture at Hopkins on "Behind Appearance: An Exploration of Art, Vision and the Brain," offered an example in a post-lecture interview: "There is a lot of knowledge contained in the face, which is why the brain devotes special visual centers to it, which in turn explains why portrait painting has had primacy in art, at least Western art. There are visual elements that do not have special cortical seats--the shoulder, the hands, the ears--and they have never had primacy in art. You could, of course, put it the other way and say that there is a lot of information available in faces because the brain has devoted special areas to it."
--Dale Keiger


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