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The Best in Sight

November 20, 2008


Eye chart.

What is sight? There's more than meets the eye.

Insights into how we see set Department of Psychology vision researchers apart

By Deane Morrison

A student sits before a computer screen in Yuhong Jiang's laboratory, watching teams of players zip a basketball around. "Count the number of passes between white players," she says. Gamely, the student tries. "Did you notice anything unusual?" asks Jiang afterward.

When she replays the scene, the student suddenly sees a girl with an umbrella walk right through the basketball game. Why didn't he see her before? His eyes did—but his attention was fixed on the ball.

The experiment is one of many by which Jiang, an assistant professor of psychology, probes the human mind's ability to manage visual information. She is a new face in the University of Minnesota Psychology Department's array of vision researchers, who are lifting the curtain on how our eyes and brains produce the miracle of vision.

The shifty human visual system

A big discovery of the past few decades is the brain's adaptability, or "plasticity," a trait well known to psychology professor and department chair Gordon Legge. Visually impaired himself, he has undergone functional magnetic resonance imaging (fMRI) studies to learn how the condition affects neurons of the visual cortex.

First responders
The retina has been called a satellite dish for the brain, but that metaphor vastly underrates this paper-thin tissue's ability to process light signals. When light strikes the retina, it is absorbed by the well-known rod and cone cells. Cones detect color and work well in daytime, but are useless at night. Rods, however, are 1,000 times more sensitive to light and allow night vision.

"Early on it was realized that nocturnal animals' retinas were dominated by rods, and those of diurnal animals by cones," says University psychology professor Dwight Burkhardt, who pioneered studies of cones. "We [humans] are both."

Only cones are found at the fovea of our eye—the part that focuses our attention—whereas peripheral areas of the retina have both rods and cones. Therefore, says Burkhardt, at night we're "functionally blind" in the foveal area and so must navigate on peripheral vision.

Dwellers in well-lit urban areas may never notice this, but ancient astronomers did. "When they wanted to see a dim star, they had to look away from it," says Burkhardt. That is still true for finding dim stars, even from a light-polluted city.

Recently, Burkhardt has focused on a key class of retinal cells called bipolars, which heighten the contrast we perceive between objects in the environment, be they stars against a dark sky or black ink on white paper.

"We've found that many bipolars are sensitive to small contrasts," he says. "Most contrasts in the environment are small. Without [enhancement by bipolars], much of what we see would look like camouflage."

That's the part of the cerebral cortex—the brain's intellectual powerhouse—that deals with visual information, starting with the "primary" visual cortex in the back of the head. But its function can be "reassigned" to other senses in people with low vision. [First sidebar about here]

"A portion of my visual cortex has been allocated to touch," says Legge. "We can see that when I read Braille, a lot of my visual cortex is activated. In the totally blind, the visual cortex seems to be taken over by touch."

And touch can be jealous of its new territory. "In some cases, 'sight restoration' surgery has not led to full restoration of visual function, possibly because touch won't let the visual cortex go," says Legge.

Sometimes, though, visual abilities can improve with effort. In his studies of how visual experience shapes vision in adults, professor Stephen Engel has found that with practice, people can begin to see very faint lines or patterns that were previously invisible to them.

A burning question is whether the adult primary visual cortex can rewire itself, and if so, how? Finding out, says Engel, could lead to optimizing people's ability to pick out everything from tumors in an X-ray to enemies in a military image.

Back in Jiang's office, a visitor watches red and green dots move randomly across a computer screen. The task is to follow the red dots, even after they suddenly turn green. Tracking one is easy, but multiple dots? Hoo boy.

It's a test of working memory, the ability to remember visual information after it's no longer in sight. People can only improve their scores a little, says Jiang, and individuals' abilities vary widely.

"Why are some people better than others? Why can't we make other people better with training? We're looking for answers," she says.

Division of labor

When we open our eyes, we usually see a seamless image that gives no hint of the effort it took: numerous cells and groups of cells in our retinas and brains, all working on different tasks, ultimately combining to form a coherent whole. Our visual system is like a high-speed factory that puts out a new product—the world as we see it—several times a second.

Our brains seem to divide the labor among separate, but physically intertwined, populations of neurons that respond to only one small aspect of our environment, such as vertical lines or motion from left to right. The brain then bases its interpretation of images largely on which neurons fire.

Also, the brain gains efficiency by organizing neurons into groups, or "centers," that act like committees, each processing a certain kind of visual information. Like faces, for example.

"Many studies show that there is a specialized mechanism to deal with faces," says professor Sheng He. It includes an area of the cerebral cortex whose main job seems to be face recognition. So fundamental is this skill that our brains can "read" emotional information from faces without our being aware of it.

"What keeps me going as a psychologist is the phenomenon that we can see a tree and immediately know it's a tree. It's amazing how strong and tangible perception is, even with weak incoming data."

He discovered this by showing images of faces displaying emotions—fear, for example—or neutral expressions to experimental subjects. Simultaneously, the subjects were distracted by "noise" in the rest of their visual fields and didn't realize any image was in front of them. Testing showed that specific areas in their brains responded differently to fearful and neutral faces, a clear indication that emotional processing of images goes on below our mind's "radar." He has also found that "invisible" images of people in erotic poses can be subconsciously perceived.

When the moon hits your eye

If science is to find treatments for blindness, dyslexia, and other conditions, more must be learned about how neurons function separately and together. Much effort focuses on the primary visual cortex, or V1. It passes information to at least 30 or 40 other areas of the cortex, including ones associated with vision, conscious perception, movement, emotion, and reasoning; even so, knowledge of how V1 "decides" what to do is in its infancy. [Second sidebar about here]

Internal compass
Most of us take navigating inside buildings for granted, but not Gordon Legge. He studies spatial mobility for vision-impaired people and has a project to help them find their way around indoors.

"One big problem is that they can't read signs," he says. "GPS has been adapted for speech output, but it doesn't work indoors."

Unlike streets, indoor passageways don't have names, so even a GPS unit that worked in a building would have trouble guiding a blind person down the right corridor. So Legge, working with Advanced Medical Electronics, has developed a prototype technology to assist them.

"Imagine every room sign had a bar code," he says. "The user carries a 'magic flashlight' sending out an infrared beam. If the beam hits a bar code, a camera [on the person] gets a reflection, reads the code, and compares it to a digital map of the building." The unit can then compute a route for the person and convert the instructions to speech.

In Legge's prototype, the technology is built into a flashlight housing and connected to a computer on a cart. In a few tests so far, low-vision people have successfully navigated indoors, he says. Legge has a grant to shrink the device, and says that in a future system a cell phone would probably handle the computing. Bar codes would be placed not only by rooms, but near all kinds of landmarks, such as drinking fountains, benches, doors, elevators, and stairs.

Adapted from an article in the fall 2008 Psychology in Minnesota, the magazine of the University of Minnesota Department of Psychology.

Sometimes our visual environment throws us a curve, and that offers a great opportunity for learning how the primary visual cortex functions. Take, for example, the well-known moon illusion: A full moon looks bigger when near the horizon than when high in the sky, even though the images in the retina are the same size.

For reasons not yet fully understood, the change in viewing context produces an apparent change in the size of the moon. Professor Daniel Kersten makes good use of the effect of context on apparent size. In experiments, he has recorded activity in the primary visual cortex of subjects viewing computer graphics of similar optical illusions and trying to judge the sizes of objects.

"My colleagues and I thought that V1 would not be depth-sensitive because it's too early [a stage in processing of visual information]," says Kersten. Yet subjects in the process of judging, and misjudging, objects showed more widespread activity in their V1 areas.

Therefore, sophisticated processing of visual information seems to begin as soon as it reaches the primary visual cortex. Or, says Kersten, V1 may only be helping higher centers in the brain interpret the information. Understanding this communication may open a window onto how the cerebral cortex works generally, and even onto what makes us intelligent.

Beyond the practical implications of their work, vision researchers can't help but have an even deeper effect: a sense of wonder at how the human eye and mind can make so much of so little.

"What keeps me going as a psychologist is the phenomenon that we can see a tree and immediately know it's a tree," says assistant professor Cheryl Olman. "It's amazing how strong and tangible perception is, even with weak incoming data."

Adapted from an article in the fall 2008 Psychology in Minnesota, the magazine of the University of Minnesota Department of Psychology.