Seeing is Not Necessarily Perceiving, Psychology Research Shows By Emil Venere If not for the brain s complex system of processing visual images, people might see reality more like the disconnected bits and pieces in a Picasso painting. Making sense of the visual world requires a mysterious interplay between the brain and the constant stream of images it receives. Now a Hopkins researcher has added a new wrinkle to the understanding of vision. Steven Yantis, an associate professor in the Department of Psychology, is studying the way the mind fills in the gaps when visual information is missing. One way to illustrate how the system works is to look at a figure that has two squares with a rectangle between them (see illustration). The brain assumes that there are actually two rectangles, one lying on top of and therefore partly covering the other so that only the ends of the bottom rectangle are visible. The same effect emerges by placing a pencil over the space between two separated rectangles. Because the observer can t see the gap that is hidden by the pencil, the two rectangles are perceived as one continuous rectangle. "Your visual system is predisposed to interpret the image in a way that is consistent with how the physical world is structured," Dr. Yantis said. Vision is no simple sense, although it would appear to be. "People don t think about it they just do it," said Dr. Yantis, who has won the 1994 Distinguished Scientific Award for Early Career Contribution to Psychology in the area of perception and motor performance. The award, which will be issued in August by the American Psychological Association, is given to outstanding researchers who have had their doctorates for no longer than nine years. His recent work deals with how the brain interprets images in motion. An example can be found by visiting a crowded ballpark. "If you are at a baseball game and a player throws a baseball and you are sitting behind somebody else so that you actually can t see the ball for some part of its trajectory, you still perceive it as a single trajectory, as one object, not as one object that disappears and a different object that appears," he said. In other words, the broken trajectory seen by the retina is experienced as continuous. Using objects on a computer screen, Dr. Yantis has taken that concept one step further. If people are shown a white dot on the screen and then the dot disappears, reappearing a fraction of a second later a short distance from its original location, the dot will be perceived as a single object moving from point A to point B. The shorter the time interval between its disappearance and reappearance, the more likely it will be perceived as a single object moving through space. But as the time gap increases, the perception of motion starts to fade, until the observer finally begins to see two distinct objects, one at point A and one at point B. Researchers have known about this kind of "apparent motion" for years. It serves as the basis for the appearance of continuous motion in such things as motion pictures and television. But Dr. Yantis has discovered that if an occluding object, much like the spectator who blocks the baseball trajectory, is placed between points A and B, the observer will continue to see the two dots as a single moving object even with long time gaps that would normally eliminate the perception of motion. In essence, the brain is tricked into thinking that the dots on the computer screen are the same object moving through space, because the occluding surface between the dots appears to be blocking the view of its trajectory. "You essentially ca see the object behind the occluder, in your mind s eye," Dr. Yantis said. "Even with a long [time] gap I can cause these objects to be perceived as stable and continuous, if I have an occluding surface that can explain the gap." His findings, based on a recent experiment, will be detailed in a paper to be published in an upcoming issue of the journal Psychological Science. Sixteen Hopkins students were used as subjects in the experiment. The students wore 3-D glasses to create the illusion that the occluding object (a rectangle) was actually floating in front of the dots on the computer screen. Each of the two lenses of the 3-D glasses acted as "shutters," quickly switching between opaque and clear, so that each eye received a slightly different view of the screen. That caused the occluding rectangle to appear to float stereoscopically in front of (or, in some cases, behind) the dots. When no occlusion was used, the students tended to see the dots as two distinct objects, flickering in place, as the time gap was increased to a tenth of a second. When the occluding surface was present, subjects reported seeing the dot as a single moving object much more often. But if the 3-D glasses were adjusted so that the occluding surface appeared to float behind the dots, the illusion of continuous motion was eliminated. This "control condition" demonstrated that placing the rectangular occlusion in front of the dots is essential for the illusion. The experiment shows how vision is not simply a matter of the retina sending signals to the brain, like so many pictures from a video camera. And although the "perceptual completion" might seem like something learned from experience, scientists believe it may be "hardwired" into the brain at birth, since past research has found that even very young infants vision works the same way as adult vision. The research also rules out reasoning and inference as an explanation because the effect happens too fast, within a few tenths of a second. "It is happening at a very low, automatic level," Dr. Yantis said.
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