Anstis, a psychologist at the University of California, San Diego, are permanently “stuck” on the young lady and cannot see the hag. Intriguingly, some people, including our colleague Stuart M. In effect, you suffer from a temporary form of agnosia for her. Yet you were not perceiving (or “knowing”) the old woman. Now, when you were perceiving the face as a young woman, you were also simultaneously seeing the lines and curves constituting the old hag. The young woman’s chin becomes the hag’s nose, and the young ear becomes the old eye. But after a while, you can mentally flip the image to see an old face. The first time you look at this illusion, you probably see the girl. What must it feel like to have such a condition, seeing yet not knowing? You can get an inkling by looking at the famous old woman/young lady illustration (a). John had a condition known as visual agnosia, a phrase coined by Sigmund Freud meaning “lack of visual knowledge.” Unlike some of Freud’s more outlandish ideas such as “penis envy” or the “Oedipus complex,” this one has survived the test of time. He might as well have been copying a meaningless jumble of lines. John could produce a faithful rendering, almost a carbon copy, of the picture but had no idea what it was. Paul’s Cathedral that was hanging on the wall. The doctors confirmed this assertion by asking him to copy a drawing of, for example, St. “They are out of focus in my mind, doctor,” he would say, “not in my eye.” Indeed, he could not visually distinguish among umbrellas, chairs or other common objects, even though he claimed to be able to see them perfectly clearly. He knew her by her voice his brain areas for hearing were unaffected, as were his memories. But when he saw his wife-or anyone else for that matter-he could not recognize her. He could observe things around him he was not blind in the usual sense. Soon after his retirement he suffered a stroke that partially damaged visual regions of both hemispheres of his brain. Humphreys, both now at the University of Birmingham in England. Or consider the strange case of John, elegantly studied in 1987 by M. Yet when asked to reach out and touch the spot, he could do so accurately he could touch a spot he couldn’t see! It seems downright spooky, but, as you will soon learn, we can explain-at least partially-his condition, known as blindsight, in terms of the multiple specialized anatomical pathways devoted to vision that we mentioned earlier. He could not consciously see anything, not even a spot of light, shown to him in that region. Damage to his visual cortex resulted in complete blindness in one half of the visual field. This approach parallels our study of “normal” illusions-by understanding misperceptions, whether for intact or damaged systems, we gain insight into brain processes involved in perception.Ĭonsider the case of a man known as GY.
It is only when parts of these different visual areas are damaged, causing selective yet often profound disturbances in perception, that we begin to appreciate the range and subtlety of normal human vision. We take our sight for granted because it usually seems so effortless. It makes sense that responsibility for processing is divided into various areas that have different computational objectives. Vision, it turns out, is a much more complex and sophisticated affair than anyone had imagined. We now know more than 30 areas in the brains of primates-including humans-are involved in handling aspects of vision such as the perception of motion, color and depth. UNTIL ABOUT 35 years ago scientists believed there was only a single visual-processing area, called the visual cortex, situated at the back of the brain.
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