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. 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. Vision, it turns out, is a much more complex and sophisticated affair than anyone had imagined. It makes sense that responsibility for processing is divided into various areas that have different computational objectives.
We take our sight for granted because it usually seems so effortless. 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. 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.
Consider the case of a man known as GY. Damage to his visual cortex resulted in complete blindness in one half of the visual field. He could not consciously see anything, not even a spot of light, shown to him in that region. 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. [For more on blindsight, see “Subconscious Sight,” by Susana Martinez-Conde; Scientific American Mind, April/May 2008.]
Or consider the strange case of John, elegantly studied in 1987 by M. Jane Riddoch and Glyn W. Humphreys, both now at the University of Birmingham in England. John had served as an air force pilot. Soon after his retirement he suffered a stroke that partially damaged visual regions of both hemispheres of his brain. He could observe things around him; he was not blind in the usual sense. But when he saw his wife—or anyone else for that matter—he could not recognize her. He knew her by her voice; his brain areas for hearing were unaffected, as were his memories. 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. “They are out of focus in my mind, doctor,” he would say, “not in my eye.”
The doctors confirmed this assertion by asking him to copy a drawing of, for example, St. Paul’s Cathedral that was hanging on the wall. John could produce a faithful rendering, almost a carbon copy, of the picture but had no idea what it was. He might as well have been copying a meaningless jumble of lines.
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.
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). The first time you look at this illusion, you probably see the girl. But after a while, you can mentally flip the image to see an old face. The young woman’s chin becomes the hag’s nose, and the young ear becomes the old eye. Now, when you were perceiving the face as a young woman, you were also simultaneously seeing the lines and curves constituting the old hag. Yet you were not perceiving (or “knowing”) the old woman. In effect, you suffer from a temporary form of agnosia for her. Intriguingly, some people, including our colleague Stuart M. Anstis, a psychologist at the University of California, San Diego, are permanently “stuck” on the young lady and cannot see the hag. Psychologist Richard L. Gregory of the University of Bristol in England refers to this inability as visual hagnosia.