DB is a 67-year-old man whose view of the world is dark from the center of his gaze leftward. He has been blind to this left part of his visual scene since age 33, when he had surgery to remove an abnormal tangle of blood vessels at the back of his brain. Unfortunately, while taking out the tangle, surgeons destroyed an important center of visual processing called the primary visual cortex, or area V1, which relays information from the eyes to higher-level brain areas dedicated to sight.
DB lost just the right half of V1. Because the right part of the brain processes visual information from the left visual field (and vice versa), his doctors were not surprised that DB became blind to the left portion of his view. But they were astounded that although DB denied seeing anything to the left of center, he was nonetheless able to accurately “guess” many properties of targets, such as shape and specific location, presented in this perceptually dark field.
DB’s ability to somehow intuit features of unseen objects and patterns is called blindsight. Researchers believe this strange phenomenon stems from the flow of information through neural pathways that bypass V1 but still convey a small amount of visual information to higher brain regions involved in sight. For unknown reasons, these secondary routes do not convey the feeling of sight.
Recent data suggest that the accuracy of a blindsight patient’s guess about what something looks like, or where it is, can improve markedly with practice, hinting that such practice might improve blindsight patients’ ability to detect objects in their everyday surroundings. And although an individual with blindsight cannot see in his or her blind field, a new study shows that DB, at least, has some object-detection abilities that surpass those of ordinary sighted people. This research also reveals that some awareness of unseen visual stimuli can accompany blindsight. DB’s and others’ cases of blindsight indicate that consciousness and visual perception can be separated in our brains.
Neurological oddities typically emerge from studies of brain-damaged people, but experiments in animals offered the earliest hints of blindsight. Beginning in the 1930s and 1940s, neurobiologists who had surgically removed V1 in monkeys noticed that the animals appeared to retain some visual skills, such as the ability to detect contrast and to tell one object from another by the objects’ shapes.
But few scientists believed that people could see without V1: the known human patients whose primary visual cortex had been destroyed were totally blind. Some exceptions to this rule included soldiers who had sustained injuries during World Wars I and II that abolished the function of V1. A few neurologists who treated these men claimed that some of them retained residual visual function. But at the time the scientific community did not take such observations seriously. Instead researchers concluded that humans and monkeys were different in this respect, despite the striking anatomical similarities in their visual pathways.
In 1973 neuroscientist Ernst Pppel, then at the Massachusetts Institute of Technology, and his colleagues reported measuring eye movements in patients who had lost area V1. The patients said that they could not see visual targets, but their eye movements were biased toward them nonetheless, hinting that their visual system was obliquely informed of the targets.
But it was the work of University of Oxford psychologist Larry Weiskrantz and his colleagues, who first examined DB in the early 1970s, that shattered the skepticism about blindsight in people. Like Pppel’s patients, DB showed eye movements biased toward visual targets. In addition, however, Weiskrantz and his co-workers unmasked other visual skills with a technique borrowed from animal experiments: they forced DB to choose between defined options instead of just asking him what he saw. That is, Weiskrantz’s team presented DB with a choice of, say, two possible colors or locations and asked him to guess which one applied to a visual target he claimed he could not see. DB’s “guesswork” was correct much more often than would have been expected by chance—matching the findings in primates.
DB himself was astonished. Because he could not see the objects, he had thought that his guesses were completely random. In the wake of these experiments, Weiskrantz coined the term “blindsight,” which appeared in a 1974 article in the journal Lancet.
Scientists then identified and examined other patients who displayed this curious ability. Although none of them so far has demonstrated detection skills as acute as those of DB, many of these patients can deduce the color or shape of an object in their blind field and predict whether it is moving or still; they can also guess the orientation of unseen lines or gratings, the timing of an object’s appearance, and the expressions on unperceived faces at better-than-chance levels. On the other hand, these patients cannot intuit fine details in their blind fields. Nor can they detect complex movements.
Whatever a patient’s ability to detect the unseen, practice can enhance it. In a 2006 study Weiskrantz, along with neuroscientists Ceri T. Trevethan and Arash Sahraie of the University of Aberdeen in Scotland and their colleagues, asked 12 patients with blindsight to repeatedly guess which of two stimuli—a flickering grating or a gray dot—had appeared in the middle of their blind field. After three months of daily practice, the patients increased the number of correct responses by up to 25 percent and could detect gratings of lower contrast than those they could detect before. They also generally reported being more consciously aware of the correct answer. The results suggest that these patients can learn to “see” in ways that could enhance their quality of life.
DB might not need much more practice, however, after having participated in numerous vision experiments over four decades. Indeed, in 2007, Weiskrantz, Trevethan and Sahraie showed that DB’s blind-field sensitivity is actually better than what normal vision can achieve. The researchers showed DB a two-second-long stimulus called a Gabor patch on a gray screen in one of two time spans. Because the patch was small and had very low contrast, even a person with normal vision would find it hard to perceive. Weiskrantz’s team asked DB to indicate, with a button press, in which time interval he thought the pattern had appeared.
In two different experiments involving more than 150 distinct presentations of the stimulus, DB performed significantly better in his blind field than in his sighted field. He consistently identified the time span containing the stimulus 87 percent of the time in his blind field as opposed to only half of the time—a rate no better than chance—in his sighted field. By varying the stimulus contrast, the researchers also learned that DB could detect stimuli of significantly lower contrast in his blind field as compared with his sighted field. Ironically, DB found the tests on the sighted field hard work, whereas the blind-field tests seemed effortless: “No problem, I’m just guessing,” he remarked.
The researchers ruled out the possibility that DB might simply have abnormally poor vision in his sighted field. When they compared DB’s performance in his sighted field with that of six age-matched participants who had normal vision, they found DB’s sighted-field vision to be equal to that of the unimpaired subjects. Thus, DB’s blind-field sensitivity is not superior merely to that of his own sighted field but also to that of normal vision.
Meanwhile DB reported no awareness of the Gabor patch when it was presented to his sighted field (confirming that he was essentially guessing about when it showed up). Nevertheless, DB had some subjective awareness of 80 percent of stimuli presented to the blind field. This awareness was nothing like vision, DB explained; he described it instead as “feeling as if a finger is pointing through the screen.”
Interestingly, DB’s awareness of the unseen pattern vanished during trials in which researchers randomly alternated its presentation to the blind or sighted field. DB reported being aware of the presence of the Gabor patch only during the trials in which the researchers first showed it repeatedly to the blind field and then switched to the sighted field for a second block of 30 trials. That is, DB’s awareness of a stimulus seems to depend on his ability to predict its appearance in the blind field—and thus on the expectation that he will not be able to actually see it.
The feeling of being aware of something is, of course, different from actually seeing it. Because DB was aware but unseeing, his damaged brain area, V1, may be essential only to the sensation of visual perception and not to subjective awareness. Thus, if you sustain damage to V1, you may be aware of much that you cannot see.
Still, not all studies of blindsight indicate that patients are aware of unseen visual stimuli. Tests of a blindsight patient known as GY showed that his talent for detecting a symbol was not accompanied by the ability to consciously predict (and bet on) his performance [see “Put Your Money Where Your Mind Is,” by Kaspar Mossman; Scientific American Mind, April/May 2007].
DB is probably a particularly gifted patient. From all his experience, he may have developed an intuitive sense for when something is going to appear—and may have learned to trust his intuition. Thus, DB may represent the pinnacle of a phenomenon in which brain damage or inborn defects that lead to amnesia, dyslexia, blindness or myriad other difficulties can nonetheless leave behind surprising residual powers. Such revelations give new meaning to the legendary rejoinder of the blind comic-book superhero Daredevil: “Yeah, tell them you got beat by a blind man, too.”