many other kinds of visual phenomena because faces are more important—but not upside-down faces, since we rarely encounter those, except when performing headstands in a yoga class. That’s why we are far better at detecting the distortion on the face that is right side up than on the one that is flipped over.
The researchers studying TN chose faces as their second series of images in the belief that the brain’s special and largely unconscious focus on faces might allow TN to improve his performance, even though he’d have no conscious awareness of seeing anything. Whether he was looking at faces, geometric shapes, or ripe peaches ought to have been a moot point, given that TN was, after all, blind. But on this test TN identified the faces as happy or angry correctly almost two times out of three. Though the part of his brain responsible for the conscious sensation of vision had obviously been destroyed, his fusiform face area was receiving the images. It was influencing the conscious choices he made in the forced-choice experiment, but TN didn’t know it.
Having heard about the first experiment involving TN, a few months later another group of researchers asked him if he would participate in a different test. Reading faces may be a special human talent, but not falling on your face is even more special. If you suddenly notice that you are about to trip over a sleeping cat, you don’t consciously ponder strategies for stepping out of the way; you just do it.15 That avoidance is governed by your unconscious, and it is the skill the researchers wanted to test in TN. They proposed to watch as he walked, without his cane, down a cluttered hallway.16
The idea excited all those involved except the person not guaranteed to remain vertical. TN refused to participate.17 He may have had some success in the face test, but what blind man would consent to navigating an obstacle course? The researchers implored him, in effect, to just do it. And they kindly offered to have an escort trail him to make sure he didn’t fall. After some prodding, he changed his mind. Then, to the amazement of everyone, including himself, he zigged and zagged his way perfectly down the corridor, sidestepping a garbage can, a stack of paper, and several boxes. He didn’t stumble once, or even collide with any objects. When asked how he’d accomplished this, TN had no explanation and, one presumes, requested the return of his cane.
The phenomenon exhibited by TN—in which individuals with intact eyes have no conscious sensation of seeing but can nevertheless respond in some way to what their eyes register—is called “blindsight.” This important discovery “elicited disbelief and howls of derision” when first reported and has only recently come to be accepted.18 But in a sense it shouldn’t have been surprising: it makes perfect sense that blindsight would result when the conscious visual system is rendered nonfunctional but a person’s eyes and unconscious system remain intact. Blindsight is a strange syndrome—a particularly dramatic illustration of the two tiers of the brain operating independently of each other.
THE FIRST PHYSICAL indication that vision occurs through multiple pathways came from a British Army doctor named George Riddoch in 1917.19 In the late nineteenth century, scientists had begun to study the importance of the occipital lobe in vision by creating lesions in dogs and monkeys. But data on humans was scarce. Then came World War I. Suddenly the Germans were turning British soldiers into promising research subjects at an alarming pace. This was partly because British helmets tended to dance atop the soldiers’ heads, which might have looked fashionable but didn’t cover them very well, especially in the back. Also, the standard in that conflict was trench warfare. As it was practiced, a soldier’s job was to keep all of his body protected by the solid earth except for his head, which he was instructed to stick up into the line of fire. As a result, 25 percent of all penetrating wounds suffered by British soldiers were head wounds, especially of the lower occipital lobe and its neighbor the cerebellum.
The same path of bullet penetration today would turn a huge swath of the brain into sausage meat and almost certainly kill the victim. But in those days bullets were slower and more discrete in their effects. They tended to bore neat tunnels through the gray matter without disturbing the surrounding tissue very much. This left the victims alive and in better condition than you might imagine given that their heads now had the topology of a doughnut. One Japanese doctor who worked under similar conditions in the Russo-Japanese War saw so many patients injured in that manner that he devised a method for mapping the precise internal brain injury—and the deficits expected—based on the relation of the bullet holes to various external landmarks on the skull. (His official job had been to determine the size of the pension owed the brain-damaged soldiers.)20
Dr. Riddoch’s most interesting patient was a Lieutenant Colonel T., who had a bullet sail through his right occipital lobe while he was leading his men into battle. After taking the hit he bravely brushed himself off and proceeded to continue leading his men. When asked how he felt, he reported being dazed but said he was otherwise just fine. He was wrong. Fifteen minutes later, he collapsed. When he woke up it was eleven days later, and he was in a hospital in India.
Although he was now conscious again, one of the first signs that something was amiss came at dinner, when Lieutenant Colonel T. noted that he had a hard time seeing bits of meat residing on the left side of his plate. In humans, the eyes are wired to the brain in such a way that visual information from the left side of your field of vision is transmitted to the right side of your brain, and vice versa, no matter which eye that information comes from. In other words, if you stare straight ahead, everything to your left is transmitted to the right hemisphere of your brain, which is where Lieutenant Colonel T. took the bullet. After he was transferred to a hospital in England, it was established that Lieutenant Colonel T. was totally blind on the left side of his visual field, with one bizarre exception. He could detect motion there. That is, he couldn’t see in the usual sense—the “moving things” had no shape or color—but he did know if something was moving. It was partial information, and of little use. In fact, it annoyed him, especially during train rides, when he would sense that things were moving past at his left but he couldn’t see anything there.
Since Lieutenant Colonel T. was consciously aware of the motion he detected, his wasn’t a case of true blindsight, as TN’s was, but still, the case was groundbreaking for its suggestion that vision is the cumulative effect of information traveling along multiple pathways, both conscious and unconscious. George Riddoch published a paper on Lieutenant Colonel T. and others like him, but unfortunately another British Army doctor, one far better known, derided Riddoch’s work. With that it virtually disappeared from the literature, not to resurface for many decades.
UNTIL RECENTLY, UNCONSCIOUS vision was difficult to investigate because patients with blindsight are exceedingly rare.21 But in 2005, Antonio Rangel’s Caltech colleague Christof Koch and a coworker came up with a powerful new way to explore unconscious vision in healthy subjects. Koch arrived at this discovery about the unconscious because of his interest in its flip side—the meaning of consciousness. If studying the unconscious was, until recently, not a good career move, Koch says that studying consciousness was, at least until the 1990s, “considered a sign of cognitive decline.” Today, however, scientists study the two subjects hand in hand, and one of the advantages of research on the visual system is that it is in some sense simpler than, say, memory or social perception.
The technique Koch’s group discovered exploits a visual phenomenon called binocular rivalry. Under the right circumstances, if one image is presented to your left eye while a different image is presented to your right eye, you won’t see both of them, somehow superimposed. Instead, you’ll perceive just one of the two images. Then, after a while, you’ll see the other image, and then the first again. The two images will alternate in that manner indefinitely. What Koch’s group found, however, was that if they present a
Employing the new technique, another group of scientists performed an experiment on normal people analogous to the one the facial expression researchers performed on patient TN.23 They exposed each subject’s right eye to a colorful and rapidly changing mosaic-like image, and each subject’s left eye to a static photograph that pictured an object. That object was positioned near either the right edge of the photograph or the left, and it was their subjects’ task to guess where the object was, even though they did not consciously perceive the static photo. The researchers expected that, as in the case of TN, the subjects’ unconscious cues would be
