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 changing image to one eye and a static one to the other, people will see only the changing image, and never the static one.22 In other words, if your right eye were exposed to a film of two monkeys playing Ping-Pong and your left to a photo of a hundred-dollar bill, you’d be unaware of the static photo even though your left eye had recorded the data and transmitted it to your brain. The technique provides a powerful tool for creating, in a sense, artificial blindsight—a new way to study unconscious vision without destroying any part of the brain.
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 powerful only if the object pictured was of vital interest to the human brain. This led to an obvious category. And so when the researchers performed this experiment, they selected, for one of the static images, pornography—or, in their scientific jargon, a “highly arousing erotic image.” You can get erotica at almost any newsstand, but where do you get scientifically controlled erotica? It turns out that psychologists have a database for that. It is called the International Affective Picture System, a collection of 480 images ranging from sexually explicit material to mutilated bodies to pleasant images of children and wildlife, each categorized according to the level of arousal it produces.
As the researchers expected, when presented with unprovocative static images and asked whether the object was on the left- or the right-hand side of the photo, the subjects’ answers were correct about half the time, which is what you would expect from completely random, uninformed guesses, a rate comparable to TN’s when he was making guesses about circles versus squares. But when heterosexual male subjects were shown an image of a naked woman, they gained a significant ability to discern on which side of the image she was located, as did females who were shown images of naked men. That didn’t happen when men were shown naked men, or when women were shown naked women—with one exception, of course. When the experiment was repeated on homosexual subjects, the results flipped in the manner you might expect. The results mirrored the subjects’ sexual preferences.
Despite their successes, when asked afterward what they had seen, all the subjects described just the tedious progression of rapidly changing mosaic images the researchers had presented to their right eye. The subjects were clueless that while their conscious minds were looking at a series of snoozers, their unconscious minds were feasting on Girls (or Boys) Gone Wild. This means that while the processing of the erotic image was never delivered to the consciousness, it did register powerfully enough in the unconscious that the subjects had a subliminal awareness of it. We are reminded again of the lesson Peirce learned: We don’t consciously perceive everything that registers in our brain, so our unconscious mind may notice things that our conscious mind doesn’t. When that happens we may get a funny feeling about a business associate or a hunch about a stranger and, like Peirce, not know the source.