Let’s first consider the two pathways by which visual information enters the cortex. The so-called old pathway starts in the retinas, relays through an ancient midbrain structure called the superior colliculus, and then projects—via the pulvinar—to the parietal lobes (see Figure 2.10). This pathway is concerned with spatial aspects of vision: where, but not what, an object is. The old pathway enables us to orient toward objects and track them with our eyes and heads. If you damage this pathway in a hamster, the animal develops a curious tunnel vision, seeing and recognizing only what is directly in front of its nose.
FIGURE 2.10 The visual information from the retina gets to the brain via two pathways. One (called the old pathway) relays through the superior colliculus, arriving eventually in the parietal lobe. The other (called the new pathway) goes via the lateral geniculate nucleus (LGN) to the visual cortex and then splits once again into the “how” and “what” streams.
The new pathway, which is highly developed in humans and in primates generally, allows sophisticated analysis and recognition of complex visual scenes and objects. This pathway projects from the retina to V1, the first and largest of our cortical visual maps, and from there splits into two subpathways, or streams: pathway 1, or what is often called the “how” stream, and pathway 2 the “what” stream. You can think of the “how” stream (sometimes called the “where” stream) as being concerned with the relationships among visual objects in space, while the “what” stream is concerned with the relationships of features within visual objects themselves. Thus the “how” stream’s function overlaps to some extent with that of the old pathway, but it mediates much more sophisticated aspects of spatial vision—determining the overall spatial layout of the visual scene rather than just the location of an object. The “how” stream projects to the parietal lobe and has strong links to the motor system. When you dodge an object hurled at you, when you navigate around a room avoiding bumping into things, when you step gingerly over a tree branch or a pit, or when you reach out to grab an object or fend off a blow, you are relying on the “how” stream. Most of these computations are unconscious and highly automated, like a robot or a zombie copilot that follows your instructions without need of much guidance or monitoring.
Before we consider the “what” stream, let me first mention the fascinating visual phenomenon of blindsight. It was discovered in Oxford in the late 1970s by Larry Weizkrantz. A patient named Gy had suffered substantial damage to his left visual cortex—the origin point for both the “how” and the “what” streams. As a result he became completely blind in his right visual field—or so it seemed at first. In the course of testing Gy’s intact vision, Weizkrantz told him to reach out and try to touch a tiny spot of light that he told Gy was to his right. Gy protested that he couldn’t see it and there would be no point, but Weizkrantz asked him to try anyway. To his amazement, Gy correctly touched the spot. Gy insisted that he had been guessing, and was surprised when he was told that he had pointed correctly. But repeated trials proved that it had not been a lucky stab in the dark; Gy’s finger homed in on target after target, even though he had no conscious visual experience of where they were or what they looked like. Weizkrantz dubbed the syndrome blindsight to emphasize its paradoxical nature. Short of ESP, how can we explain this? How can a person locate something he cannot see? The answer lies in the anatomical division between the old and new pathways in the brain. Gy’s new pathway, running through V1, was damaged, but his old pathway was perfectly intact. Information about the spot’s location traveled up smoothly to his parietal lobes, which in turn directed the hand to move to the correct location.
This explanation of blindsight is elegant and widely accepted, but it raises an even more intriguing question: Doesn’t this imply that only the new pathway has visual consciousness? When the new pathway is blocked, as in Gy’s case, visual awareness winks out. The old pathway, on the other hand, is apparently performing equally complex computations to guide the hand, but without a wisp of consciousness creeping in. This is one reason why I likened this pathway to a robot or a zombie. Why should this be so? After all, they are just two parallel pathways made up of identical-looking neurons, so why is only one of them linked to conscious awareness?
Why indeed. While I have raised it here as a teaser, the question of conscious awareness is a big one that we will leave for the final chapter.
Now let’s have look at pathway 2, the “what” stream. This stream is concerned mainly with recognizing what an object is and what it means to you. This pathway projects from V1 to the fusiform gyrus (see Figure 3.6), and from there to other parts of the temporal lobes. Note that the fusiform area itself mainly performs a dry classification of objects: It discriminates Ps from Qs, hawks from handsaws, and Joe from Jane, but it does not assign significance to any of them. Its role is analogous to that of a shell collector (conchologist) or a butterfly collector (lepidopterist), who classifies and labels hundreds of specimens into discrete nonoverlapping conceptual bins without necessarily knowing (or caring) anything else about them. (This is approximately true but not completely; some aspects of meaning are probably fed back from higher centers to the fusiform.)
But as pathway 2 proceeds past the fusiform to other parts of the temporal lobes, it evokes not only the name of a thing but a penumbra of associated memories and facts about it—broadly speaking the semantics, or meaning, of an object. You not only recognize Joe’s face as being “Joe,” but you remember all sorts of things about him: He is married to Jane, has a warped sense of humor, is allergic to cats, and is on your bowling team. This semantic retrieval process involves widespread activation of the temporal lobes, but it seems to center on a handful of “bottlenecks” that include Wernicke’s language area and the inferior parietal lobule (IPL), which is involved in quintessentially human abilities as such as naming, reading, writing, and arithmetic. Once meaning is extracted in these bottleneck regions, the messages are relayed to the amygdala, which lies embedded in the front tip of the temporal lobes, to evoke feelings about what (or whom) you are seeing.
In addition to pathways 1 and 24 there seems to be an alternate, somewhat more reflexive pathway for emotional response to objects that I call pathway 3. If the first two were the “how” and “what” streams, this one could be thought of as the “so what” stream. In this pathway, biologically salient stimuli such as eyes, food, facial expressions, and animate motion (such as someone’s gait and gesturing) pass from the fusiform gyrus through an area in the temporal lobe called the superior temporal sulcus (STS) and then straight to the amygdala.5 In other words, pathway 3 bypasses high-level object perception—and the whole rich penumbra of associations evoked through pathway 2—and shunts quickly to the amygdala, the gateway to the emotional core of the brain, the limbic system. This shortcut probably evolved to promote fast reaction to high-value situations, whether innate or learned.
The amygdala works in conjunction with past stored memories and other structures in the limbic system to gauge the emotional significance of whatever you are looking at: Is it friend, foe, mate? Food, water, danger? Or is it just something mundane? If it’s insignificant—just a log, a piece of lint, the trees rustling in the wind—you feel nothing toward it and most likely will ignore it. But if it’s important, you instantly feel something. If it is an intense feeling, the signals from the amygdala also cascade into your hypothalamus (see Figure Int.3), which not only orchestrates the release of hormones but also activates the autonomic nervous system to prepare you to take appropriate action, whether it’s feeding, fighting, fleeing, or wooing. (Medical students use the mnemonic of the “four Fs” to remember these.) These autonomic responses include all the physiological signs of strong emotion such as increased heart rate, rapid shallow breathing, and sweating. The human amygdala is also connected with the frontal lobes, which add subtle flavors to this “four F” cocktail of primal emotions, so that you have not just anger, lust, and fear, but also arrogance, pride, caution, admiration, magnanimity, and the like.