The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

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.)