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

First, you can record from single nerve cells in a monkey’s MT areas. The cells signal the direction of moving objects but don’t seem that interested in color or shape. Second, you can use microelectrodes to stimulate tiny clusters of cells in a monkey’s MT area. This causes the cells to fire, and the monkey starts hallucinating motion when the current is applied. We know this because the monkey starts moving his eyes around tracking imaginary moving objects in its visual field. Third, in human volunteers, you can watch MT activity with functional brain imaging such as fMRI (functional MRI). In fMRI, magnetic fields in the brain produced by changes in blood flow are measured while the subject is doing or looking at something. In this case, the MT areas lights up while you are looking at moving objects, but not when you are shown static pictures, colors, or printed words. And fourth, you can use a device called a transcranial magnetic stimulator to briefly stun the neurons of volunteers’ MT areas—in effect creating a temporary brain lesion. Lo and behold, the subjects become briefly motion blind like Ingrid while the rest of their visual abilities remain, to all appearances, intact. All this might seem like overkill to prove the single point that MT is the motion area of the brain, but in science it never hurts to have converging lines of evidence that prove the same thing.

Likewise, there is an area called V4 in the temporal lobe that appears to be specialized for processing color. When this area is damaged on both sides of the brain, the entire world becomes drained of color and looks like a black-and-white motion picture. But the patient’s other visual functions seem to remain perfectly intact: She can still perceive motion, recognize faces, read, and so on. And just as with the MT areas, you can get converging lines of evidence through single-neuron studies, functional imaging, and direct electrical stimulation to show that V4 is the brain’s “color center.”

Unfortunately, unlike MT and V4, most of the rest of the thirty or so visual areas of the primate brain do not reveal their functions so cleanly when they are lesioned, imaged, or zapped. This may be because they are not as narrowly specialized, or their functions are more easily compensated for by other regions (like water flowing around an obstacle), or perhaps our definition of what constitutes a single function is murky (“ill posed,” as computer scientists say). But in any case, beneath all the bewildering anatomical complexity there is a simple organizational pattern that is very helpful in the study of vision. This pattern is a division of the flow of visual information along (semi)separate, parallel pathways (Figure 2.10).

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.