In addition to the extreme agility of our fingers, the human thumb developed a unique saddle joint allowing it to oppose the forefinger. This feature, which enables the so-called precision grip, may seem trivial, but it is useful for picking small fruits, nuts, and insects. It also turns out to be quite useful for threading needles, hafting hand axes, counting, or conveying Buddha’s peace gesture. The requirement for fine independent finger movements, opposable thumbs, and exquisitely precise eye-hand coordination—the evolution of which was set in motion early in the primate line—may have been the final source of selection pressure that led us to develop our plethora of sophisticated visual and visuomotor areas in the brain. Without all these areas, it is arguable whether you could blow a kiss, write, count, throw a dart, smoke a joint, or—if you are a monarch—wield a scepter.
This link between action and perception has become especially clear in the last decade with the discovery of a new class of neurons in the frontal lobes called canonical neurons. These neurons are similar in some respects to the mirror neurons I introduced in the last chapter. Like mirror neurons, each canonical neuron fires during the performance of a specific action such as reaching for a vertical twig or an apple. But the same neuron will also fire at the mere sight of a twig or an apple. In other words, it is as though the abstract property of graspability were being encoded as an intrinsic aspect of the object’s visual shape. The distinction between perception and action exists in our ordinary language, but it is one that the brain evidently doesn’t always respect.
While the line between visual perception and prehensile action became increasingly blurred in primate evolution, so too did the line between visual perception and visual imagination in human evolution. A monkey, a dolphin, or a dog probably enjoys some rudimentary form of visual imagery, but only humans can create symbolic visual tokens and juggle them around in the mind’s eye to try out novel juxtapositions. An ape can probably conjure up a mental picture of a banana or the alpha male of his troop, but only a human can mentally juggle visual symbols to create novel combinations, such as babies sprouting wings (angels) or beings that are half-horse, half-human (centaurs). Such imagery and “off-line” symbol juggling may, in turn, be a requirement for another unique human trait, language, which we take up in Chapter 6.
IN 1988 A sixty-year-old man was taken to the emergency room of a hospital in Middlesex, England. John had been a fighter pilot World War II. Until that fateful day, when he suddenly developed severe abdominal pain and vomiting, he had been in perfect health. The house officer, Dr. David McFee, elicited a history of the illness. The pain had begun near the navel and then migrated to the lower right side of his abdomen. This sounded to Dr. McFee like a textbook case of appendicitis: an inflammation of a tiny vestigial appendage protruding from the colon on the right side of the body. In the fetus the appendix first starts growing directly under the navel, but as the intestines lengthen and become convoluted the appendix gets pushed into the lower right quadrant of the abdomen. But the brain remembers its original location, so that is where it experiences the initial pain—under the belly button. Soon the inflammation spreads to the abdominal wall overlying it. That’s when the pain migrates to the right.
Next Dr. McFee elicited a classic sign called rebound tenderness. With three fingers he very slowly compressed the lower right abdominal wall and noted that this caused no pain. But when he suddenly withdrew his hand to release the pressure, there was a short delay followed by sudden pain. This delay results from the inertial lag of the inflamed appendix as it rebounds to hit the abdominal wall.
Finally, Dr. McFee applied pressure in John’s lower left quadrant, causing him to feel a sharp twinge of pain in the lower right, the true location of the appendix. The pain is caused by the pressure displacing the gas from the left to the right side of the colon, which causes the appendix to inflate slightly. This tell-tale sign, together with John’s high fever and vomiting, clinched the diagnosis. Dr. McFee scheduled the appendectomy right away: The swollen, inflamed appendix could rupture anytime and spill its contents into the abdominal cavity, producing life-threatening peritonitis. The surgery went smoothly, and John was moved to the recovery room to rest and recuperate.
Alas, John’s real troubles had only just begun.2 What should have been a routine recovery became a waking nightmare when a small clot from a vein in his leg was released into his blood and clogged up one of his cerebral arteries, causing a stroke. The first sign of this was when his wife walked into the room. Imagine John’s astonishment—and hers—when he could no longer recognize her face. The only way he knew who he was talking to was because he could still recognize her voice. Nor could he recognize anyone else’s face—not even his own face in a mirror.
“I know it’s me,” he said. “It winks when I wink and it moves when I do. It’s obviously a mirror. But it doesn’t look like me.”
John emphasized repeatedly that there was nothing wrong with his eyesight.
“My vision is fine, Doctor. Things are out of focus in my mind, not in my eye.”
Even more remarkably, he couldn’t recognize familiar objects.
When shown a carrot, he said, “It’s a long thing with a tuft at the end—a paint brush?”
He was using fragments of the object to intellectually deduce what it was instead of recognizing it instantly as a whole like most of us do. When shown a picture of a goat, he described it as “an animal of some kind. Maybe a dog.” Often John could perceive the generic class the object belonged to—he could tells animals from plants, for example—but could not say what specific exemplar of that class it was. These symptoms were not caused by any limitation of intellect or verbal sophistication. Here is John’s description of a carrot, which I’m sure you will agree is much more detailed than what most of us could produce:
A carrot is a root vegetable cultivated and eaten as human consumption worldwide. Grown from seed as an annual crop, the carrot produces long thin leaves growing from a root head. This is deep growing and large in comparison with the leaf growth, sometimes gaining a length of twelve inches under a leaf top of similar height when grown in good soil. Carrots may be eaten raw or cooked and can be harvested during any size or state of growth. The general shape of a carrot is an elongated cone, and its color ranges between red and yellow.
John could no longer identify objects, but he could still deal with them in terms of their spatial extent, their dimensions, and their movement. He was able to walk around the hospital without bumping into obstacles. He could even drive short distances with some help—a truly amazing feat, given all the traffic he had to negotiate. He could locate and gauge the approximate speed of a moving vehicle, although he couldn’t tell if it was a Jaguar, a Volvo, or even a truck. These distinctions prove to be irrelevant to actually driving.