How does this explain Mikhey’s laughter? I didn’t know this at that time, but many years later I saw another patient named Dorothy with a similar “laughter from pain” syndrome. A CT (computed tomography) scan revealed that one of the pain pathways in her brain was damaged. Even though we think of pain as a single sensation, there are in fact several layers to it. The sensation of pain is initially processed in a small structure called the insula (“island”), which is folded deep beneath the temporal lobe on each side of the brain (see Figure Int.2, in the Introduction). From the insula the pain information is then relayed to the anterior cingulate in the frontal lobes. It is
And the same holds for tickling. The huge adult approaches the child menacingly. She is clearly outmatched, prey, completely at the mercy of a hulking Grendel. Some instinctive part of her—her inner primate, primed to flee from the terrors of eagles and jaguars and pythons (oh my!)—cannot help but interpret the situation this way. But then the monster turns out be gentle. It deflates her expectation of danger. What might have been fangs and claws digging fatally into her ribs turn out to be nothing but firmly undulating fingers. And the child laughs. It may well be that tickling evolved as a early playful rehearsal for adult humor.
The false-alarm theory explains slapstick, and it is easy to see how it might have been evolutionarily coopted (exapted, to use the technical term) for
Lastly, consider that universal greeting gesture in humans: the smile. When an ape is approached by another ape, the default assumption is that it is being approached by a potentially dangerous stranger, so it signals its readiness to fight by protruding its canines in a grimace. This evolved further and became ritualized into a mock threat expression, an aggressive gesture warning the intruder of potential retaliation. But if the approaching ape is recognized as a friend, the threat expression (baring canines) is aborted halfway, and this halfway grimace (partly hiding the canines) becomes an expression of appeasement and friendliness. Once again a potential threat (attack) is abruptly aborted—the key ingredients for laughter. No wonder a smile has the same subjective feeling as laughter. It incorporates the same logic and may piggyback on the same neural circuits. How very odd that when your lover smiles at you, she is in fact half-baring her canines, reminding you of her bestial origins.
And so it is that we can begin with a bizarre mystery that could have come straight from Edgar Allan Poe, apply Sherlock Holmes’s methods, diagnose and explain Mikhey’s symptoms, and, as a bonus, illuminate the possible evolution and biological function of a much treasured but deeply enigmatic aspect of the human mind.
CHAPTER 2
Seeing and Knowing
—SHERLOCK HOLMES
THIS CHAPTER IS ABOUT VISION. OF COURSE, EYES AND VISION ARE not unique to humans—not by a long shot. In fact, the ability to see is so useful that eyes have evolved many separate times in the history of life. The eyes of the octopus are eerily similar to our own, despite the fact that our last common ancestor was a blind aquatic slug-or snail-like creature that lived well over half a billion years ago.1 Eyes are not unique to us, but vision does not occur in the eye. It occurs in the brain. And there is no other creature on earth that sees objects quite the way we do. Some animals have much higher visual acuity than we do. You sometimes hear factoids like the fact that an eagle could read tiny newsprint from fifty feet away. But of course, eagles can’t read.
This book is about what makes humans special, and a recurring theme is that our unique mental traits must have evolved from preexisting brain structures. We begin our journey with visual perception, partly because more is known about its intricacies than about any other brain function and partly because the development of visual areas accelerated greatly in primate evolution, culminating in humans. Carnivores and herbivores probably have fewer than a dozen visual areas and no color vision. The same holds for our own ancestors, tiny nocturnal insectivores scurrying up tree branches, little realizing that their descendents would one day inherit—and possibly annihilate!— the earth. But humans have as many as thirty visual areas instead of a mere dozen. What are they doing, given that a sheep can get away with far fewer?
When our shrewlike ancestors became diurnal, evolving into prosimians and monkeys, they began to develop extrasophisticated visuomotor capacities for precisely grasping and manipulating branches, twigs, and leaves. Furthermore, the shift in diet from tiny nocturnal insects to red, yellow, and blue fruits, as well as to leaves whose nutritional value was color coded in various shades of green, brown, and yellow, propelled the emergence of a sophisticated system for color vision. This rewarding aspect of color perception may have subsequently been exploited by female primates to advertise their monthly sexual receptivity and ovulation with estrus—a conspicuous colorful swelling of the rumps to resemble ripe fruits. (This feature has been lost in human females, who have evolved to be continuously receptive sexually throughout the month—something I have yet to observe personally.) In a further twist, as our ape ancestors evolved toward adopting a full-time upright bipedal posture, the allure of swollen pink rumps may have been transferred to plump lips. One is tempted to suggest—tongue in cheek—that our predilection for oral sex may also be an evolutionary throwback to our ancestors’ days as frugivores (fruit eaters). It is an ironic thought that our enjoyment of a Monet or a Van Gogh or of Romeo’s savoring Juliet’s kiss may ultimately trace back to an ancient attraction to ripe fruits and rumps. (This is what makes evolutionary psychology so much fun: You can come up with an outlandishly satirical theory and get away with it.)
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
