The Pleasure Instinct: Why We Crave Adventure, Chocolate, Pheromones, and Music (16 page)

—Paul Klee

—Marcel Proust

 

 

 

Martin watched the first flurries of snow begin to fall and finally relaxed as he switched off the outside light and urged his body upstairs to bed. The workday had been a frenzy of meetings and deadlines blurred into a whirl that began when he entered the freeway on-ramp in the morning and subsided only after a glass of wine before dinner. His drive home had been the pièce de résistance of the day. Another car seemed to emerge from thin air as he was changing lanes, causing him to veer suddenly. The remainder of his commute was accompanied by white knuckles and a sickening cold sweat—the holidays always seemed to bring out the amateurs.

No matter how tense his day, Martin never seemed to have any difficulty getting to sleep once his head hit the pillow. His last thoughts on this cold December evening were of his beautiful wife and his four-year-old son’s warm, crooked smile—a sight that always overwhelmed him.

Well before daybreak Martin was roused from sleep for his customary middle-of-the-night trip to the bathroom and immediately felt the gripping headache. Stumbling into the bathroom, he noticed a strange numbness in his left leg and the left side of his face. In the dim glow of the bathroom night-light, he saw that something was also wrong with his face. It was hard to say exactly what was different at first, but slowly he realized that his mouth seemed at odds with the rest of his face—the left side kind of drooping placidly. Worse yet, when he moved his right arm toward the drug cabinet, his hand seemed to vanish from sight, only to reappear once it came in contact with the mirror. Later that morning in the hospital, his doctors had a hard time convincing him that although just forty-one years old, he had experienced a stroke that could have ended his life.

After six months of physical therapy, Martin began to feel like himself again.The stroke had been a warning, but he’d survived and come through fairly unscathed save for occasional slurred speech. The new Martin had been molded into a different person altogether—finding more time for relaxing with his family, and even for a daily jog around his neighborhood each morning before work. All of his attending doctors pronounced him fit—and lucky—yet he often felt odd sensations during the day. More and more frequently during his morning jogs, Martin seemed to have brief moments of intense fear and panic. These fleeting episodes occurred especially when he ran through the park at the end of the street and were often triggered by the sudden appearance of a dog—any dog, no matter how small—or even another person walking toward him.

Although by all standard tests Martin had excellent vision, he mentioned during a visit with his doctor that he occasionally had trouble estimating the distance of approaching cars while crossing a street—a problem that had led to several close calls in the past few months. When asked about any other difficulties, he reluctantly confessed that sometimes when talking with a coworker he had difficulty understanding what she was saying, because her mouth seemed to “fade in and out” of sight when she spoke. This must have been a surprise to the attending neurologist, but it was enough of a clue to raise the possibility that his patient was suffering from
akinetopsia
, or visual motion blindness.

Martin was referred to a specialist that week for a battery of visual acuity, motion, and perimetry tests. His static visual acuity was completely normal. In tests using moving stimuli, however, Martin seemed to have severe deficits in specific portions of his visual field. Objects at rest that began moving proved to be particularly troublesome.

A common test for akinetopsia involves what psychologists and other vision scientists refer to as object tracking.The patient is seated directly in front of a computer display and asked to focus his or her gaze and follow the movement of a colored circle.The circle begins in the middle of the screen, and the tester can manipulate the object’s gradual movement to different corners of the screen. People with normal vision see the circle move smoothly from the center to the corner locations, and grow bored quickly. Patients with akinetopsia have an entirely different experience. They report seeing the still object disappear from the center and reappear in one of the corners. Once the circle moves, it is as good as gone until it comes to rest. Martin had a similar experience in his object tracking test, so his neurologist immediately ordered a second MRI scan of his injured brain. The damage was concentrated on one side of Martin’s posterior parietal lobe, an area toward the back top of the brain that is deeply involved in—not surprisingly—motion perception.

The fact that Martin had stroke damage to his posterior parietal lobe provides an explanation for his akinetopsia, yet the particular way this disorder impacted his life—creating a sudden, blinding fear of dogs and people coming toward him—can only be understood if we consider how the pleasure instinct and experience guide the development of the visual brain.

The emergence of the earliest primates from the mammalian branch some sixty million years ago came with dramatic changes in the sensory systems of this lineage. Based on the abundance of fossils these animals left behind, we know that they were rather small—probably weighing only a few ounces—and closely resembled modern-day prosimians such as galagos, tarsiers, and lemurs. They were adapted to a much warmer climate than we have today, with tropical rain forests covering a significant portion of the planet. Their small size and prehensile hands and feet allowed them to grab onto and forage among the fine terminal branches that make up the rain forest canopy. This unique niche came with its own challenges in terms of identifying potential foods, usually fruit, seeds, and insects camouflaged against a background of green leaves and thickets, and for recognizing potential predators. These two key selection factors—needing to locate hidden fruit and predators—promoted the gradual shift in a sensory system dominated by smell in most mammals to a new model where vision reigned supreme in the emerging early primates. These new creatures had large, forward-facing eyes with a high density of photoreceptors in the center of their retina, an area called the fovea. In early primates this high concentration of photoreceptors came with new brain-stem circuitry that evolved to focus their visual gaze frontally toward anything that moved, and a dramatic increase in the size of the brain areas devoted to vision relative to those devoted to olfaction.

With the shift toward frontal vision, early primates sacrificed some ability to detect the presence of food or predators using smell, yet these anatomical changes gave them distinct advantages over other groups of mammals. In particular, the shift from side-oriented eyes to a frontal position permitted the evolution of binocularity and stereoscopic vision, two functions critically important for fine visual acuity and determining the size and distance of an object.

The favoring of vision over smell in early primates was not just a matter of the visual cortex getting bigger—entirely new brain areas devoted to specialized visual functions evolved in these animals that never existed in other mammals. One important innovation was the evolution of brain areas in the posterior parietal lobe and medial temporal regions that were devoted to the visual guidance of muscle movement. Evolutionary biologists have argued that the presence of improved frontal visual acuity and a propensity to live among fine tree branches necessitated the development of neural systems designed to improve eye-hand coordination. The emergence of posterior parietal areas for perceiving visual motion is the result of this evolutionary ratchet effect (chapter 2), where one adapted function served as a selection factor for yet another adaptation. In this case, the development of increased frontal visual acuity in combination with prehensile hands and feet made the rain-forest canopy a viable niche for early primates. This shift from ground-level to tree-branch foraging incurred a high survival cost on primates with poor eye-hand coordination. Simply moving from one swaying branch to another might prove fatal if the distance to the next limb or its degree of movement was miscalculated. Such conditions served as strong selection factors in driving the evolution of specialized brain regions devoted to perceiving object motion.

The evolution of brain regions in the posterior parietal lobe and medial temporal areas that facilitate object tracking and eye-hand coordination, in turn, created the conditions in which selection factors arose for the development of color vision. It is thought that until about forty million years ago early primates had only one primary photoreceptor type tuned to a single distribution of light wavelengths. In terms of function, this mechanism allowed a primate to see the world in basic shades of gray. Most modern-day prosimians have two distinct types of photoreceptor (dichromatic) that are maximally sensitive to different light wavelengths.These animals have a rudimentary capacity for color vision. During the early period in the evolutionary history of primates, a mutation caused the genes that normally control photoreceptor development to duplicate.This process resulted in a higher density of photoreceptors, which eventually diverged into two and then three distinct classes, each sensitive to a different wavelength of visible light. Recent work has shown that the advance from dichromacy to trichromatic color vision specifically enhanced the ability of primates to distinguish nutrient-rich fruits from the background coloration of leaves.Thus the evolution of improved eye-hand coordination and frontal vision that gave early primates a novel niche created the conditions where identifying colored fruits against a uniformly green background of leaves served as a selection pressure for color vision.

The evolutionary history of primate vision is a story riddled with ratchet effects, and so too is the ontogenetic development of vision in humans. By the fifth week of gestation, human embryos show the first signs of an early eyecup that begins to differentiate into a lens and a retina. At this point in development, the eyes face laterally to each side, much like an early mammal. The retina itself is derived from the same neural ectoderm that comprises the central nervous system and is therefore considered part of the brain. Each retina is attached to the brain-stem by a broad group of fibers called the optic nerve. By the fourteenth week of gestation, the eyes begin to face forward in the familiar primate form, and photoreceptor cells start to form in the center of the retina and gradually fill in from the fovea outward. Development continues from the retina inward to brain areas, advancing along the same path as normal sensory input. Retinal development is followed by the emergence of several brainstem sites such as the superior colliculus (involved in controlling eye movements), then the visual cell groups in the thalamus (e.g., lateral geniculate nucleus), and finally neocortical areas such as the primary visual cortex (also known as V1).

Primates have tremendous developmental investment in vision. There are more than forty known cortical areas devoted specifically to visual information processing. By the end of the second trimester, a human fetus has a fairly mature primary visual cortex, and most secondary and tertiary visual centers have undergone rapid growth both in terms of cell numbers and synaptic connections between the areas. A human’s visual system, however, does most of its development after birth and is even more dependent on experience for normal maturity than the other senses. While the other sensory systems can be stimulated fairly early in the womb and thus undergo considerable experience-expectant growth before birth, vision is the exception to the rule. A scarcity of light makes its way to the fetus, and consequently most of the visual system’s fine-tuning must be guided by the pleasure instinct after birth.

Any parent will tell you that infants are hungry for visual experiences, yet they are rather particular in their choices. Much like the other sensory systems, there seems to be a characteristic sequence of preferred stimulation types that is attractive for infants. Little Kai, who is now crawling, has followed a pattern of visual development that is an echo of primate phylogeny. At birth his visual system was relatively immature when compared to the sensory systems responsible for touch, taste, smell, and hearing. For the first few weeks, Kai could only lock onto faces and high-contrast objects that were within about five to ten inches of his face. Infants this age have great difficulty focusing on objects outside this range, since the muscles that control lens shape (which affects light refraction) are so immature. By four months, however, Kai could focus his vision across a much larger range of distances and was fascinated by anything that moved. He was particularly fond of ceiling fans, but the love affair always ended once it was turned off, suggesting that it was the motion that piqued his interest. Toy manufacturers are, of course, sensitive to these developmental milestones. One might argue tongue in cheek that the preference of newborns for things that move was an important (albeit artificial) selection factor in the evolution of the mobile (insofar as parents tend to select toys that are attractive to newborns).

 

 

Similar to what we have seen with the other sensory systems, the neocortical areas devoted to vision tend to specialize. Once information enters the first neocortical area dedicated to vision (primary visual cortex or V1), it diverges to a number of additional cortical regions, each with a different specialty. Some areas are responsible for processing object motion and location. Others process object form, color, texture, shading, shape, and so forth. Parallel processing is a general operating principle for all sensory systems in the brain.The strength of parallel processing lies in the fact that it facilitates speed of information transfer through the brain and adds to the robustness of the information through built-in redundancy (that is, multiple channels are used).