What criteria would you use to determine whether synesthesia is a disorder or a variation of normal sensation and perception?

n a 3-page Microsoft Word document, create a work sheet by answering the Questions for Research and Discussion provided for each case study.

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CHAPTER 1 Overview of Genetics Senses Working Overtime

Eighteen-year-old Sean Maxwell has always perceived the world in an unusual way. To most people, color is a characteristic of an object—a cherry is red; a hippo, gray. To Sean, colors are much more. When he plays a note on his guitar, or hears it from another instrument, a distinctively colored shape pops into his mind. His brain, while perceiving the note as an E flat or a C sharp, creates an overwhelming feeling of iridescent orange-yellow diamonds, or a single, shimmering sky blue crescent. Soaring crescendos of sound become detailed landscapes, peppered with alternating black and white imagery that parallels the staccato notes. These images flash by his consciousness in such rapid succession that he is barely aware of them, yet they seem to burst through his fingers in the patterns of notes that he plays. Sean has experienced these peculiar specific sound-color-shape associations for as long as he can remember, but never thought much about it. Didn’t everyone link music to imagery? Then he reads a science blog about a condition called synesthesia that mixes up the senses. Synesthesia was once thought to be extremely rare, affecting only about 1 in 2000 people. But as more and more “synesthetes” are finding one another through shared strange sensory stories on the Internet, it is becoming clear that possibly as many as 1 in 23 people has some form of the condition. Rather than experiencing it as a disability, synesthetes report that they can actually harness their sensory associations to enhance learning. It isn’t surprising that the condition is about eight times more common among artists and novelists than among people in other fields. Sean is so excited by what he’s read about synesthesia that he decides to talk about it at dinner when he’s home from college one weekend. It’s easy to slip it into a discussion, for the Maxwells are a very musical family. Sean’s dad, Peter, sings in various cover bands, and Sean is in a band too, playing lead guitar. “For me, notes have colors. But for most synesthetes, letters or numbers have colors. Or time is colored, maybe days of the week, or months. It gets even stranger. Some people taste triangles or smell colors,” he says between bites. Sean looks around at his oddly quiet family, who usually interrupt one another constantly. His mom, Ellie, is focusing on her salad, while his 16-year-old sister Keri twirls her finger against her head, as if Sean’s lost his mind. But his dad and 12-year-old sister Anna are each holding their forks still and are simply staring at him, their mouths agape, eyes wide. “What?” says Sean. “Do you think I’m weird? What is it?” Peter and Anna are silent a bit longer, as if deciding what to say. Then Peter pushes his long red hair back and says, “Not exactly. I understand.” “You do?” says Sean, astonished. “Yes,” Peter says, looking embarrassed. “Notes have always been colored for me. I see the colors vividly when I play. The notes have textures, too. Some notes are shiny, while others seem to have a matte finish. But I never told anyone. Actually, I never even thought about it until now, and I never heard of syn whatever you called it. Sounds like an acid trip.” “Synesthesia. And you’re right, LSD does cause it, temporarily. But an acid trip will give you different colors for the same notes at different times. Synesthesia doesn’t work that way. It’s consistent. Also, most people who have synesthesia remember it from early childhood. Speaking of which, are you sure you didn’t mention it to me, like when I was six and you taught me to play guitar? Maybe I just subconsciously copy you.” “No, I’m quite sure I never said anything. I just thought it was some quirk, maybe even normal. A B flat minor chord is shiny green, and G major seventh speckled indigo. Notes have shapes, too.” Peter looks sheepish. “Yes! Shapes! But you’ve got the colors all wrong,” exclaims Sean, jumping up. “B flat minor is pink, and G major seventh lavender, sometimes with stripes.” Father and son continue to compare their synesthetic perceptions, growing more and more excited, until Anna speaks up. “I’ve got it too.” “What?” ask Sean and Peter. “But not like you two,” Anna continues. “Maybe that’s because I’m not musical, like you are. Instead, I see letters and words as colors. I thought it was just a little trick I use to study—it’s easier to memorize colored words.” Now father and son gaze at Anna in amazement, as Ellie and Keri, the only ones still eating, look puzzled. “Well, I’m afraid my sensory life is boring—everything’s what it’s supposed to be to me! No colors to sounds, or anything like that. Must’ve come from your side, Peter,” concludes Ellie. “But maybe I’m missing out,” she adds. “You are. Dad and I aren’t the only musical synesthetes,” says Sean. “Tori Amos says that although specific groups of chords have colors, no two songs are alike. John Mayer has synesthesia too, and so did Franz Liszt, Duke Ellington, and Leonard Bernstein.” Peter’s mind is racing. He can’t wait to tell his bandmates. Synesthesia comes from the Greek: syn for “together” and aesthesis for “to perceive.” The sensual associations of synesthesia are involuntary and highly specific, and they persist over a lifetime. The condition has been recognized since at least 1883, when Sir Francis Galton, a cousin of Charles Darwin who was an early supporter of eugenics, described it in an article in Nature magazine as a “mingling of the senses” that “runs in families.” Studies since then have shown that a blood relative of a person with synesthesia has a 4 in 10 chance of also having the condition. How does synesthesia arise? Are mixed up senses merely a matter of taking a metaphor too far, such as a sharp cheese or bittersweet symphony? Or do persistent mixed senses reflect childhood associations, formed at a critical period in brain development, such as remembering the colors of letters in a book from which a child learned to read, or recalling colored refrigerator magnets in the shapes of letters. Brain imaging studies and genetics have shed light on the biological basis of synesthesia, but it is still not well understood. For example, functional magnetic resonance imaging of the brain focuses on neighboring parts of the cerebral cortex that process numbers and color. When a nonsynesthete looks at a string of numbers or letters, only one brain center lights up; when a synesthete who associates numbers or letters to colors watches, both brain parts light up. Perhaps synesthesia arises in the fetus, when extra connections (synapses) between brain neurons form and would normally be trimmed back. In synesthetes these extra neural links may remain, similar to a bush that hasn’t been sufficiently pruned. The discovery of colorblind synesthetes localizes the phenomenon clearly to the brain. The eyes of these men lack the receptors for color vision, but their brains fill in colors for visual images. The different manifestations of synesthesia may reflect the fact that different neurons are pruned in different individuals. The degree to which genetics causes synesthesia isn’t known. One study found that four specific parts of the human genome vary in a certain way among synesthetes much more frequently than among nonsynesthetes. The results of this genome-wide association study led researchers to several genes already known to be associated with autism, seizures, dyslexia, and long-term memory and learning. In fact, many people with autism who are “savants,” possessing incredible talents, are synesthetes too. Researchers now think that inheriting combinations of variants in several genes, plus environmental influences, causes synesthesia. For example, if Peter and Sean weren’t musically gifted, they might not have noticed their synesthesia. Because synesthesia can differ within a family, such as Anna’s coloring of language instead of music, the genetic cause is likely a fundamental brain change that is expressed differently depending upon other gene variants and experiences.

Answer questions below


7. What criteria would you use to determine whether synesthesia is a disorder or a variation of normal sensation and perception?

8. Why do you think that synesthesia is more common today than it was 20 years ago?

9. Why might it be possible for infants to have synesthesia, but the ability is gradually lost?

10. Would you want to take a genetic test for synesthesia? Cite a reason for your answer.

11. Do you think that synesthesia should be regarded as a learning disability, an advantage, or neither?


First cousins Sheila and Anika look so much alike, with their curly blond hair and startlingly blue eyes, that people often mistake them for twins. Now, at age 24, they are becoming mothers at the same time. Sheila has just given birth to Mallory, while Anika is in her first trimester of pregnancy. Both young women were biology majors, and so they are intrigued with nursing their babies, perhaps more so than most new mothers. Anika watches as Sheila responds to her baby’s fussing. As soon as Mallory cries in hunger, her mother’s brain sends hormonal signals into her bloodstream that trigger production, secretion and ejection of milk, a process called lactation. Hormones had begun remodeling Sheila’s breasts months earlier, replacing fat with glandular tissue. The system of milk ducts in the breasts, thin branches when Sheila was a child, grew into a lush network of ducts with grapelike tips called alveoli. A day after Mallory’s birth the alveoli swelled, filling Sheila’s breasts. The cells that line the alveoli which now make up most of Sheila’s breasts are specialized forms of epithelium called lactocytes. They secrete milk into the ducts that deliver it to the areola, the pigmented area that supports the nipple. Milk squirts forcefully from 15 to 25 holes when the baby feeds as other specialized cells, called myoepithelial cells, contract. Rare stem cells within the ducts divide, helping to reconfigure the fatty gland into a milk production facility. Milk is a highly complex and variable mixture tailored to anatomy, physiology, and lifestyle. The milks of all mammals consist of the major nutrients—proteins, fats, carbohydrates, vitamins, and minerals—suspended in water. Genes provide the specific recipe. Human milk has a higher proportion of fats compared to other species, which insulates cells of the developing brain, enabling them to communicate. In contrast, cow’s milk has much more protein than human milk, which a calf uses to rapidly build muscle. Yet marine mammals have even more fat than human milk, which they need to stay warm in frigid waters. Making milk takes a lot of energy. Following the secretion of milk illustrates the functions of organelles (Figure 1). The process begins in the nucleus, where genes encoding various proteins are transcribed into mRNA. Casein proteins are abundant—they provide a variety of amino acids and are therefore highly nutritious. They are also easy to digest. The genetic instructions also specify which antibody proteins line Mallory’s digestive and urinary tracts, protecting her against certain bacterial infections. Other mRNAs made in a lactocyte represent enzymes required to produce the nonprotein parts of milk, such as the sugar lactose and fats.

The mRNAs exit the nucleus and travel to the rough endoplasmic reticulum (ER), where they bind ribosomes. Transfer RNAs arrive and protein synthesis ensues—both to make milk components and to carry on the “housekeeping” functions necessary to keep any cell alive. The milk proteins move within the tubules of the ER out toward the plasma membrane, picking up lipids at the smooth ER and sugars at the Golgi apparatus. The tubules of this secretory network narrow and end at the plasma membrane. Here, the proteins and sugars exit the cell in membrane-bounded, saclike vesicles, like a fleet of bubbles. Lipids pass directly through the lipid-rich plasma membrane, taking bits of the outer layer with them. The milk accumulates outside the cells, until Mallory’s cries stimulate the myoepithelial cells to contract and eject the milk. Mitosis and apoptosis oversee the changes in Sheila’s breasts. Rapid division of lactocytes began just before and during puberty, resumed early in pregnancy, and increased again just after Mallory’s birth. Once mother and daughter establish a regular feeding schedule, however, most of the lactocytes stop dividing, but they still use cellular energy to maintain the milk supply. When Sheila cuts the number of daily feedings to wean Mallory, many of the lactocytes will die by apoptosis, triggered by the increasingly longer times between hormonal signals. At the same time, the number of lysosomes increases. They degrade the glandular tissue as it is no longer needed. Sheila’s breasts shrink, but stem cells will enable them to produce milk for future children.

Answer questions below


10. Historical references as well as current anecdotal reports suggest that under very unusual circumstances, males can breastfeed. The Talmud, a book of Jewish law, discusses a man whose wife died and who had no money to pay a wet nurse (a woman who breastfeeds another woman’s child). He was able to nourish the child with his own body. The writings of other religions report similar tales. In agriculture, male goats can receive hormonal treatments and make milk. Do you think that it is possible for a human male to breastfeed, and if so, what conditions must be provided to coax his body to produce and secrete milk?

11. Select one of the mammals whose milk composition is listed in Table 1, read about the animal’s characteristics and activities on the Internet, and hypothesize why the proportion of the milk components is consistent with the animal’s behavior.

Table 1 Milk Composition in Different Mammals Species % Fat % Protein % Lactose % Total Solids

Human 4.5 1.1 6.8 12.6

Cow 3.5 3.1 4.9 15.0

Cat 10.9 5.9 4.3 21.5

Deer 19.7 10.4 2.6 34.1

Polar bear 31.0 10.2 0.5 42.9

Kangaroo 2.1 6.2 Trace 9.5

Seal 53.2 11.2 2.6 67.7

12. Compare the roles of mitosis and apoptosis in remodeling Sheila’s breast from a fatty sac to an active milk gland.

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