My enduring thanks also go to my merry band of neuroscience collaborators not already mentioned, past and present, from whom I have learned so much. They include (in alphabetical order) Joe Andreano, Shir Atzil, Moshe Bar, Larry Barsalou, Marta Bianciardi, Kevin Bickart, Eliza Bliss-Moreau, Emery Brown, Jamie Bunce, Ciprian Catana, Lorena Chanes, Maximilien Chaumon, Sarah Dubrow, Wim van Duffel, Wei Gao, Talma Hendler, Martijn van den Heuvel, Jacob Hooker, Ben Hutchinson, Yuta Katsumi, Ian Kleckner, Phil Kragel, Aaron Kucyi, Kestas Kveraga, Kristen Lindquist, Dante Mantini, Helen Mayberg, Yoshiya Moriguchi, Suzanne Oosterwijk, Gal Raz, Carl Saab, Ajay Satpute, Lianne Scholtens, Kyle Simmons, Jordan Theriault, Alexandra Touroutoglou, Tor Wager, Larry Wald, Mariann Weierich, Christi Westlin, Susan Whitfield-Gabrieli, Christy Wilson-Mendenhall, and Jiahe Zhang. And I remain deeply grateful to my intrepid engineering and computer scientist collaborators, who continue to teach me about dynamical systems, complexity, and other topics in computation that make me a better neuroscientist, including Dana Brooks, Sarah Brown, Jaume Coll-Font, Jennifer Dy, Deniz Erdogmus, Zulqarnain Khan, Madhur Mangalam, Jan-Willem van de Meent, Sarah Ostadabbas, Misha Pavel, Sumientra Rampersad, Sebastian Ruf, Gene Tunik, Mathew Yarossi, and the rest of the PEN group at Northeastern University. Thanks also to statisticians Tim Johnson and Tom Nichols.
This book also wouldn’t exist were it not for the boundless enthusiasm and expert guidance from my editor at Houghton Mifflin Harcourt, Alex Littlefield. I’m particularly grateful for his careful reading and his encouragement to combine complicated observations about the brain with big ideas of what it means to be a human being. In this regard, I’m also indebted to James Ryerson at the New York Times for his guidance as I developed my voice while navigating choppy waters between neuroscience, psychology, and philosophy.
The book also greatly benefited from the artistic skills and inquisitive nature of Van Yang, whose team’s ingenious illustrations bring the science to life; I especially appreciate his deep desire to communicate science to a wide audience. Thanks also to Aaron Scott for his design consultations; his expertise, careful eye, and creativity have helped me translate complex scientific ideas into understandable images for over a decade.
Thank you to the production and marketing teams at HMH, including Olivia Bartz, Chloe Foster, Tracy Roe, Chris Granniss, Emily Snyder, Heather Tamarkin, and especially Michelle Triant, PR maven extraordinaire. Thanks also to my agent, Max Brockman, for his continued enthusiasm and support, and to his crew at Brockman Inc., Thomas Delaney, Evelyn Chavez, Breana Swinehart, and Russell Weinberger.
This book was notably improved by valuable comments, criticisms, and ideas offered by early readers, many of whom are dear friends and extraordinary scientists in their own right. They are (in alphabetical order) Kevin Allison, Vanessa Kane Alves, Eliza Bliss-Moreau, Dana Brooks, Lindsey Drayton, Sarah Dubrow, Peter Farrar, Barb Finlay, Ludger Hartley, Katie Hoemann, Ben Hutchinson, Peggy Kalb, Tsiona Lida, Micah Kessel, Ann Kring, Batja Mesquita, Karen Quigley, Sebastian Ruf, Aaron Scott, Scott Sleek, Annie Temmink, Kelley Van Dilla, and Van Yang. And for close reviews of the science in specific lessons, I give special thanks to Olaf Sporns and Sebastian Ruf for lesson no. 2, Dima Amso for lesson no. 3, and Ben Hutchinson and Sarah Dubrow for lesson no. 4.
I also offer heartfelt thanks to my colleagues and trainees in the Interdisciplinary Affective Science Laboratory at Northeastern University and Massachusetts General Hospital. Much of the material in these essays has been the topic of ongoing discussion and research in our community of talented young scientists. All the members (past and present) are listed at affective-science.org. I’m particularly grateful to Sam Lyons for ultra-fast retrieval of a never-ending torrent of research papers on request and to Karen Quigley, who co-directs our lab. Karen has deep expertise in peripheral physiology of the body, interoception, and allostasis. We like to joke that, with her knowledge of the body and my knowledge of the brain, between the two of us, we make up a whole person.
I am also especially grateful to the Martinos Center for Biomedical Imaging at the Massachusetts General Hospital and its director, Bruce Rosen, as well as to the psychology department at Northeastern University, and in particular to our chair, Joanne Miller. Their support and patience make it possible for me to be both a neuroscientist and a psychologist, not to mention a communicator of science to the public.
This book was made possible with a fellowship from the John Simon Guggenheim Foundation and a book grant from the Alfred P. Sloan Foundation. I am deeply grateful to both for their generous support.
And above all, I offer a stream of continuous thanks and unbounded appreciation to the two brains I love best—my daughter, Sophia, and my husband, Dan—for their inspiration, forbearance, and general balancing of my body budget.
Appendix
The Science Behind the Science
This appendix adds crucial scientific details for certain topics in my essays, explains that certain points are still debated by scientists, and gives credit to scientists whose ideas and turns of phrase I’ve incorporated. Full references for the book can be found at sevenandahalflessons.com. (Most appendix entries also include a direct link to the relevant web page.)
The biggest challenge of science writing is deciding what to leave out. A science writer, like a sculptor, chips away at complex material until something compelling and comprehensible takes shape. The end result is necessarily incomplete from a strict scientific perspective, but (one hopes) still correct enough not to offend most experts.
An example of “correct enough” is saying that a human brain is made of approximately 128 billion neurons. This estimate may differ from some others that you’ve seen, because I include the neurons that make up the cerebellum—a brain structure that’s important for using sensations like touch and vision to coordinate physical movements, among other things. Some research papers may underestimate neurons in the cerebellum. Even so, my estimate of brain cells is incomplete, because the brain is also made of 69 billion other cells that are not neurons, called glial cells, which have a surprising number of biological functions. But the 128 billion figure serves to make the point that the brain is a complex network of parts, which is a pivotal concept in lesson no. 2.
The challenge of writing about science for the public
The Half-Lesson: Your Brain Is Not for Thinking
Amphioxi populated the oceans about 550 million years ago: These ancient creatures, also called lancelets, are still around today. Amphioxi are our evolutionary cousins in the following way: Humans are vertebrates, meaning that we possess a backbone, which we call a spine, and a nerve cord, which we call a spinal cord. Amphioxi are not vertebrates, but they have a nerve cord running stem to stern. They also have a backbone of sorts, called a notochord, made of a fibrous material and muscle instead of bone. Amphioxi and vertebrates belong to a larger group of animals known as chordates (phylum Chordata), and we share a common ancestor. (More on this ancestor shortly.)
Amphioxi lack all sorts of features that distinguish vertebrates from invertebrates. They have no heart, liver, pancreas, or kidneys, nor the internal bodily systems that go with these organs. They do have some cells that regulate a circadian rhythm and produce a cycle of sleeping and waking.
Amphioxi do not have a distinct head or any of the visible sense organs that are found in a vertebrate head, such as eyes, ears, a nose, and so on. At its most anterior tip, an amphioxus has a small group of cells on one side, called an eyespot. These cells are photosensitive and can detect gross changes in light and dark, so if a shadow falls on the animal, the animal moves away. The cells of this eyespot share some genes in common with a vertebrate retina, but amphioxi do not have eyes and cannot see.
Also, amphioxi cannot smell or taste. They have some cells in their skin to detect chemicals in the water, and these cells contain some genes that are similar to those found in a vertebrate olfactory bulb, but it is not clear that the genes function in the same way. An am
phioxus also has a cluster of cells with hairs in them that enable it to orient and balance its body in water and perhaps sense acceleration when it swims, but amphioxi do not have inner ears with hair cells to hear with, as vertebrates do.
Amphioxi also cannot locate food and approach it; they dine on whatever stream of little creatures the ocean currents deliver. They have cells to detect the absence of food and wriggle away in a random direction that hopefully will lead to a meal (in effect, the cells signal, Anyplace is better than here). See 7half.info/amphioxus.
a teeny clump of cells that was not quite a brain: Scientists continue to debate whether amphioxi have brains. It all comes down to where you draw the dividing line between “brain” and “not a brain.” The evolutionary biologist Henry Gee sums up the situation well: “Nothing like the vertebrate brain is seen in either tunicates [sea squirts] or the amphioxus, although there are traces of its ground plan . . . if one looks hard enough.”
Scientists pretty much agree that a sketch of the genetic outlines of the vertebrate brain can be found in anterior end of the amphioxus notochord, and these outlines are at least 550 million years old. This does not necessarily mean that the genes found in the anterior end of the notochord work in the same way or produce the same structures that they do in the brain of a vertebrate. (For more details on what it means for two species to have similar genes, see the appendix entry for lesson no. 1, “reptiles and nonhuman mammals have the same kinds of neurons that humans do.”) And this is where the scientific debates begin. Amphioxi have some of the molecular patterns that organize the vertebrate brain into major segments, but scientists debate which segments are sketched out and which segment instructions are absent. It is also debatable whether the actual segments are present in amphioxi. Similarly, an amphioxus has the rudimentary genetic foundations necessary for a head, even though it has no head per se.
For a more detailed discussion of amphioxi, see Henry Gee’s Across the Bridge: Understanding the Origin of the Vertebrates, and the evolutionary neuroscientists Georg Striedter and Glenn Northcutt’s book Brains Through Time: A Natural History of Vertebrates. See 7half.info/amphioxus-brain.
you behold a creature very similar to your own ancient, tiny ancestor: Scientists believe that our common ancestor with amphioxi resembled modern amphioxi very closely, because amphioxi’s environment (their niche) has barely changed in the past 550 million years, so they wouldn’t have had to adapt much. In contrast, vertebrates have undergone tremendous evolutionary changes, as have other chordates, such as sea squirts. Therefore, scientists assume that by studying modern amphioxi, we can learn about the common ancestor of all chordates.
Still, some scientists continue to debate these assumptions—it’s unlikely that amphioxi have not changed at all in half a billion years! For example, the amphioxus notochord (its central nervous system) extends the entire length of its body, from tip to tail, whereas in vertebrates, the spinal cord ends where the brain begins. Scientists debate whether our shared ancestor had an amphioxus-like notochord that became shorter in conjunction with evolving a vertebrate brain, or a shorter notochord that extended during evolution. Several similar debates (e.g., the evolution of olfaction) exist as well.
For a more detailed discussion of our amphioxus-like ancient ancestor, see Henry Gee’s Across the Bridge. See 7half.info/ancestor.
Why did a brain like yours evolve: Statements like “Your brain is for this” and “Your brain evolved to do that” are examples of teleology, from the Greek word telos, meaning “end,” “purpose,” or “goal.” Several types of teleology are discussed in science and philosophy. The most common type, which is generally discouraged by scientists and philosophers, is a statement that something was intentionally designed for a purpose with an ultimate end point. An example is suggesting that brains evolved in some kind of upward progression—say, from instinctual to rational, or from lower animals to higher animals. That is not the form of teleology I’m using in this lesson.
A second type of teleology, which I have employed in this lesson, is a statement that something is a process that embodies a goal with no ultimate end point. In stating that the brain is not for thinking but for regulating a body in a particular niche, I am not implying that body budgeting—allostasis—has some final end state. Allostasis is a process that anticipates and deals with ever-changing environmental input. All brains manage allostasis. There’s no orderly progression from a worse way to a better way.
The psychologists Bethany Ojalehto, Sandra R. Waxman, and Douglas L. Medin study how people across cultures reason about the natural world. Their research suggests that teleological statements of the sort employed in this lesson reflect an appreciation of the relationships among living things and their environments. They call it “contextual, relational cognition.” A statement like “A brain is not for thinking” is inherently relational (it refers to the relationship among the brain, various bodily systems, and stuff in the environment) and does not reflect that the brain was intentionally designed for a purpose with a final end point.
My phrasing (e.g., “Your brain is not for thinking”) also appears in a particular context—in a nontechnical essay that describes aspects of brain function. The phrasing achieves its full meaning only in the context in which it’s employed. If you strip away the context, it’s easy to mistake the statement as the first, problematic type of teleology. Allostasis is of course not the sole cause of brain evolution and did not drive evolution in some orderly fashion. Brain evolution was largely driven by natural selection, which is haphazard and opportunistic. Brain evolution may also be influenced by cultural evolution, which I discuss in lesson no. 7. See 7half.info/teleology.
The scientific name for body budgeting is allostasis: Allostasis is not the only factor influencing how brains evolve and how they work, but it’s a big one. Allostasis is a predictive balancing process over time, not a process that seeks a single, stable point for the body to maintain (it’s not like a thermostat). The word for seeking a single, stable point is homeostasis. See 7half.info/allostasis.
The movement should be worth the effort, economically speaking: The idea of worthwhile movement is well studied in the field of economics, where it’s called value. See 7half.info/value.
the insides of bodies became more sophisticated: The organs inside your body, such as your heart, stomach, and lungs, are called viscera, and they are part of broader visceral systems below your neck, such as your cardiovascular system, your gastrointestinal system, and your respiratory system, respectively. Movements that happen inside your heart, gut, lungs, and other organs are called visceromotor movements. Your brain controls your visceral systems (i.e., it performs visceromotor control). In the same way that your brain has a primary motor cortex and a whole system of structures in your subcortex for controlling your muscle movements, it also has a primary visceromotor cortex and a whole system of subcortical structures for controlling your viscera. Some visceral organs, like your lungs, require your brain in order to function. Your heart and your gut, however, have their own intrinsic rhythms, and the visceromotor system in your brain fine-tunes them. One last note: your body has other systems not typically linked to any visceral organ, such as the immune system and endocrine system, and their changes are also broadly referred to as visceromotor.
In the same way that the motor movements of your arms, legs, head, and torso produce sense data that is relayed to your brain (specifically, to the somatosensory system), visceromotor movements produce sensory changes, called interoceptive sense data, that are sent to your brain (to the interoceptive system). All this sense data helps your brain better control your motor and visceromotor movements.
The best scientific estimates today suggest that the evolution of visceral and visceromotor systems in vertebrates was accompanied by the evolution of sensory systems. After conception, when an embryo is building its brain and body, the visceral systems and the sensory systems both emerge from the sam
e temporary cluster of cells, called the neural crest. So does the segment of the vertebrate brain that contains the visceromotor and interoceptive systems, which is known as the forebrain. The neural crest is unique to vertebrates and can be seen in all vertebrate species, including humans.
Visceromotor and interoceptive systems play a key role in determining the value of any movement, but we cannot say they evolved for that reason. Other selection pressures contributed to the evolution of the body’s visceral systems and the brain’s visceromotor system, such as the evolution of larger bodies that needed new kinds of tending and maintenance. For example, most animals on this planet are small in diameter, with only a few cells that span from the inside of the body to the outside world. This arrangement makes certain physiological functions easier, like the exchange of gases (in breathing) and removal of waste products. In a larger body, the inside of the body is farther away from the outside world, so new systems have evolved, like one that pumps water over gills to facilitate gas exchange, and the kidneys and an extended gut to excrete waste. These new systems allowed vertebrates to become more powerful swimmers and, accordingly, more successful predators. See 7half.info/visceral.
Lesson 1. You Have One Brain (Not Three)
Your human mind, wrote Plato: Plato wrote about the psyche, which differs from our modern idea of a mind. I am following the colloquial tradition of using psyche and mind as synonyms. See 7half.info/plato.
scientists later mapped Plato’s battle onto the brain: The triune brain idea fused neuroscience with Plato’s writings about the human psyche. In the early twentieth century, the physiologist Walter Cannon proposed that emotions were triggered and expressed (respectively) by two brain regions, the thalamus and the hypothalamus, which sit directly beneath the supposedly rational cortex. (Today, we know that the thalamus is the main gateway for all sense data, except for chemicals that become smells, to reach the cortex. The hypothalamus is critical for regulating blood pressure, heart rate, respiratory rate, sweating, and other physiological changes.) In the 1930s, the neuroanatomist James Papez proposed a “cortical circuit” dedicated to emotion. His circuit went beyond the thalamus and hypothalamus to include cortical regions that border the subcortical regions (the cingulate cortex) and were therefore assumed to be ancient. This segment of cortex was dubbed the limbic lobe by the neurologist Paul Broca fifty years earlier. (He used the term limbic, which comes from the Latin word meaning “border,” limbus. This tissue abuts the brain’s sensory systems and the motor system that moves your arms, legs, and other body parts. Broca thought the limbic lobe housed primitive survival faculties, like the sense of smell.) In the late 1940s, the neuroscientist Paul MacLean transformed Papez’s “cortical circuit” into a full-fledged limbic system and embedded it within a three-layered brain that he named the triune brain. See 7half.info/triune.
Seven and a Half Lessons About the Brain Page 10