The outermost layer, part of the cerebral cortex: The many brain terms that include cortex can be confusing. The cerebral cortex is a sheet of neurons arranged in layers that covers the subcortical (meaning “below the cortex”) parts of your brain. It is popularly believed that one part of the cerebral cortex is evolutionarily old and belongs to the limbic system (e.g., the cingulate cortex) and another part is evolutionarily new, which is why it’s called the neocortex. This distinction derives from a misunderstanding of how the cortex evolved, which is the topic of this lesson.
one of the most successful and widespread errors in all of science: Scientists normally try to avoid saying that something is a fact or is definitively true or false. In the real world, facts have some probability of being true or false in a particular context. (As Henry Gee says in his book The Accidental Species: Misunderstandings of Human Evolution, science is a process of quantifying doubt.) In the case of the triune brain, however, it’s justified to use more absolute language. By the time MacLean published his magnum opus, in 1990, The Triune Brain in Evolution: Role in Paleocerebral Functions, the evidence was already clear that the triune brain idea was wrong. Its continued popularity is an example of ideology rather than scientific inquiry. Scientists work hard to avoid ideology, but we are also people, and people are sometimes guided by belief more than data. (See Richard Lewontin’s book Biology as Ideology: The Doctrine of DNA.) Mistakes are part of the normal process of science, and when scientists acknowledge them, they are great opportunities for discovery. Learn more in Stuart Firestein’s books Failure: Why Science Is So Successful and Ignorance: How It Drives Science. See 7half.info/triune-wrong.
genes were most likely present in our last common ancestor: This assumption depends on there not having been much evolutionary change in the cells of animals we’re comparing.
More generally, genes are not the whole story when it comes to inferring whether two animals have brain features that can be traced back to a common ancestor even when those features look different to the naked eye. Sometimes genes can be misleading. And some scientists use other sources of biological information, such as the connections between neurons, to determine whether two brain structures have a common ancestry. For a more detailed discussion of this topic, which is called homology, see Georg Striedter’s Principles of Brain Evolution and Striedter and Northcutt’s Brains Through Time. See 7half.info/homology.
as brains become larger over evolutionary time, they reorganize: This idea comes from the neurobiologist Georg Striedter. He likened brains to companies, which reorganize to scale up their business. See Striedter’s Principles of Brain Evolution. It is also possible for brains to lose complexity over evolutionary time or during development; an example is tunicates (sea squirts). See 7half.info/reorg.
segregating and then integrating: Here is an analogy to reinforce my comparison of the primary somatosensory cortex in rats and humans. The author and chef Thomas Keller explains that if you cook a bunch of vegetables together in a pot, the mixture will have a single, blended flavor. No individual ingredient stands out. But, Keller explains, there’s a better, tastier way to make your dish: cook each vegetable separately and assemble them in the pot at the end. Now every spoonful is a different complex medley of flavors. The difference between these two techniques is essentially the difference between the primary somatosensory cortex in rats and humans. The rat’s single region is like a single pot containing all the ingredients, and the four human regions are like four pots with separate ingredients. In the language of lesson no. 2, the four-pot technique has higher complexity. See 7half.info/keller.
reptiles and nonhuman mammals have the same kinds of neurons that humans do: By this, I mean the neurons have the same molecular identity—a specific gene or sequence of genes—that performs the same genetic activities (e.g., they make the same proteins). A given gene does not necessarily make the same proteins in every animal where it’s found. Two animals can have the same genes, but those genes can function differently or produce different structures. And even within the same animal, a network of genes can perform different genetic activities at different times in development. (For a clear explanation and examples, see Henry Gee’s book Across the Bridge.) The important observation here is that two creatures can have neurons with some of the same genes that function the same way in both creatures, and yet those neurons can differ in how they are organized, resulting in very different-looking brains. See 7half.info/same-neurons.
The common brain-manufacturing plan: This research originated with evolutionary and developmental neuroscientist Barbara Finlay, who calls it the “translating time” model. Finlay built a mathematical model that predicts the timing of 271 events in developing animal brains. Some of these events include when neurons are created, when axons begin to grow, when connectivity is established and refined, when myelin starts to form over the axons, and when brain volume starts to change and expand. Finlay’s model calculates the equivalent number of days for any developmental event across eighteen mammalian species that have been studied and even some animal species not included in the original model. If one compares her model’s predicted timing to the actual timing of brain formation, the correlation is an astounding 0.993 (on a scale of -1.0 to 1.0). This means the ordering of events is close to identical for all species studied, because they’re all described by a single model.
Additionally, the genes found in various mammalian brain cells provide molecular genetic evidence that is consistent with the translating time model. The brain cells of jawed fish contain those genes as well. Some genes go all the way back to amphioxus and very likely to its common ancestor with humans. So, based on the genetic evidence alone, it’s reasonable to infer that the common manufacturing plan (or part of it) holds for all jawed vertebrates. See 7half.info/manufacture.
the human brain has no new parts: As a neuroscientist, I am persuaded by the evidence that supports Finlay’s hypothesis of a common brain-manufacturing plan. Interested readers should be aware, however, that some scientists continue to hold to the idea that certain features of the human brain, such as the prefrontal cortex, have evolved to become larger than expected for a scaled-up primate brain. My view is that some of the distinctive capacities of a human brain come from a combination of a big cerebral cortex (not bigger than expected for the overall brain size, mind you, just big in absolute terms) and souped-up connections between neurons in certain parts of the cortex, including upper layers of the prefrontal cortex. Some scientists, myself included, hypothesize that these features give humans the ability to understand things by their function rather than their physical form, as I discuss in lesson no. 7 and in my earlier book, How Emotions Are Made: The Secret Life of the Brain. See 7half.info/parts.
There is no such thing as a limbic system dedicated to emotions: Even though the limbic system is a myth, your brain does contain something called limbic circuitry. Neurons in limbic circuitry connect to the brain stem nuclei that regulate your autonomic nervous system, immune system, endocrine system, and other systems whose sense data create interoception, your brain’s representation of the sensations in your body. Limbic circuitry is not exclusive to emotion and is distributed across multiple brain systems. It includes subcortical structures, such as the hypothalamus and the central nucleus of the amygdala; allocortical structures, such as the hippocampus and the olfactory bulb; and parts of the cerebral cortex, such as the cingulate cortex and the anterior part of the insula. See 7half.info/limbic.
The triune brain idea and its epic battle between emotion, instinct, and rationality is a modern myth: The triune brain belongs to a long history of entrenched myths in science. Here are some more to amuse you. In the eighteenth century, serious scholars believed that heat was created by a mythical fluid called caloric and that combustion was caused by an imaginary substance called phlogiston. Physicists of the nineteenth century insisted that the universe was filled with an invisible substance called luminiferous ether that perm
itted light waves to propagate. Their medical colleagues attributed illnesses such as the plague to smelly vapors called miasmas. Each of these myths survived and substituted for scientific fact for one hundred years or more before it was overturned. See 7half.info/myths.
we’re just an interesting sort of animal: This idea comes from Henry Gee’s book The Accidental Species. See 7half.info/interesting.
Lesson 2. Your Brain Is a Network
Your brain is a network: Your brain network is made of smaller networks, or subnetworks, of interconnected neurons. Each subnetwork is a loose collection of neurons that constantly join in and leave as the subnetwork functions. Think of a basketball team that has twelve to fifteen players but only five of them participate at a time. Players switch in and out of the game, but we still view the people on the court as the same team. Likewise, a subnetwork is maintained even though the actual neurons that create it switch in and out. This variability is an example of degeneracy, when structurally dissimilar elements (such as groups of neurons) perform the same function. See 7half.info/network.
a network of 128 billion neurons: My count of 128 billion neurons in the average human brain is higher than you may find in other sources, which commonly cite about 85 billion neurons. The difference is due to the fact that neurons can be counted by different methods. In general, scientists estimate the number of neurons in a brain using stereological methods, which employ probability and statistics to estimate the three-dimensional structure of neurons from two-dimensional images of brain tissue. The 128 billion figure comes from a paper that used a stereological method called optical fractionator that counted about 19 billion neurons in the human cerebrum, including the cerebral cortex, the hippocampus, and the olfactory bulb, and another 109 billion or so granule cells in the cerebellum, plus 28 million or so Purkinje neurons in the cerebellum. The more common figure of 85 billion neurons comes from another method called isotropic fractionator, which is simpler and quicker but systematically omits some neurons. See 7half.info/neurons.
A brain network is not a metaphor: The brain isn’t symbolically like a network—it really is a network, meaning it functions similarly to other networks. The term network is a concept here, not a metaphor. It helps call to mind other networks that you know to help you understand better what a brain network is and how it works.
Generally speaking, each neuron looks like a little tree: The human brain has different types of neurons of various shapes and sizes. The kind of neuron I’ve described in our lesson is a pyramidal neuron in the cerebral cortex.
I’ll refer to this whole arrangement as the “wiring” of your brain: The simple term wiring, as I use it, stands in for more specific structural details. In general, a neuron consists of a cell body, some branch-like structures on the top called dendrites (think the crown of a tree), and one long, slender projection with a root-like structure on the bottom called an axon. Each axon is much thinner than a human hair and has little balls on the end, called axon terminals, that are filled with chemicals. Dendrites are riddled with receptors to receive the chemicals. Typically, the axon terminals of one neuron are close to the dendrites of thousands of other neurons, but they do not touch, and the intervening spaces are called synapses. When a neuron’s dendrites detect the presence of chemicals, the neuron “fires” by sending an electrical signal down its axon to its axon terminals, which release their neurotransmitters into the synapses; the neurotransmitters then attach to receptors on the other neurons’ dendrites. (Other cells, called glial cells, help the process along and prevent chemical leaks.) This is how neurochemicals excite or inhibit the receiving neurons and change their rate of firing. Through this process, one individual neuron influences thousands of others, and thousands of neurons can influence one, all simultaneously. This is the brain in action. See 7half.info/wiring.
the area is routinely called the visual cortex: What does it mean to “see”? Your conscious experience of things in the world, like seeing your hand or your phone, is created in part by neurons in your occipital cortex. It is possible to navigate the world if these neurons are damaged, however. If you place an obstacle in front of a person with damage to the primary visual cortex, the person won’t consciously see the obstacle but will walk around it. This phenomenon is called blindsight. See 7half.info/blindsight.
if you blindfold people with typical vision: The study of blindfolded people who learned braille is another demonstration that neurons have multiple functions. When the scientists disrupted neural firing in the primary visual cortex (V1) using a technique called transcranial magnetic stimulation, blindfolded test subjects had a harder time reading braille, although that difficulty disappeared twenty-four hours after the blindfold was removed and visual input was available again to be processed by V1. See 7half.info/blindfold.
A system has higher or lower complexity: Complexity does not imply an orderly progression of brains on some phylogenetic scale or scala naturae from less complex to ever more complex, culminating in the human brain. The brains of other animals, such as monkeys and worms, also have complexity. See 7half.info/complexity.
Meatloaf Brain: I drew inspiration for this name from the book The Blank Slate by psychologist Steven Pinker; in it, he described a “uniform meatloaf” mind as “a homogeneous orb invested with unitary powers.” See 7half.info/meatloaf.
Pocketknife Brain: This name was inspired by evolutionary psychologists Leda Cosmides and John Tooby, who described a human mind as like a Swiss Army knife. See 7half.info/pocketknife.
A real pocketknife with, say, fourteen tools: Here’s a bit more mathematical detail behind the complexity of a fourteen-tool pocketknife. In a particular configuration of the pocketknife’s tools, which I’ve called a pattern, each tool has two possible states: used or unused. Fourteen tools with two states each yields about 16,000 possible patterns for the whole pocketknife:
2×2×2×2×2×2×2×2×2×2×2×2×2×2=214=16,384
Adding a fifteenth tool doubles the number of patterns:
2×2×2×2×2×2×2×2×2×2×2×2×2×2×2=215=32,768
If each tool is given an additional function, it now has three possible states instead of two—its first function, its second function, or unused. This yields far more total patterns for the pocketknife:
3×3×3×3×3×3×3×3×3×3×3×3×3×3=314=4,782,969
Tools with four functions would yield 414 or 268,435,456 patterns, and so on.
Neurons aren’t literally wired together: This observation is courtesy of my colleague Dana Brooks in the Department of Electrical and Computer Engineering at Northeastern University.
Physicists sometimes say that light travels in waves: In this metaphor, I am not referring to wave-particle duality but to the myth of luminiferous ether described in an appendix entry in lesson no. 1. See 7half.info/wave.
Lesson 3. Little Brains Wire Themselves to Their World
many newborn animals are more competent than newborn humans: Of course, many newborn animals are less competent than newborn humans, such as the blind, bald little peanuts that are born to rats, guinea pigs, and other rodents.
“Neurons that fire together, wire together”: This saying is attributed to the neuroscientist Donald Hebb, and the phenomenon is more formally known as Hebb’s principle or Hebbian plasticity. Strictly speaking, the firing is not simultaneous—one neuron fires just before another. See Hebb’s book The Organization of Behavior: A Neuropsychological Theory. See 7half.info/hebb.
It has more of a lantern: The wonderful metaphor of a “lantern of attention” is courtesy of psychologist Alison Gopnik, who studies the cognitive development of children. See her book The Philosophical Baby: What Children’s Minds Tell Us About Truth, Love, and the Meaning of Life.
Besides sharing attention, other abilities are probably important to developing a spotlight of attention. One is the brain’s control of the head, an ability that develops over the first few months of life. Another is control of the muscles of the eye, calle
d oculomotor control, which improves during the first few months of life.
Seven and a Half Lessons About the Brain Page 11