The Accidental Mind: How Brain Evolution Has Given Us Love, Memory, Dreams, and God
Citation (Chicago Style): Linden, David J.. The Accidental Mind: How Brain Evolution Has Given Us Love, Memory, Dreams, and God. Harvard University Press, 2012. Kindle edition.
One. The Inelegant Design of the Brain
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After much careful scientific thought, this master hormone was given the compelling name “growth hormone releasing hormone” (endocrinologists, like many scientists, are not known for their literary flair).
Two. Building a Brain with Yesterday’s Parts
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Glutamate and GABA are fast-acting neurotransmitters: when they bind their receptors, the electrical changes they produce occur within a few milliseconds. They are the dominant fast neurotransmitters in brain, but there are some other fast ones.
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quintessentially American
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solution: burn that junk in the front yard. For example, acetylcholine is destroyed in the synaptic cleft by an enzyme specifically built for that purpose. Most other neurotransmitters get the European treatment: they are recycled.
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whole trip will have been in vain, and no neurotransmitters will be released. What a bum deal! These constraints may have been tolerable for the simple problems solved by the nervous system of a worm or a jellyfish, but for the human brain, the constraints imposed by (ancient) neuronal electrical function are considerable.
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wiring of the brain is guided by patterns of activity, which allows the strength and pattern of synaptic connections to be molded by experience, a process called synaptic plasticity
Three. Some Assembly Required
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In other words, genes influence general intelligence but to a lesser degree than they influence personality.
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Sense of humor is another. Identical twins raised apart tend not to find the same things humorous, whereas they do share a sense of humor with their adoptive siblings.
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Proteins form the important structural and functional units of the cell. For example, they make all of the important neuronal molecules discussed so far. These include ion channels (such as the voltage-sensitive sodium channels that underlie the upstroke of the spike), enzymes that direct chemical reactions to produce or break down neurotransmitters (like the enzyme acetylcholinesterase, which breaks down the neurotransmitter acetylcholine), and neurotransmitter receptors (such as glutamate receptors), as well as the structural molecules, the cables, tubes, and rods of protein that give neurons their shape.
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“homeotic” genes that are master regulators of early development. Homeotic genes code for proteins, and these proteins are,
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aberrant neuronal migration can result in cerebral palsy, mental retardation, and epilepsy.
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result supports the former model, in which developing neurons are derived from multipotent progenitors.
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This finding supports the latter model, in which neuronal fate is determined by cell lineage.
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In the case of humans, the period when brain wiring affects fine-scale brain development starts in the later stages of pregnancy and continues through the first few years of life.
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Strong activation of a synapse not only preserves and strengthens it, but also makes its neighbors weaker and ultimately can cause them to be eliminated. I’ll talk a lot about the molecular basis of how this happens in Chapter 5, when I consider memory storage that reuses these same mechanisms.
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“selectionist theory” or “neural Darwinism.”
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The ability of the brain to be modulated by experience is called neural plasticity.
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Rather than showing that extra enrichment beyond normal experience can boost brain growth, what this experiment shows is that severe environmental deprivation can, at least temporarily, cause a reduction in the complexity of cortical circuits.
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The flip side of this phenomenon is that babies exposed to two languages can develop perfectly accented speech in both languages.
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This is what can happen when a tiny bit of science finds it way into a policy debate.
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Learning is a process by which new experiences are integrated with previous experiences. Therefore, early experience may be important, not because it is written into neural circuitry more effectively, but rather because it is the basis for subsequent learning.
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Four. Sensation and Emotion
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the sensory world, our brains are messing with the data.
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Milder cases of this can involve inability to recognize a particular object within a class—the inability to pick out one’s own car in a full parking lot is typical.
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These neurons with a dual sensory and motor function were called mirror neurons.
Five. Learning, Memory, and Human Individuality
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modulatory neurotransmitter dopamine.
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This may explain why schizophrenics and patients with Parkinson’s disease, people whose ailments are associated with defects in dopamine signaling, perform poorly on tests of working memory.
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his splendid book The Seven Sins of Memory, the Harvard University psychologist Daniel Schacter speaks of three of these “sins of commission” in declarative memory retrieval: misattribution, suggestibility, and bias.
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if a particular pattern of neuronal activity results in a lasting modification of, say, voltage-sensitive sodium channels located at the axon hillock, such that the threshold for firing a spike was moved closer to the resting potential, then this could produce a lasting change in the firing properties of that neuron, thereby contributing to an engram. This is only one of many possible changes that would affect neuronal spiking. For example, modifying the voltage-sensitive potassium channels that underlie the downstroke of the spike could change their average time to open. This would result in alterations to the rate and number of spikes fired in response to synaptic drive. Indeed, changes in voltage-sensitive ion channels can persistently alter the intrinsic excitability of neurons and, in animal experiments, these changes can be triggered by learning.
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Experience-dependent modification of synaptic function is a general mechanism that is thought by most brain researchers to underlie a large part of memory storage.
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a microscopic level, the synapses of the brain are not static. They grow, shrink, morph, die off, and are newly born, and this structural dynamism is likely to be central to memory storage.
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In the years that followed, thousands of papers were published about LTP. One of the most interesting things scientists learned is that, although LTP was initially found in the hippocampus, it is actually a phenomenon that occurs throughout the brain.
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This is the ultimate example of “when life gives you lemons, make lemonade.” Our memory, which is the substrate of our consciousness and individuality, is nothing more than the accidental product of a work-around solution to a set of early evolutionary constraints. Put another way, our very humanness is the product of accidental design, constrained by evolution.
Six. Love and Sex
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In our modern world, some very recent changes relevant to sexual behavior, such as the availability of contraception and assisted fertility, and changes in social conventions, political systems, and technologies, have allowed women to live independently. Most of these changes have only appeared in the last generation. So, the genes that help to instruct the parts of our brains involved in sexual behavior have not yet undergone selection by many of the forces operating in modern society.
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lordosis, a posture that presents the genitals.
Seven. Sleeping and Dreaming
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memory has been nicely articulated by Robert Stickgold of the Harvard Medical School, who writes “the unique physiology of sleep and perhaps even more so, of REM sleep, shifts the brain/ mind into an altered state in which it pulls together disparate, often emotionally charged and weakly associated memories into a narrative structure and . . . this process of memory reactivation and association is, in fact, also a process of memory consolidation and integration that enhances our ability to function in the world.”
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Eight. The Religious Impulse
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Confabulation in anterograde amnesia is not a process under voluntary control. Rather, it’s what the brain does when confronted with a problem it cannot begin to solve: it makes a story from whatever bits of experience it can dredge up, in much the same way that narrative dreams are created from scraps of memory.
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Holiness the Dalai Lama has said, “If science proves Buddhism is wrong, then Buddhism must change.”
Nine. The Unintelligent Design of the Brain
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“Intelligent design readily embraces the sacramental nature of physical reality. Indeed, intelligent design is just the Logos theology of John’s Gospel restated in the idiom of information theory”
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which codes for a protein in the mitotic spindle (a structure used to organize the chromosomes during cell division), seems to determine how many times cortical progenitor cells divide before they become committed to becoming cortical neurons. As a result, this gene is crucial for determining cortical size.
Epilogue. That Middle Thing
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first glance, this would appear to be a fairly complete explanation, but it’s not. What’s missing is that middle thing. How is it that changing the strength of some synapses in the hippocampal circuit actually gives rise to memories for facts and events, as recalled during behavior?