By Norman Doidge May 22, 2022 ⋅ 25 min read ⋅ Books
Preface
This book is about the remarkable discovery that the brain can change itself.
It was believed that the brain doesn’t change after childhood except to decline.
Three reasons for this belief
Brain-damaged patients rarely make a full recovery.
Inability to see the living brain’s microscopic activities.
The idea that the brain is like a machine.
While machines do many extraordinary things, they don’t change and grow.
Review of the history of neuroplasticity.
Neuroplasticity is a double-edged sword as it can both enable plasticity but also rigidity.
E.g. Some of our most stubborn habits and disorders come from plasticity.
Chapter 1: A Woman Perpetually Falling …
Patient C.S. feels like she’s forever falling and because of this feeling, she actually falls.
For her, she feels like she’s constantly on the edge of a bridge.
After falling, when C.S. is on the floor, the feeling is still there and is described as “an imaginary trapdoor opens up and swallows me.”
We have senses we don’t know we have, such as our sense of balance controlled by the vestibular system.
A functioning vestibular system has a strong link to our visual system.
In C.S.’s case, her vestibular system is damaged and no longer functions.
So, the researcher Paul Bach-y-Rita and colleagues developed a vestibular substitution device that sends electric shocks through her tongue to her brain.
C.S.’s condition is due over-prescribing the drug “gentamicin” which permanently damages the inner ear, hence why her vestibular system is broken.
Since the link between C.S.’s vestibular and visual system is damaged, her eyes can’t follow a moving target smoothly.
E.g. “Everything I see bounces like a bad amateur video.”
She also suffers from extreme mental fatigue as it takes a lot of attention and energy to consciously maintain an upright position.
Coming back to the vestibular substitution device, it works by measuring acceleration signals using an accelerometer and maps those signals onto an array of 144 electrodes implanted in a plastic strip on the tongue.
E.g. When a person tilts their head forwards, there’s a tingling sensation at the front of their tongue similar to champagne bubbles. When tilted backwards, there’s a tingling at the back of the tongue.
When the author tested out the device with his eyes closed, he soon forgot that the sensory information was coming from his tongue.
When C.S. tried the device for the first time, she started to cry. The sense of perpetual falling that she’s felt for five years is finally gone.
With the device, C.S. looks peaceful. The jerking has stopped, the mysterious demons that seemed to be inside her, pushing her off balance, have disappeared.
Her first goal is to stand upright without support for twenty minutes, which she eventually achieved.
In her brain, she’s decoding the signals from the artificial vestibular apparatus. This is only possible due to neuroplasticity since the signals that normally make their way to the brain’s sensory cortex are now redirected to the area that processes balance.
However, the second neuroplastic marvel appears after the device is removed.
Without the device, C.S. can stand without falling and hugs the researchers.
E.g. “I feel anchored and solid. I don’t have to think where my muscles are. I can actually think of other things.” - C.S.
The interesting and remarkable finding is that without the device, C.S. has no artificial and no natural vestibular apparatus, so how is she able to balance?
The first time C.S. wore the device, it was only for a minute. However, the researchers noticed a residual effect after removing the device where C.S. could maintain balance for about twenty seconds.
For the second time, C.S. had the device on for two minutes and the residual effect lasted for about forty seconds.
The third time she had the device on was for twenty minutes, expecting a residual effect of around seven minutes following the pattern of the previous experiments. However, the effect lasted an hour.
Something amazing was happening here for the residual effect to occur and to persist, sometimes even longer than the training period.
Furthermore, C.S. behaves almost normally now and the link between the visual and vestibular system is also recovered as she can track moving targets.
Possible explanations for the residual effect
The artificial vestibular device may help to recalibrate and reinforce the disorganized and noisy signals from her damaged vestibular system.
The device might also recruit other pathways using neuroplasticity. It might unmask unused or older pathways, making them more efficient by shortening them.
How far can the residual effect be pushed? Is there a limit where the residual time never grows longer?
Over the next year, C.S. wore the device more often to get relief and to build up her residual effect.
Her residual effect progressed from multiple hours, to days, and then to four months.
And now, she doesn’t use the device at all and may be cured of her condition.
A similar sensory substitution device was built for congenitally blind people where a camera and computer translated light signals into vibration signals felt in the person’s back.
The stimulators acted like pixels, vibrating for darker shades and freezing for lighter shades.
This tactile-vision device enabled blind subjects to read, make out faces, and distinguish objects.
This suggests that the blind subjects experienced the space in front of the camera as 3D and not as skin stimuli.
Their mental perceptual experience took place not on the skin’s surface but in the world.
E.g. If tickled near the stimulators, the subjects didn’t confuse the tickle with a visual stimulus.
The idea that our brain structure is fixed and hardwired by genetics is called “localizationism”.
Paul Bach-y-Rita rejected the claims made by localizationism and argued that our senses have an unexpected plastic nature to them where damage to one sensory system can be compensated for by another sense.
“We see with our brains, not with our eyes” - Paul Bach-y-Rita
This claim goes against our intuitive belief that we see with our eyes.
How a sensation enters the brain isn’t important.
Review of the discovery of Broca’s and Wernicke’s area.
Sensory substitution is possible because all of our sense receptors translate different kinds of energy from the external world into the common language of electrical patterns.
The tongue is the ideal brain-machine interface because it has no insensitive layer of dead skin.
Paul’s journey of learning about neuroplasticity started with his father who had a massive stroke that damaged major parts of his brain and brainstem, leaving him paralyzed.
However, Paul’s father was able to recover most of his mobility and autopsy analysis showed that the damaged was extensive. This suggests that the brain can reorganize itself to recover functions after devastating strokes.
One of the most amazing plasticity experiments of our time was done by Mriganka Sur, a neuroscientist who surgically rewired the optic nerves from a ferret’s visual cortex to their auditory cortex and discovered that the ferret learned to see.
This is evidence that the auditory cortex can change to handle visual information.
Implicit in Paul Bach-y-Rita’s work is the idea that we’re all born with a brain that’s more adaptable and opportunistic than we’ve believed.
The brain survives in a changing world by changing itself.
Chapter 2: Building Herself a Better Brain
We start with the case of B.A., a woman who’s body is asymmetrical.
E.g. Longer right leg than left, larger right side than left, left eye less alert, asymmetrical spine.
B.A. also has a confusing set of serious learning disabilities.
E.g. Trouble pronouncing words, impaired spatial reasoning, and constantly losing her possessions.
In graduate school, B.A. came across neuropsychologist Aleksandr Luria’s work.
Luria observed and documented the case of Lyova Zazetsky, a man with symptoms similar to B.A. after a gunshot wound to his head.
The wound was at the junction of the temporal, occipital, and parietal lobe.
Luria realized that Zazetsky couldn’t relate and integrate different units into wholes.
E.g. Relating symbols to meaning or the size of objects to each other.
Around this time, B.A. came across a paper by Mark Rosenzweig that showed rat brain’s in a stimulating vs impoverished environment had more neurotransmitters, were heavier, and had better blood supply than those from the impoverished environment.
Also around this time, B.A. came across a paper by Mark Rosenzweig showing how rats raised in stimulating environments had heavier brains with more neurotransmitters, and had better blood supply than those from impoverished environments.
This evidence shows how neuroplasticity could produce changes in the structure of the brain.
B.A.’s breakthrough was to link Luria’s and Rosenzweig’s research by creating a training program to work on her weakest function: relating symbols to each other.
E.g. Relating the hands of an analog clock to time.
After training, she noticed improvements in her other difficulties related to symbols that she hadn’t trained on.
No notes on how B.A. formed the Arrowsmith school to help other people with cognitive disabilities using brain training. Whether brain training improves cognition is controversial and the evidence either inconclusive or doesn’t exist.
In people, postmortem examinations have shown that training and education increases the number of branches between neurons, pushing neurons farther apart leading to increases in brain volume and thickness.
Chapter 3: Redesigning the Brain
Michael Merzenich invents neuroplastic devices and one neuroscientist described him as “the world’s leading researcher on brain plasticity.”
Merzenich has dozens of ongoing experiments and collaborations, and has started several companies.
Merzenich claims that when learning is consistent with the laws of brain plasticity, our mental capabilities can be improved so that we learn and perceive with greater precision, speed, and retention.
To understand how brain maps can be changed, we first need to understand what brain maps are.
Review of Wilder Penfield’s experiments at the Montreal Neurological Institute in the 1930s.
Brain maps are locations in the brain where different body parts are represented and where their activities are processed.
Penfield mapped the sensory and motor parts of the brain by stimulating the brain using electrodes and asking the patient what they felt or moved.
By stimulating different parts of the motor cortex, Penfield could trigger movements in different body parts.
Penfield discovered that the sensory and motor brain maps are topographical, meaning that adjacent body areas are generally adjacent on the brain’s maps.
It’s was believed that these brain maps were fixed and unchanging, though Penfield himself never made this claim.
Merzenich discovered that these maps are neither unchanging within a brain nor universal across brains, but vary in their borders and size from person to person.
He showed that the shape of our brain maps changes depending on what we do over the course of our lives.
Review of Hubel and Wiesel’s work on micromapping the visual cortex and their discovery of the critical period for vision.
Their work on the visual cortex indicated that the brain is plastic in the critical period and showed how lack of visual stimulation during this period resulted not only in deficits towards the closed eye, but also that the open eye’s brain territory invaded the closed eye’s brain territory.
Further studies suggest that each neural system has a different critical period during which it is especially plastic and sensitive to the environment.
Hubel and Wiesel’s discovery forever changed medical practice as interventions and corrective surgery were performed during critical periods to maximize the chances of success.
Microelectrodes had shown that plasticity is an indisputable fact of childhood.
Using microelectrodes in Mountcastle’s lab in the 1960s, Merzenich showed that plasticity also occurs in adulthood.
Review of central and peripheral nervous system, dendrite, soma, and axon, excitatory and inhibitory signals, synapse. The axon is a living cable.
Merzenich discovered that cortical maps were dynamic by severing the peripheral nerves and seeing how the brain changed in response to input changes.
Neuroscientists were willing to accept Hubel and Wiesel’s discovery that plasticity exists in infancy, but they rejected Merzenich’s discovery that plasticity continues into adulthood.
Merzenich would go on to become a professor and research the auditory cortex, helping to invent the cochlear implant.
In one of Merzenich’s experiments, he cut the nerve relaying sensory information from the middle of a monkey’s hand (median nerve) and saw that the cortical map for that area was invaded by the surrounding hand areas.
This showed that if the median nerve was cut, other functioning nerves would take over the unused map space to process their input.
Thus, brain maps are governed by an ongoing competition for brain area.
Competitive plasticity in adults explains some of our limitations.
E.g. It explains why it’s difficult for most adults to learn a second language and why it’s difficult to break bad habits.
Plasticity doesn’t require nervous system damage but is a constant force.
Our brain maps are constantly changing.
It’s reasonable to assume that if new maps are forming, then new connections must also be forming between neurons.
To explain the formation of new maps, Merzenich used Hebb’s learning rule to explain that simultaneous inputs become more strongly connected, thus forming a map.
To test this hypothesis, Merzenich sewed together two monkey fingers and mapped their cortical representation. What he found was that simulation of either finger would light up a new single map because all movements and sensations in those fingers always occurred simultaneously, thus forming a map.
This shows that the timing of inputs to neurons in the brain is key to forming cortical maps; neurons that fire together in time wired together to make a map.
The reverse is also true: if you separate the signals to neurons in time, you create separate brain maps.
E.g. Neurons that fire apart wire apart or neurons out of sync fail to link.
In his final experiment, Merzenich grafted a small patch of skin from one finger, with its nerve, onto an adjacent finger.
According to the anatomical-hardwiring model, the signals from that patch of skins should light up the map of the original finger. Instead, when the patch was stimulated, the map of its new finger lit up.
Thus, the map for the patch of skin migrated from the brain map of the original finger to its new one, because both the patch and the new finger were simultaneously stimulated.
A brain map is topographically organized if the map is ordered as the body itself is ordered.
Topographic organization is efficient because brain areas that often work together are close together, reducing the distance signals need to travel.
How does topographic organization emerge in a brain map?
Like how a brain map forms, topographic organization emerges from the coincident timing of signals.
E.g. Since thumb and pinkie finger signals tend to arrive at separate times, they have more distant brain maps.
With training and learning, the area of maps can grow to represent more of that body part.
E.g. Monkeys trained on a fine touch discrimination task using their index finger had an enlarged index finger representation in their brain.
As brain maps grow, the neurons in the map become more efficient in two stages.
The first stage is when the map grows to take up more space, using more neurons.
The second stage is when the map uses fewer neurons to perform the task.
E.g. When a child first learns to play the piano, they tend to use their entire upper body, such as wrist, arm, shoulder, to play each note. With practice, however, the pianist stops using irrelevant muscles and only uses the correct finger to play the note.
This more efficient use of neurons happens whenever we become proficient at a skill and explains why we don’t quickly run out of map space as we practice or add skills to our repertoire.
Neurons also become more sensitive as their receptive fields shrink, making the map more precise.
Merzenich also discovered that paying close attention is essential for long-term plastic change as not paying attention did lead to changed brain maps, but that the changes didn’t last.
No notes on Merzenich’s and Paula Tallal’s collaboration on children with delayed language development.
Autism: a human mind that can’t conceive of other minds.
Brain-derived neurotrophic factor (BDNF) is crucial in reinforcing plasticity changes during the critical period in four different ways.
When neurons fire together, they release BDNF which consolidates the connections between neurons and helps to wire them together so they fire together in the future.
BDNF also promotes the growth of myelin sheathing that speeds up the transmission of electrical signals.
During the critical period, BDNF turns on the nucleus basalis, which, together with the attention system, modulates plasticity in the brain.
The final function of BDNF is to help terminate the critical period as once the main neuronal connections are stabilized, there’s less of a need for plasticity.
No notes on how BDNF may explain all of the symptoms of autism.
Chapter 4: Acquiring Tastes and Loves
No notes on sexual plasticity and how some people may switch sexual orientation or gain a new orientation.
Perhaps the ultimate sign of sexual plasticity is that sexuality, as an instinct, can be satisfied without accomplishing its biological goal: reproduction.
As with sex, love is also flexible and its expression has changed throughout history.
The brain structures that regulate instinctive behaviors, the hypothalamus and amygdala, are plastic, so it’s no surprised that the behaviors they’re responsible for, such as sex and emotion, are also plastic.
Neuroplasticity is a property of all nerve tissue, including the brain and spinal cord.
It’s impossible to have just one part of the nervous system be plastic because the systems connected to that system would have to change as well.
No notes on Freud’s sexual stages of development and addiction to pornography.
Review of long-term potentiation (LTP) and long-term depression (LTD).
Unlearning and weakening connections is just as plastic as learning.
If we only strengthened connections, our neural circuits would eventually become saturated.
Neuromodulators are different from neurotransmitters as neurotransmitters can excite or inhibit neurons, while neuromodulators can enhance or diminish the overall effectiveness of synapses.
Neurotransmitter-level implements plasticity and neuromodulator-level implements metaplasticity.
Hardcore masochists interviewed by a researcher must have formed a pathway that linked painful sensations to their sexual pleasure systems, resulting in a new composition experience: pleasurable pain.
Chapter 5: Midnight Resurrections
Details the case of Dr. Michael Bernstein, an eye surgeon that had a stroke that devastated the left side of his body.
After physiotherapy, Dr. Bernstein was able to almost make a full recovery except for a few instances where his left arm feels abnormal and he has some weakness in his left leg.
Stroke is one of the leading causes of disability in adults and there were no effective treatments until Edward Taub invented a new plasticity-based treatment.
Review of behaviorism and how it was weak in explaining voluntary actions.
Reflexological theory of movement: the idea that all of our movement happens in response to stimuli and that we move, not because of our brains, but because of spinal reflexes.
E.g. Knee jerk reflex.
The reflexological theory was used to explain all movement as some reflex not involving the brain.
E.g. If the sensory nerves in a monkey limb were cut, the monkey stopped using that limb. This seems strange because the motor nerves still functioned. One answer is that since movement is a reflex and a reflex is initiated by sensory information, losing sensory information means movement is impaired.
However, another answer is that the monkey might not use their impaired limb because they could use their other good limb, thus compensating for the impairment.
Taub experimented by cutting the sensory nerves in one of the monkey’s arms and put the other good arm in a sling, forcing the monkey to use the impaired arm.
Would putting the good arm in a sling force the monkey to use the impaired arm?
Yes, thus disproving the reflexological theory of movement.
If we cut the sensory nerves in both arms, would the monkey use both arms?
Yes, because the monkey has to survive.
This finding is paradoxical because if one arm was impaired, the monkey wouldn’t use it. However, if both arms were impaired, then the monkey would use both.
Even cutting all of the sensory inputs to the spinal cord resulted in monkeys using their limbs.
Interestingly, if the impaired arm is put into a sling and then removed three months later, the monkey was able to use the impaired limb, suggesting that there’s a period of “learned nonuse” right after cutting the sensory nerves.
Learned nonuse might be masking a patient’s ability to recover from stroke.
By preventing stroke patients from using their good limb, Taub showed that 80 percent of stroke patients who had lost arm functionality can improve substantially with training of their weak arm. This method of therapy is called constraint-induced (CI) therapy.
These improvement show that the brain is plastic and is capable of reorganization after damage.
Constraint-induced therapy doesn’t work well with reinforcement learning because if the reward is only given after performing the task, the monkey wouldn’t use their impaired limb. Instead, behavior must be shaped by first rewarding small movements, and then rewarding progressively larger movements.
After a stroke and disuse of a limb, movement is harder not only because of muscle atrophy but also because of brain atrophy.
Brain scans of stroke patients show that the brain map for the affected limb shrinks by about half.
After CI therapy, the size of the brain map returns to its original size.
Currently, Taub is exploring the optimal length of CI therapy and results suggest that three hours a day with many movements is better than an exhausting six hours of treatment with fewer movements.
Training principles
Training is more effective if the skill closely relates to everyday life
Training should be done in increments
Training should be concentrated
Merzenich found that when the sensory input from a finger was cut off, the brain map associated with it typically changes in a 1 to 2 millimeter area of cortex.
However, is this the limit of neuroplasticity?
Further experiments revealed that neuroplastic change can occur up to 14 millimeters away, showing that the brain can massively rewire itself.
Chapter 6: Brain Lock Unlocked
Some people, the ones that excessively worry, are so constantly traumatized by their own brain that they often consider suicide.
There are many kinds of worriers and many kinds of anxiety.
E.g. Phobias, PTSD, panic attacks, and OCD.
OCD often worsens over time, gradually altering the structure of the brain.
The agony of the obsessive worrier is that whenever something bad is possible, it feels inevitable.
After the obsessive worries begin, OCD patients typically do something to diminish the worry, a compulsive act.
E.g. If they worry about germs, they’ll constantly wash themselves and disinfect their environment.
E.g. In one case, a man feared being contaminated by the battery acid spilled in car accidents and would drive to the site of nearby accidents. After the responders left, he would scrub the road with a brush for hours to remove the danger.
OCD is very difficult to treat as medications and behavior therapy are only partially successful.
Three steps in correcting a mistake
First we get a feeling that something is wrong.
Second we become anxious which drives us to fix the mistake.
Third when we’ve corrected the mistake, our brain allows us to move on.
In the brain of OCD people though, they don’t complete the third step allowing them to move on.
Three parts of the brain are involved in obsessions
Orbital frontal cortex: detects mistakes and is what causes the “mistake feeling”.
Cingulate gyrus: triggers the anxiety that something bad is going to happen unless we correct the mistake.
Caudate nucleus: allows our thoughts to flow from one to the next.
In OCD, brain scans show that all three brain areas are hyperactive.
One psychiatrist researcher, Jeffrey M. Schwartz, set out to develop a treatment for OCD that would change the activity of these three areas.
Schwartz wondered whether patients could shift the caudate nucleus manually by paying constant attention to something besides the worry, such as a new, pleasurable activity.
The first step is to label the obsession as the problem and not the object of the obsession.
E.g. Having OCD patients relabel their problem not as germs or battery acid, but as OCD itself.
One issue with current OCD treatments is that they also focus on the object of obsession, but the patient knows that they’re being obsessive. The problem is that they feel compelled to act on such obsessions.
The second step is to refocus to another activity during an obsessive episode, thus weakening the link between the compulsion and the reward from reducing anxiety.
Schwartz has had good results with severe cases and when he and his team scanned the brains of their improved patients, they found that the three “locked” parts of the brain had begun to fire separately.
Chapter 7: Pain
Review of V. S. Ramachandran’s work.
Ramachandran hates crowds in science and doesn’t like large scientific meetings.
He also has a disdain for complicated fancy equipment due to the time required to learn how to use it and that the conclusions drawn from such equipment are often too far removed from the brain.
Review of phantom limb syndrome.
Phantom limbs are a problem because they can result in chronic phantom pain in 95 percent of amputees, often for their lifetime.
Phantom pain can also be due to amputation of the internal organs.
E.g. The need to urinate even though the bladder has been removed. Menstrual cramps and labor pains after removing the uterus.
If normal pain signals body damage, then phantom pain signals both body and pain system damage.
Our pain maps get damaged and signal false alarms, making us believe the problem even though it’s long gone.
After reading about Edward Taub’s plasticity experiments on monkeys, Ramachandran immediately thought that plasticity might explain phantom limbs because the brain maps for both monkeys and patients have been deprived of stimuli from their limbs.
For Taub’s monkeys, stroking the impaired arm activated the face brain region. So, would stroking the monkey’s face feel like their face was being stroked or that their impaired arm was being stroked?
In one of Ramachandran’s experiments on a phantom limb patient, T.S., stroking the patient’s cheek felt both like touching their face and touching their phantom limb.
When Ramachandran put a drop of warm water on T.S.’s cheek, they felt a warm trickle move down their cheek and down their phantom limb.
Using MEG, Ramachandran mapped T.S.’s arm and hand and found that their hand map was now being used to process facial sensations.
In T.S., their hand and face maps had blurred together.
Thus, neural plasticity and map invasion explains the phantom limb syndrome.
Our skin has unique receptors to detect temperature, vibration, and pain. Sometimes after injury, the nerve fibers conveying touch information can be cross wired, resulting in feeling pressure as pain.
Phantom maps, and all brain maps, are constantly changing borders and area.
Phantom limbs can not only cause phantom pain, but also the feeling that the phantom limb is frozen, unable to move.
This feeling of paralysis might be due to the lack of feedback the motor system receives from the amputated limb.
To fix this, Ramachandran developed an ingenious method to generate phantom sensation.
Using a mirror box, the patient can fool their own brain by placing their working limb on one side of the mirror and believing that the mirror image is the amputated limb.
Similar to the rubber hand illusion, the brain plugs in the illusion, making it feel like the actual limb.
One patient used the mirror box to remove his phantom limb and associated phantom pain.
So, V. S. Ramachandran became the first physician to successfully amputate a phantom limb.
The mirror box appears to cure pain by changing the patient’s perception of their body image.
Pain and body image are closely related because we always experience pain as projected onto the body.
We don’t need a physical body or even pain receptors to feel pain as we only need the body image produced by our brain maps.
People with actual limbs don’t realize this because their body image is perfectly projected onto their physical body, making it impossible to distinguish between body image and physical body.
However, phantom limbs enable us to make this distinction as physical body is gone while body image remains.
Body image can be thought of as a copy or representation of the body in the brain. But if one copy changes without notifying the other, there’s a mismatch between copies leading to phantoms.
Distorted body images are common and are further evidence that body image and physical body are separate.
E.g. Anorexia, body dysmorphia.
Review of the rubber hand experiment and the gate control theory of pain.
Chapter 8: Imagination
Review of transcranial magnetic stimulation (TMS) and the researcher Alvaro Pascual-Leone.
Pascual-Leone was one of the first to use TMS to map the brain.
Pascual-Leone’s TMS experiments on people learning Braille suggest different plastic mechanisms for learning.
E.g. One for strengthening existing neuronal connections and one for creating brand new connections.
Maintaining improvement and making a skill permanent requires the slow and steady work of forming new connections.
It’s well known that the blind can develop superior nonvisual senses and that Braille readers gain extraordinary sensitivity in their Braille-reading fingers.
Pascual-Leone reasoned that if the visual cortex helped blind people read Braille, then blocking it with TMS should interfere with Braille reading. It does.
Blocking TMS applied to the visual cortex of sighted people had no effect on their ability to feel, suggesting that something unique was happening to the blind Braille readers.
E.g. A part of the brain devoted to one sense had become devoted to another.
Pascual-Leone also showed that the better a person could read Braille, the more their visual cortex was used.
Review of mental practice and it’s impact on performance.
Mental practice does improve performance, but not to the same level as physical practice.
One reason we can change our brains simply by imagining is because imagining an act and doing it aren’t so different.
Brain scans show that in action and imagination, many of the same brain regions are activated.
Thought translation machines work because the brain is plastic and physically changes its state and structure as we think, which can be measured and decoded by electronic measurements.
Similar behaviors, performed at different times, use different circuits due to plasticity.
This is the difference between plasticity and elasticity.
The plastic brain is perpetually altered by every encounter, every interaction, but this leads to the problem of how we can perform stable behaviors.
If we view the brain as analogous to a snowy hill, and performing a behavior as going down the hill with a sled, then over time, certain paths will become used more than others.
Once these tracks have been laid down, it’s difficult to get out of, thus creating stability.
To take a different path becomes increasingly difficult.
When Pascual-Leone blindfolded normal people for five days, he was surprised to find just how rapidly the brain reorganized itself in a few days.
Using brain scans, he found that it could take as few as two days for the visual cortex to start processing tactile and auditory input.
Absolute darkness was vital to the change because if any light was received, the visual cortex preferred to process it over sound and touch.
When the blindfolds were removed, the subjects’ visual cortex stopped responding to sound and touch within twelve to twenty-four hours.
Given the rapid switch in visual cortex processing, Pascual-Leone believed that there wasn’t enough time in two days for the brain to grow new connections, meaning the connections must have already existed.
Instead, preexisting paths were unmasked by blocking vision, a sort of neural Darwinism where different brain regions compete to process sensory signals.
Imagining an act engages the same motor and sensory programs involved in doing it.
Chapter 9: Turning Our Ghosts into Ancestors
Details the case of Mr. L who had depression and various other mental problems.
Using psychoanalysis as a neuroplastic therapy, Mr. L was able to change important parts of his character.
Review of Eric Kandel’s work on neuroplasticity.
Kandel was the first to show that when we form long-term memories, neurons change their anatomical shape and increase the number of synaptic connections they have to other neurons.
E.g. In the sea snail Aplysia, stimulation to the exposed gills causes a gill-withdrawal reflex. Repeated stimulation can either sensitize or habituate the reflex, and this learned behavior was detected as a change in connection strength between neurons. This was the first proof that learning and neuroplastic strengthening are causally connected.
Furthermore, Kandel, James Schwartz, and colleagues showed that for short-term memories to become long-term, a new protein had to be made in the cell that changes the structure of nerve endings, causing it to grow new connections between neurons.
Kandel argues that psychotherapy changes people by learning, which causes changes in gene expression that modify the strength of synaptic connections and the structural pattern of connections.
Review of Sigmund Freud’s work such as law of association and free association.
Free association is based on the belief that all of our mental associations, even seemingly random ones, are expressions of links formed in our memory networks.
Review of procedural/implicit and declarative/explicit memory.
Plasticity paradox: that the same neuroplastic properties that allow us to change our brains and produce more flexible behaviors can also allow us to produce more rigid behaviors.
As Pascual-Leone’s metaphor shows, neuroplasticity is like fresh snow on a hill.
The first few times we go down, we can choose any path thus representing flexibility.
However, if we choose the same path again and again, tracks start to develop that push our sled into established paths thus representing stability.
Our route will now be quite rigid as neural circuits, once established, tend to become self-sustaining.
Chapter 10: Rejuvenation
We start with the case of Dr. Stanley Karansky, a ninety-year-old family doctor that does mental and physical exercises to maintain his cognitive and physical functions.
Other organs make new tissues from stem cells, but no stem calls are found in the brain.
E.g. Cut skin can heal itself, fractured bones can mend themselves, our liver and intestines can repair themselves, and blood can be created in bone marrow.
One reason why the human brain can’t repair itself is that our neurons evolved to be so complex and specialized that it lost the power to produce replacement cells.
However, the first human neuronal stem cells were discovered by Frederick Gage and Peter Eriksson in 1998 in the hippocampus.
Neurogenesis: the process by which new neurons are formed.
Neuronal stem cells were long overlooked, in part, because they went against the idea that the brain is like a complex machine or computer, and machines don’t grow new parts.
In the 1980s, Fernando Nottebohm was surprised to find that birds sing new songs every season. After examining their brains, he found that every year, during bird-singing season, birds grow new brain cells in the area responsible for song learning.
Through the long history of the discovery of neurogenesis, we now know that new neurons are formed in the hippocampi and olfactory bulb.
Long-term enrichment, such as new toys and new environments, was found to have a significant effect on promoting neurogenesis in the mouse brain.
Two ways to increase neuron numbers
Create new neurons
Extend the life of existing neurons
New environments may trigger neurogenesis, while new toys may cause neurons to live longer.
Thus, these two processes work in complementary ways to first create new stem cells and then to prolong their survival.
Although neurogenesis is one way to improve brain function, another paradoxical way is by losing neurons.
E.g. The massive pruning of unused neurons and synapses during adolescence provides the remaining neurons with more resources such as blood, oxygen, and energy.
As we age, we tend to perform cognitive activities in different lobes of the brain compared to when we were young.
This shift within the brain is another sign of plasticity.
Cognitive reserve: the theory that people with more education seem better protected from mental decline because they have more networks devoted to mental activity.
Chapter 11: More than the Sum of Her Parts
Michelle Mack never developed a left hemisphere and is missing half of her brain.
She lives a normal live because her right hemisphere took over for her left hemisphere.
However, she does have difficulties such as easily getting lost in unfamiliar surroundings and has trouble with certain kinds of abstract thought.
Furthermore, the lack of a left hemisphere has Michelle using a brace to support her right leg, having an impaired right visual field, and being sensitive to sensory stimuli.
Four kinds of neuroplasticity
Map expansion: when a map’s borders grow due to continued use and practice.
Sensory reassignment: when one sense is blocked, the brain area responsible for that sense can receive new input from another sense.
Compensatory masquerade: compensating for lack of skill in one sensory domain by using another sense.
Mirror region takeover: when part of one hemisphere fails, the mirror region in the opposite hemisphere adapts and takes over the function.
Michelle may have developed a superior registration for events because her right hemisphere was never inhibited by her nonexistent left hemisphere.
It would seem that the most frightening thing about brain disease is that we lose our mental functions.
But an equally frightening thought is that brain disease leads us to express parts of ourselves we wish didn’t exist.
Parts of the brain are inhibitory and when we lose that inhibition, unwanted drives and instincts emerge.
Appendix 1: The Culturally Modified Brain
What’s the relationship between culture and the brain?
Both influence each other in a feedback loop.
E.g. Brains produce culture, culture changes brains.
Playing music requires extraordinary demands from the brain and in musicians who practice stringed instruments, the brain map for their active left hand becomes larger.
Review of the increased volume of hippocampus in London taxi drivers.
Plasticity creates a new way for organisms to evolve beyond genetic mutation and variation.
Change in one brain area is propagated to its connected areas.
E.g. Learning, practice, and lesions.
To a larger extent than we believed, culture determines what we can and can’t perceive.
The idea that culture can change fundamental brain functions such as sight and listening is a radical one.
As we use an electronic medium, our nervous system extends outward while the medium extends inward.
Appendix 2: Plasticity and the Idea of Progress
Any change in how we understand the brain ultimately affects how we understand human nature.