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Brain Plasticity 
In 1861 French neurosurgeon Paul Broca correlated injured or surgically affected regions of the brain with certain lost 
skills, such as fine motor skills or language ability. For more than a century scientists believed these regions were 
hardwired for specific tasks. Although certain brain areas do tend to be used for particular types of skills, we now 
understand that such assignments can be changed in response to brain injury such as a stroke. In a classic 1965 study, 
D. H. Hubel and T. N. Wiesel showed that extensive and far-reaching reorganization of the brain could take place after 
damage to the nervous system, such as from a stroke.
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Moreover, the detailed arrangement of connections and synapses in a given region is a direct product of how 
extensively that region is used. As brain scanning has attained sufficiently high resolution to detect dendritic-spine 
growth and the formation of new synapses, we can see our brain grow and adapt to literally follow our thoughts. This 
gives new shades of meaning to Descartes' dictum "I think therefore I am." 

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In one experiment conducted by Michael Merzenich and his colleagues at the University of California at San 
Francisco, monkeys' food was placed in such a position that the animals had to dexterously manipulate one finger to 
obtain it. Brain scans before and after revealed dramatic growth in the interneuronal connections and synapses in the 
region of the brain responsible for controlling that finger. 
Edward Taub at the University of Alabama studied the region of the cortex responsible for evaluating the tactile 
input from the fingers. Comparing nonmusicians to experienced players of stringed instruments, he found no 
difference in the brain regions devoted to the fingers of the right hand but a huge difference for the fingers of the left 
hand. If we drew a picture of the hands based on the amount of brain tissue devoted to analyzing touch, the musicians' 
fingers on their left hand (which are used to control the strings) would be huge. Although the difference was greater 
for those musicians who began musical training with a stringed instrument as children, "even if you take up the violin 
at 40," Taub commented, "you still get brain reorganization."
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A similar finding comes from an evaluation of a software program, developed by Paula Tallal and Steve Miller at 
Rutgers University, called Fast ForWord, that assists dyslexic students. The program reads text to children, slowing 
down staccato phonemes such as "b" and "p," based on the observation that many dyslexic students are unable to 
perceive these sounds when spoken quickly. Being read to with this modified form of speech has been shown to help 

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such children learn to read. Using fMRI scanning John Gabrieli of Stanford University found that the left prefrontal 
region of the brain, an area associated with language processing, had indeed grown and showed greater activity in 
dyslexic students using the program. Says Tallal, "You create your brain from the input you get." 
It is not even necessary to express one's thoughts in physical action to provoke the brain to rewire itself. Dr. 
Alvaro Pascual-Leone at Harvard University scanned the brains of volunteers before and after they practiced a simple 
piano exercise. The brain motor cortex of the volunteers changed as a direct result of their practice. He then had a 
second group just think about doing the piano exercise but without actually moving any muscles. This produced an 
equally pronounced change in the motor-cortex network.
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Recent fMRI studies of learning visual-spatial relationships found that interneuronal connections are able to 
change rapidly during the course of a single learning session. Researchers found changes in the connections between 
posterior parietal-cortex cells in what is called the "dorsal" pathway (which contains information about location and 
spatial properties of visual stimuli) and posterior inferior-temporal cortex cells in the "ventral" pathway (which 
contains recognized invariant features of varying levels of abstraction);
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significantly, that rate of change was directly 
proportional to the rate of learning.
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Researchers at the University of California at San Diego reported a key insight into the difference in the formation 
of short-term and long-term memories. Using a high-resolution scanning method, the scientists were able to see 
chemical changes within synapses in the hippocampus, the brain region associated with the formation of long-term 
memories.
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They discovered that when a cell was first stimulated, actin, a neurochemical, moved toward the neurons 
to which the synapse was connected. This also stimulated the actin in neighboring cells to move away from the 
activated cell. These changes lasted only a few minutes, however. If the stimulations were sufficiently repeated, then a 
more significant and permanent change took place. 
"The short-term changes are just part of the normal way the nerve cells talk to each other," lead author Michael A. 
Colicos said. 
The long-term changes in the neurons occur only after the neurons are stimulated four times over the course 
of an hour. The synapse will actually split and new synapses will form, producing a permanent change that 
will presumably last for the rest of your life. The analogy to human memory is that when you see or hear 
something once, it might stick in your mind for a few minutes. If it's not important, it fades away and you 
forget it 10 minutes later. But if you see or hear it again and this keeps happening over the next hour, you are 
going to remember it for a much longer time. And things that are repeated many times can be remembered for 
an entire lifetime. Once you take an axon and form two new connections, those connections are very stable 
and there's no reason to believe that they'll go away. That’s the kind of change one would envision lasting a 
whole lifetime. 
"It's like a piano lesson," says coauthor and professor of biology Yukiko Goda. "If you playa musical score over 
and over again, it becomes ingrained in your memory." Similarly, in an article in Science neuroscientists S. Lowel and 
W. Singer report having found evidence for rapid dynamic formation of new interneuronal connections in the visual 
cortex, which they described with Donald Hebb's phrase "What fires together wires together."
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Another insight into memory formation is reported in a study published in Cell. Researchers found that the CPEB 
protein actually changes its shape in synapses to record mernories.
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The surprise was that CPEB performs this 
memory function while in a prion state. 
"For a while we've known quite a bit about how memory works, but we've had no clear concept of what the key 
storage device is," said coauthor and Whitehead Institute for Biomedical Research director Susan Lindquist. "This 
study suggests what the storage device might be—but it's such a surprising suggestion to find that a prion-like activity 
may be involved....It ... indicates that prions aren't just oddballs of nature but might participate in fundamental 
processes." As I reported in chapter 3, human engineers are also finding prions to be a powerful means of building 
electronic memories. 

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Brain-scanning studies are also revealing mechanisms to inhibit unneeded and undesirable memories, a finding 
that would gratify Sigmund Freud.
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Using fMRI, Stanford University scientists asked study subjects to attempt to 
forget information that they had earlier memorized. During this activity, regions in the frontal cortex that have been 
associated with memory repression showed a high level of activity, while the hippocampus, the region normally 
associated with remembering, was relatively inactive. These findings "confirm the existence of an active forgetting 
process and establish a neurobiological model for guiding inquiry into motivated forgetting," wrote Stanford 
psychology professor John Gabrieli and his colleagues. Gabrieli also commented, "The big news is that we've shown 
how the human brain blocks an unwanted memory, that there is such a mechanism, and it has a biological basis. It gets 
you past the possibility that there's nothing in the brain that would suppress a memory—that it was all a misunderstood 
fiction." 
In addition to generating new connections between neurons, the brain also makes new neurons from neural stem 
cells, which replicate to maintain a reservoir of themselves. In the course of reproducing, some of the neural stem cells 
become "neural precursor" cells, which in turn mature into two types of support cells called astrocytes and 
oligodendrocytes, as well as neurons. The cells further evolve into specific types of neurons. However, this 
differentiation cannot take place unless the neural stem cells move away from their original source in the brain's 
ventricles. Only about half of the neural cells successfully make the journey, which is similar to the process during 
gestation and early childhood in which only a portion of the early brain's developing neurons survive. Scientists hope 
to bypass this neural migration process by injecting neural stem cells directly into target regions, as well as to create 
drugs that promote this process of neurogenesis (creating new neurons) to repair brain damage from injury or 
disease.
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An experiment by genetics researchers Fred Gage, G. Kempermann, and Henriette van Praag at the Salk Institute 
for Biological Studies showed that neurogenesis is actually stimulated by our experience. Moving mice from a sterile, 
uninteresting cage to a stimulating one approximately doubled the number of dividing cells in their hippocampus 
regions.
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