Transcript Eagleman Ch 4. Neuroplasticityx

4: Neuroplasticity
Cognitive Neuroscience
David Eagleman
Jonathan Downar
Chapter Outline
The Brain Dynamically Reorganizes to
Match Its Inputs
 The Brain Distributes Resources Based on
Relevance
 The Brain Uses the Available Tissue
 A Sensitive Period for Plastic Changes
 Hardwiring versus World Experience
 The Mechanisms of Reorganization
 Changing the Input Channels
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The Brain Dynamically
Reorganizes to Match Its Inputs
Changes to the Body Plan
 Changes to Sensory Input
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Changes in the Body Plan
The brain is constantly changing,
reorganizing with each new experience.
 Plasticity is the ability to change and to
retain that new structure.
 Plastic changes must be relevant.
 Some systems have a sensitive period
early in life when they have greater
plasticity.
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Changes in the Body Plan
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The homunculus is the map of the body
within the sensory and motor cortices.
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Changes in the Body Plan
Changes to the body, such as losing a
limb, can result in changes to the
representation of the body in the brain.
 Sensory areas that responded to the
damaged part of the body are taken over
by adjacent sensory areas.
 Phantom limb pain is pain that seems to
come from the missing body part.
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Changes in the Body Plan
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Changes to Sensory Input
Removing or altering sensory input, even
on a temporary basis, can cause a
remapping of the brain.
 The speed of this remapping suggests that
there are existing connections that can be
unmasked.
 Such reorganization has been observed in
auditory and visual systems.
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Changes to Sensory Input
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The Brain Distributes Resources
Based on Relevance
The Role of Behavior
 The Role of Relevance: Gating Plasticity
with Neuromodulation
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The Role of Behavior
The brain uses adaptive coding, altering
the amount of resources assigned to a
function depending on how important it is.
 Sensory and motor representations will
reorganize based on the particular skills
and needs of the person.
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The Role of Behavior
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The Role of Relevance: Gating
Plasticity with Neuromodulation
The behavior must be relevant to the
organism to result in plasticity.
 Plasticity can be turned on or turned off
(gated) in particular places at particular
times.
 Neuromodulators, such as acetylcholine,
control this gating.
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The Role of Relevance: Gating
Plasticity with Neuromodulation
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The Brain Uses the Available
Tissue
Maps Adjust Themselves to the Available
Brain Tissue
 Cortical Reorganization after Brain
Damage
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Maps Adjust Themselves to the
Available Brain Tissue
Maps will make use of the available
amount of brain tissue.
 Research with the visual system of
tadpoles found that the input makes use of
the available brain area, whether there is
less brain area or more input.
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Maps Adjust Themselves to the
Available Brain Tissue
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Cortical Reorganization after
Brain Damage
Following injury to the central nervous
system, some function tends to be
recovered as swelling decreases.
 Cortical reorganization can occur over a
longer period of time to allow further
recovery of function.
 The language problems of aphasia tend to
decline as the right hemisphere takes
over.
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A Sensitive Period for Plastic
Changes
A Window of Time to Make Changes
 The Sensitive Period in Language
 Neuromodulation in Young Brains
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A Window of Time to Make
Changes
Plasticity is greatest during periods of
development known as sensitive periods.
 After the sensitive period has passed,
plasticity is still possible, but not as easy.
 The success of treatment for strabismus
(lazy eye) early in life is an example of
these sensitive periods.
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The Sensitive Period in
Language
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Acquisition of a second language supports
the idea of sensitive periods.
 If
you are exposed to a second language
before age 7, you will be as fluent as a native
speaker.
 If exposed between 8 – 10 years, it will be
harder to achieve fluency.
 If exposed after age 17, fluency will be low.
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The Sensitive Period in
Language
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Neuromodulation in Young
Brains
In humans, young people have greater
brain plasticity.
 There is a tradeoff between plasticity and
efficiency, and, as your brain gets better at
some tasks, it becomes less able to
perform other tasks.
 Young animals show widespread plasticity
without needing attentional focus.
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Hardwiring versus World
Experience
Aspects of the Brain Are Preprogrammed
 Experience Changes the Brain
 Brains Rely on Experience to Unpack
Their Programs Correctly
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Aspects of the Brain Are
Preprogrammed
We are born with certain reflexes, such as
grasping and sucking.
 Sperry conducted studies of the newt
visual system and developed the
chemoaffinity hypothesis.
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 Connections
within the visual system are
preprogrammed to follow chemical cues to
reach their target.
 Chemical cues can be attractive or repulsive.
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Aspects of the Brain Are
Preprogrammed
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Experience Changes the Brain
The environment alters the brain and
affects the brain’s ability to learn.
 Rats in an enriched environment have
more extensive dendrites.
 Neurons in the language area known as
Wernicke’s Area have more elaborate
dendrites in college-educated individuals.
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Experience Changes the Brain
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Brains Rely of Experience to
Unpack Their Programs Correctly
The environment not only influences brain
development, but is necessary for
development.
 The encoding discussed previously is only
at a general level.
 Experience is required to refine the
connections.
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Brains Rely of Experience to
Unpack Their Programs Correctly
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Kittens raised with strabismus do not
develop binocular vision because they do
not get appropriate input from both eyes.
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Brains Rely of Experience to
Unpack Their Programs Correctly
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The Mechanisms of
Reorganization
Neurons Compete for Limited Space
 Competition for Neurotrophins
 Rapid Changes: Unmasking Existing
Connections
 Slow Changes: Growth of New
Connections
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Neurons Compete for Limited
Space
Neurons, axons, and dendrites need to
compete for resources to survive.
 The initial connections to the muscles and
the visual system are refined over time by
activity-depended plasticity.
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Neurons Compete for Limited
Space
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Neurons Compete for Limited
Space
Pruning is the process of removing
neurons and processes that are not
needed.
 Apoptosis is a form of cell death that is
normal in development and enables the
cells to die without affecting adjacent
neurons.
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Competition for Neurotrophins
Neurotrophins are chemicals that help to
sustain the neurons.
 Generally, they are secreted by the target
to promote survival in the neurons that
reach the target.
 They allow the cell to differentiate.
 In young cells, they prevent apoptosis in
cells that make appropriate connections.
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Competition for Neurotrophins
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Rapid Changes: Unmasking
Existing Connections
Many existing connections are masked by
activity within the nervous system.
 Other connections predominate and inhibit
the weaker connections that exist.
 Following damage or loss of input, this
inhibition is lost, unmasking these
connections.
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Rapid Changes: Unmasking
Existing Connections
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Slow Changes: Growth of New
Connections
Longer-term changes, over weeks or
months, are likely due to the growth of
new connections.
 If the short-term changes are
advantageous, then growth of new
connections will follow.
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Slow Changes: Growth of New
Connections
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Changing the Input Channels
The plasticity of the brain enables new
forms of input.
 For example, the brain can learn to
interpret input from a retinal implant.
 The BrainPort enables different sensations
to be delivered tactilely to the tongue.
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Changing the Input Channels
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