S114'740 Special Course in Communication and Cognition: Neural Plasticity

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S-114.740 Special Course in Communication and
CognitionNeural Plasticity

• Iiro P. Jääskeläinen, Ph.D., Professor
• Cognitive Science and Technology
• Laboratory of Computational Engineering

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What is plasticity?

• Functional organization of the brain reflects
  adaptation to environment
• As long as the environment (and the neural
  systems) stay approximately the same, functional
  organization remains the same
• Changes in the environment and in the neural
  systems (such as after a lesion) trigger plastic
  changes to facilitate re-adaptation

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Different kinds of plasticity

• Developmental plasticity (immature brain first
  begins to process sensory information)
• Activity-dependent plasticity (changes in sensory
  input due to, e.g., eyesight problems)
• Plasticity of learning and memory (e.g.
  discrimination training)
• Injury-induced plasticity (following brain damage)

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Plasticity and developing nervous system
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Development and plasticity

• Critical sensitivity periods in development
• Language acquisition (1st and 2nd)
• Pruning as an underlying mechanism?
• initially more connections than in the mature CNS
• Damage early during development ? relatively
  minimal adverse effects (e.g., hydrocephalus
  findings)

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Critical sensitivity periods
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Pruning neurons that fire together, wire
together
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NMDA-receptors and synaptic plasticity

• Convergent pre-synaptic activity leads to
  strenghtening of synaptic connections
• Magnesium blockade of NMDA receptors is removed
  by depolarization ? Ca2 influx ? plasticity

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Plastic changes after loss of sensory input
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Cross-modal plasticity in congenitally deaf

• These PET/MR images show increased neural
  activity in the superior temporal gyrus in
  congenitally deaf subjects when they viewed signs
  or sign-like movements, suggesting that auditory
  cortical regions may contribute to the processing
  of visual information in the deaf

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... and in congenitally blind
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Changes in sensory input induce plastic changes
in somatotopy

• Spinal cord injuries in adult monkeys result in
  somatosensory reorganization of the topographic
  map in area 3b. The region of the map that
  normally processes sensory information from the
  hand now receives sensory inputs from the face.

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Following removal of sensory input
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3rd example
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Plastic changes induced experimentally

• Changing the external stimulus environment
• Reversal of the visual world with goggles ? after
  a period of days, switching of the view to normal
  despite goggles
• Sensory deprivation and hallucinations
• Somatosensory two-point discrimination training ?
  changes in somatosensory homonculus

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Short-term plasticity
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Short-term plasticity a short definition

• Influence of previous stimuli (i.e., memory),
  top-down effects (e.g., attention), and learning
  (longer-term plasticity), on how the sensory
  systems filter stimuli, enabling tracking of and
  reacting to relevant objects

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Paired-pulse effects

• The simplest form of short-term plasticity is
  perhaps manifested in paired-pulse effects
• paired-pulse depression
• paired-pulse facilitation
• Short-lived changes in amplitude and latency of
  responses to the second stimulus of a pair
• Sensory memory?

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Neural tuning

• Auditory-system neurons exhibit selective
  responses to certain stimulus attributes over
  others
• Combined with PPD/PPF, neural tuning can explain
  short-term sensory memory

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Differential adaptation of N1m(a) and N1m(p)
explain the mismatch response
Differential adaption of anterior and posterior
sources contributing to the overall N1m
response explains the differences in ECD loci
between the MMNm and N1m Anterior N1m slower in
latency, sharp frequency tuning. Related to the
what processing stream? Posterior N1m fast,
only coarse frequency tuning. Related to the
where processing stream?
Jääskeläinen et al. PNAS 101 68096814, 2004
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Selective attention tunes responses to 3-D vs.
phonetic
Stimulus pairs varying in both phonetic (/ö/ vs.
/ä/) and 3-D location features Task of the
subject is the pair same or different
with respect to the preceding pair in 3-D
location or phonetic content? Passive ignore
condition
Ahveninen, Jääskeläinen et al. in preparation
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Combined 3-T fMRI (Siemens Trio) and 306-channel
MEG (Neuromag VectorView) data suggested sharper
neural tuning in areas posterior to primary
auditory cortex. Selective attention to 3-D
significantly augmented this.
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Corroborating macaque findings on the what and
where

• Monkey studies suggesting anterior (AL) what
  and posterior (CL) where processing pathways in
  the auditory cortex
• Spatial location vs. species-specific
  vocalizations
• Visual system analogy?

Rauschecker Tian PNAS 97 1180011806, 2000
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Gain vs. tuning an open question

• Several studies have contrasted the hypotheses of
  gain vs. tuning as the neural basis for selective
  attention
• Possible tuning mechanisms include narrowing of
  and shifts in tuning curve

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Is there tonotopy at all?

• While BFs to pure tones disclose tonotopic
  organization, the responses even at BF are not
  vigorous
• Stimulation sweeping at certain speed over the BF
  elicit most robust responses in AC neurons
• Spectrotemporal receptive fields

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Dynamic STRF changes in AC
Fritz J et al. Nature Neuroscience 61217-1223,
2003
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Modulation of primary auditory cortex activity by
visual speech
During continous scanner noise, seeing movies of
visual articulations vs. a still-face baseline
significantly activated the human primary
auditory cortex Dynamic modulation of primary
auditory cortex STRFs aiding speech perception?
Pekkola et al. submitted
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AC vs. subcortical structures

• Corticofugal influence electrical stimulation of
  auditory cortex causes modulation of STRFs at
  lower auditory system structures, MGB, IC, even
  cochlea!
• Animal data suggest that the lower one goes, the
  longer time it takes to see such changes
• AC as the initiator of modulatory effects?

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Short-term plasticity and the somatosensory system

• Local anesthesia of a finger causes relatively
  rapid changes in cortical representation areas
• These changes are quickly reversed to normal upon
  normalization of stimulation
• Dormant connections between areas as underlying
  neural mechanism?

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Attention and gain in somatosensation

• When attention is directed to the tactile
  stimulus, the response of the neurons in the
  somatosensory cortex is enhanced, compared to
  when attention is directed to visual stimuli.

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Attention and plastic changes

• Attention to certain stimulus features required
  for short-term plastic changes to occur
• Transfer of short-term plastic changes to
  long-term ones?

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Neurochemistry and plasticity

• Selective lesions of central noradrenergic
  pathways impair recovery after a subsequent
  injury to the cerebral cortex. Drugs that deplete
  central norepinephrine, block alpha 1-adrenergic
  receptors, or decrease norepinephrine release
  (alpha 2-adrenergic receptor agonists) impede
  recovery whereas drugs that increase
  norepinephrine release (alpha 2-adrenergic
  receptor antagonists) or sympathomimetics
  (amphetamine) facilitate recovery
• N.B. NE is a neurochemical correlate of
  attention!
• Also, acetylcholine suggested to be vital for
  plasticity

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Brain injury, rehabilitation and recovery

• How quickly does the injury occur?
• Brain tumors, hydrocephalus ? slow destruction of
  brain matter, time for adaptive / plastic changes
• Brain tumors can be large before any symptoms are
  noticed
• Stroke sudden loss of areas, drastic behavioral
  / cognitive effects

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Spontaneous recovery

• Spontaneous recovery from, e.g., stroke
• Quick recovery of functions during the first
  three months after injury
• Slower recovery thereafter

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Re-occurrence of injury

• After having sufffered brain damage (e.g.,
  stroke), another stroke usually has significantly
  larger detrimental effects
• Plastic reserve has been drained

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Rehabilitation

• Circumventing the problem
• anterograde amnesia after stroke learning to use
  notebook
• relatively effortless way to correct problem
• Rehabilitation of function
• anterograde amnesia after stroke performing
  highly specific memory tasks, thus enhancing
  memory performance
• diagnostics problems, persistence

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Rehabilitation

• Needs to be specific
• loss of visual field (scotoma)
• attention to to stimulation at the edges of the
  scotoma result gradually in smaller scotoma size
• attention required!
• How to design specific rehabilitation of e.g.
  executive functions?
• Symptom self-recognition low

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Imaginary training in rehabilitation?

• Paralysis due to stroke may prevent early
  participation in a rehabilitation program
• Similar network of cerebral structures (e.g.,
  premotor cortex) is activated when normal control
  subjects execute physically or imagine a sequence
  of up-down foot movements ? mental practice with
  motor imagery can be used as a therapeutic
  approach to keep active the neural circuits
  involved in locomotion, facilitating the
  rehabilitation of patients who sustained damage
  to the brain(?)

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Neurogenesis in the brain

• Traditionally thought that new neurons are not
  produced in the brain
• Recent studies have yielded tentative evidence
  for neurogenesis in, for instance, hippocampus
  even in adult brain
• N.B. glia form impenetrable scars after brain
  injury
• Also, methods are being developed wherein stem
  cells are injected to brain that develop into
  neurons

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Stem cells

• Adult stem cells exists in the brain in small
  numbers, remaining quiescent (non-dividing) for
  many years until activated by e.g. disease /
  tissue injury.
• Effort to find ways to grow adult stem cells in
  cell culture and manipulate them to generate
  specific cell types so they can be used to treat
  injury or disease. Some examples of potential
  treatments include replacing the
  dopamine-producing cells in the brains of
  Parkinson's patients.