Neurology of Memory Systems

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Neurology of Memory Systems
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Review

• Conditioning
• Habituation/Sensitization
• Working Memory
• Phonologic
• Visual/Spatial Sketchpad
• Central Executive
• Long-Term-Memory
• Implicit
• Explicit

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Brain Systems

• Conditioning

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The Aplysia

• Also called a sea slug
• It is a mollusk with nervous system containing
  only about 5000 neurons
• Has specific reflexes touching the animal causes
  the gill to be pulled inward
• This animal has made it easy to study the basic
  neural systems of learning

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Habituation

• Due to Synaptic Depression
• There is a decrease in the release of
  neurotransmitter
• The transmitter is not used up but rather the the
  repeated stimulation causes a change in the ion
  flow which would normally trigger the transmitter
  to be released

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Picture
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Sensitization

• Occurs at the same synapse but by the actions of
  another neuron terminal (interneuron)
• A shock to the tail activates the release of a
  (stimulant) neurotransmitter that causes the
  terminal to change the flow of ions
• This time the change in ions causes more
  neurotransmitter to be released

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Picture
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Duration

• A single training session of 10 touches give rise
  to learning that lasts minutes (STM-LTP and no
  protein)
• Four training sessions of 10 trials a day (spaced
  over four days) give rise to learning that lasts
  at least three weeks (LTM-requires protein
  synthesis)
• Overall it seems that learning takes places
  relatively quickly

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Conditioning of Fear

• Typically a shock will be used to evoke a fear
  response (increased heart rate)
• A small lesion was made in the hypothalamus
  (believed to be involved in the control of the
  cardiovascular system)
• There was no longer an increase in heart rate due
  to the tone. There was still an increase in
  heart rate due to shock
• The behavior stayed the same, but the
  physiological learned fear response no longer
  existed
• Amygdala projects to the hypothalamus

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Conditioning Motor Response

• Eyeblink conditioning involves a learned motor
  response
• A small lesion in the cerebellum abolishes the
  learning and memory of a conditioned motor
  response
• Is the memory actually stored there? Yes!!
• This is the only long-term memory trace that has
  been localized in the mammalian brain

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Brain Systems

• Working memory

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Visual Sketchpad and the Brain

• Monkey learned a STM task where the food was
  under the new object
• Lesions of the hippocampus disrupted STM of what
  they had seen and they could not tell the
  difference between old and new object
• They could discriminate between objects, so
  visual perception was intact

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Spatial Sketchpad and the Brain

• Monkey learned a spatial STM task
• Destruction of a region of frontal cortex
  severely disrupts the ability to perform the task
• The monkey can still perform the visual STM task

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Continuous Brain Activity

• Monkeys were trained to remember a color for 16
  seconds (the temporal lobe activated during color
  perception)
• Recording brain activity showed that during the
  16 sec, there was continuous activity in the
  temporal lobe
• Continuous activity has also been found during
  remembering tasks in other sensory areas of the
  brain

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Frontal Lobe

• The frontal lobe is also continuously active
  during a remembering task
• It is possible to disrupt the continuous activity
  in (say) the temporal lobe during a color
  remembering task, but continuous activity in the
  frontal lobe remains continuous
• This is neurologic evidence that the frontal
  lobes have executive control over other
  processing systems

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Memory Systems

• Long-Term-Memory

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LTM STM Storage Areas

• How do the short-term and long-term forms of
  memory relate to each other, and do they occur at
  the same or different locations?
• It seems that short-term learning results in
  connections simply being depressed or stimulated.
  Long-term learning involves inactivation or
  forming of new connections
• Thus, the same synaptic connections that are used
  in a STM are also used in a LTM

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Medial Temporal Lobe

• This structure seems to be the gateway into LTM
  from STM
• However, the structure is not the repository of
  LTM
• LTMs are thought to be stored in the same
  distributed set of structures that perceive,
  process, and analyze what is to be remembered

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LTM Storage Areas

• Activity in the inferior temporal lobe was
  recorded while monkeys viewed visual patterns.
  Found that specific neurons responded best to
  specific pictures
• Monkeys learned 12 picture pairs
• Before one neuron responded to one picture.
  After learning, a neuron responded to the
  original picture and the picture that was paired
  with it

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Hippocampal Damage

• Damage to the hippocampal system not only impairs
  new learning but also can disrupt recent memory
• Thus, memory is not fixed at the time of learning
  but takes time to develop a permanent form

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Establishing LTM

• Memory does not start in the hippocampus and
  diffuse out to the cortical areas, but rather is
  always in the cortex
• The medial temporal lobe stores some aspect of
  the information for a significant period after
  learning
• One idea is that the medial temporal lobe stores
  links or pointers that connect multiple cortical
  areas

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Implicit/Explicit and the Brain

• Using PET, brain activity was recorded during
  implicit and explicit memory tasks
• Subjects learned a list of words In the explicit
  condition, they had to remember the words during
  the 40 sec scan. In the implicit condition a
  word stem completion task was used
• In the explicit test there was more activity in
  the hippocampus, but in the implicit test there
  was more activity in the visual areas
• Thus, implicit and explicit memory seem to be in
  different brain regions

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Memory Systems

• The Adaptable Neuron

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LTP

• The process that lead to long-term changes in the
  neuron is called long-term-potentiation (LTP)
• Found in the lab when a brief high-frequency
  period of electrical activity was applied
  artificially to pathways in the hippocampus
• The result was an increase in synaptic strength
  lasting for days or weeks (depending)

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How Do LTPs Work?

• It is very complicated and it dose not work the
  same in all regions of the brain
• LTP is associated with an increased probability
  that neurotransmitters will be released
• Oddly enough, increasing the probability of
  neurotransmitter release results from a chemical
  message (nitric oxide?) sent from the
  post-synaptic nerve terminal

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Need More Than LTP to Make a Long-Term-Memory
--Protein

• The switch from STM to LTM is complicated
• Communication between neurons will trigger
  special a regulatory protein that regulates gene
  (DNA) expression in the nucleus of the neuron
• DNA produce new proteins that add to the physical
  structure of the neuron

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Growing New Synapses

• Protein production builds new synapses between
  neurons
• As time passes and there is no further use of the
  synapses the new synaptic terminals are lost