Neural Representation

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Neural Representation
Biomedical engineering Group School of Electrical
Engineering Sharif University of Technology

• How World is Mapped onto the Mind

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NEURAL SYSTEMS

• Amazingly profesion at solving problems
• Seagulls and Shellfish
• Bees and finding their ways
• Rats and sense of direction
• Explanation Representation
• Serving to relate the internal state of the
  animal to its environment
• Can be manipulated internally without
  manipulating the actual, external, represented
  object.
• Penfild Observations
• Transformation
• Exploiting representations
• Updating
• Manipulating
• Relating
• Explaining how neurobiological systems represent
  the world, and how they use those
  representations, via transformations, to guide
  behavior

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NEURAL REPRESENTATION

• The main problem is to determine the exact nature
  of the representation relation that is, to
  specify the relation between things inside the
  head and things outside the head.
• We define
• The representational relationship
• To see if it does the explanatory work that is
  needed
• A close tie between neural representations as
  understood by neuroscientists and codes as
  understood by communications engineers
• Codes in Engineering Encode Decode
• Encode/Decode A procedure between 2 alphabets
• Neural firings encode properties of external
  stimuli

A,B,..
Encode
Decode
Morse
Stimulus
Encode
Neural Firing
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REPRESENTATION

• Representation one/more Neural Firing
• Example A neuron Firing for Face orientation
• Graded Representation Firing more or less
  strongly
• Preferred Stimulus One/More Neurons
• A Relation between Stimului and Firing
• Decoding Inferring from Firing

90 degree orientation
45 degree orientation
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Relevant Alphabets

• So many Different Alphabets
• Ex Retinal Ganglion Cells
• light intensities
• certain retinal locations
• spike trains of single neurons
• Ex An entire cortical area, like the primary
  visual cortex
• Color
• spatial frequency
• Intensity
• spike trains of large populations of neurons
• Relating neural responses (alphabet 1) and
  physical properties (alphabet 2)
• Neural Alphabets
• average production rate of neural spikes (i.e., a
  rate code)
• specific timings of neural spikes (i.e., a timing
  code)
• population-wide groupings of neural spikes
  (population code)
• synchrony of neural spikes across neurons
  (synchrony code)
• Distances of Spikes in a Neural Loop
• Number of Spikes in a Neural Loop
• Of these possibilities, arguably the best
  evidence exists for a combination of timing codes
  and population codes

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Physical properties

• Encoded by physicists and Neurons
• Displacement
• Velocity
• Acceleration
• Wavelength
• Temperature
• Pressure
• Mass, ..
• Only Encoded by neurons
• Red
• Hot
• Square
• Dangerous
• Edible
• Object
• Conspecific
• These latter higher-order properties are
  inferred on the basis of (i.e., are the results
  of transformations of) representations
• For the time being we focus our attention on
  characterizing more basic physical properties,
  where we believe successes can be more
  convincingly demonstrated

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Different Coding

• Engineering Specified
• Neurons Discovered
• A lot of debate concerning what is actually
  represented
• what is represented depends in part on how it is
  subsequently used
• Have to know how the system works in order to
  know what it represents.
• we have a fairly comprehensive understanding of
  what is actually represented in the brain
• Information encoded by a neural population may be
  decoded in a variety of ways

How it Works
Obstacle
Representation
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The Single Neuron
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Synapse
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(No Transcript)
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STRUCTURE

• They have three distinct parts
• (1) Cell body,
• (2) Dendrites, and
• (3) the Axon
• The particular type of neuron that stimulates
  muscle tissue is called a motor neuron.
• Dendrites receive impulses and conduct them
  toward the cell body.

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Myelinated Axons

• The axon is a single long, thin extension that
  sends impulses to another neuron.
• They vary in length and are surrounded by a
  many-layered lipid and protein covering called
  the myelin sheath, produced by the schwann cells.

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Resting Potential

• In a resting neuron (one that is not conducting
  an impulse), there is a difference in

electrical charges on the outside and inside of
the plasma membrane. The outside has a positive
charge and the inside has a negative charge.
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Contribution of Active Transport

• There are different numbers of potassium ions
  (K) and sodium ions (Na) on either side of the
  membrane. Even when a nerve cell is not
  conducting an impulse, for each ATP molecule
  thats hydrolysed, it is actively transporting 3
  molecules Na out of
• the cell and 2 molecules
• of K into the cell, at
• the same time by
• means of the
• sodium-potassium pump.

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Contribution of facilitated diffusion

• The sodium-potassium pump creates a concentration
  and electrical gradient for Na and K, which
  means that K tends to diffuse (leak) out of
  the cell and Na tends

to diffuse in. BUT, the membrane is much more
permeable to K, so K diffuses out along its
concentration gradient faster. Conversely, the
electric field causes both ions tend to come in.
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RESULTS IN

• a net positive charge outside a net negative
  charge inside. Such a membrane is POLARISED

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Action Potential

• When the cell membranes are stimulated, there is
  a change in the permeability of the membrane to
  sodium ions (Na).
• The membrane becomes more permeable to Na and
  K, therefore

sodium ions diffuse into the cell down a
concentration gradient. The entry of Na disturbs
the resting potential and causes the inside of
the cell to become more positive relative to the
outside.
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All-or-None Principle

• Throughout depolarisation, the Na continues to
  rush inside until the action potential reaches
  its peak and the sodium gates close.
• If the depolarisation is not great enough to
  reach threshold, then an action potential and
  hence an impulse are not produced.
• This is called the All-or-None Principle.

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Speed of Nerve Impulses

• Impulses travel very rapidly along neurones. The
  presence of a myelin sheath greatly increases the
  velocity at which impulses are conducted along
  the axon of a neuron. In unmyelinated fibres, the
  entire axon membrane is exposed and impulse
  conduction is slower.

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Speed of Nerve Impulses

• Impulses travel very rapidly along neurons. The
  presence of a myelin sheath greatly increases the
  velocity at which impulses are conducted along
  the axon of a neuron. In unmyelinated fibres, the
  entire axon membrane is exposed and impulse
  conduction is slower.

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Equivalent Model for Dendrites and Axons
dx
Rdx
Cdx
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Equivalent Model for an excited Neuron
dx
Rdx
v0(t)
Cdx
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Transmission of Action Potential/ Dendrite
potential