SUMMATION AND INHIBITION IN THE FROG

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SUMMATION AND INHIBITION IN THE FROGS RETINA

• H. B. Barlow, 18th June, 1952

Presented by Joel Tan
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The Question

• How does sub-threshold summation work in the
  retina?
• Why does the brain seem to sacrifice so much
  resolution by convergence?
• Hartline (1940) showed ganglion cells have
  receptive fields up to 1mm in diameter (1/128 of
  retina) why so big?
• Human 100m PR ? 1 m Ganglion cells
• Frog 1m PR ? 30k Ganglion cells

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Alternatives

• Hartlines results were wrong.
• Receptive fields of ganglion cells are actually
  smaller than Hartline thought
• Hartlines research had technical errors (eg.
  scattered light, etc.)
• Hartline was right ganglion cell receptive
  fields are quite large
• There is integration of input from many
  photoreceptors, meaning a further two
  alternatives
• Frogs just dont see as well the frogs retinas
  have a couple photoreceptors too many
• Large receptive fields are somehow adaptive.

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Logic

• By using a stimulus spot, and tracking
  sensitivity threshold (in terms of position, size
  and intensity of light), the receptive field of
  individual ganglion cells can be measured.

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Methods

• Preparation electrode placed in opened, excised
  retina of a frog
• Preparations lasted 1 4 hrs (though usually
  about 1 hr)
• Estimation of sensitivity threshold by varying
  parameters of light (size, intensity, position)
• Threshold determined by Action Potential firing.
• Stimulator focused on preparation, intensity
  adjusted until threshold found
• In cases where on and off thresholds needed,
  lower threshold taken first

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Methods (continued)

• Hartlines experiment (1940)
• Stimulus was presented as a single spot at high
  intensity, moved to determine extent of receptive
  field
• Barlows experiment (1952)
• Hartlines method used to find center of
  receptive field
• Spots of increasing size, starting from center of
  receptive field, were used to determine effect of
  summation

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Results

• Size of Receptive Fields
• Central Plateau of Sensitivity noted in receptive
  fields
• Receptors of different sizes found
• Ganglion cells with largest receptive fields
  found at periphery of retina
• largest ganglion cell receptive fields matched
  Hartlines findings (1 mm receptive fields)

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More Results

• Activity within a receptive field
• Off fibers unaffected by light outside of
  receptive field
• On-off fibers showed inhibition when stimulus
  spot extended beyond receptive field
• inhibition proportional to intensity of light
  outside receptive field
• Inhibition demonstrated again by experiment using
  side spot and centre spot

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Yet More Results

• Movement Sensitivity
• Off units unable to differentiate between
  different areas of receptive field
• On-off units showed response to change in
  spatial patterns of light

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Interpretation

• Sub-threshold summation works by convergence of
  many photoreceptors onto a given ganglion cell.
• The brain is more sensitive to contrast than to
  luminance values
• Economical way of visual image construction
  helps to parse object boundaries, etc.

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Interpretation

• Contrast is emphasized by an inhibitory surround
  of ganglion cells.
• Corresponds to assumption of a contrast
  extracting computation in the brain
• Therefore, the brain translates actual luminance
  values, representing visual images in terms of
  contrast.

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Interpretation

• Makes ecological / evolutionary sense
• Size of receptive field matches size of image of
  flies at 2 inches
• Could be used to track movement of flies, with
  maximally excited ganglion indicating a flys
  position
• On-off units may be used for tracking movement
  / detail (less summation, more sensitive to
  movement)
• On-off units as Bug detectors

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Problems

• Retina was excised problems of deterioration?
• Coarse measurement of ganglion cell activity
  (on/off)
• Inability to measure firing rates, etc. makes
  quantitative assessment impossible

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• THE END