Psychology 4051

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Psychology 4051
Spatial Vision
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The Contrast Sensitivity Function

• Provides an evaluation of real-world vision
• Everything under the CSF is visible
• Everything above the CSF is invisible

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Landmarks

• The reduction is CS at high SF is high frequency
  roll-off
• The x-axis intercept can be extrapolated.
• Provides an estimate of the highest SF that can
  be detected at maximum contrast.

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Landmarks

• Can provide a measure of visual acuity.
• Good agreement between this measure and
  traditional resolution acuity (Drover et al.
  2006).

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Landmarks

• Peak CS is at 2 5 cpd.
• This may correspond to average receptive field
  size of retinal ganglion cells.

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Landmarks

• The stripewidth of the grating may correspond to
  the center/surround size of receptive fields.
• This would provide near maximal stimulation of
  the retinal ganglion cell.
• Less contrast is required to detect the grating

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Landmarks

• The reduction in CS at low SF is low frequency
  attenuation.
• May be due to lateral inhibition throughout the
  visual system

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Landmarks

• The low SF grating illuminates both the center
  and surround.
• The surround inhibits the center of the receptive
  field

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Spatial Frequency Channels

• The shape of the CSF may reflect underlying
  spatial frequency channels.
• Cell groups in the visual system that respond to
  a small range of spatial frequencies only.
• The CSF is simply an envelope that covers all SF
  channels.
• Evidence comes from selective adaptation
  experiments.

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Spatial Frequency Channels
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Clinical Significance of the CSF

• Provides a measure of real world vision.
• Disorders may selectively affect certain SF
  channels.
• This will not be picked up the visual acuity
  testing.

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Clinical Significance of the CSF

• If a single spatial frequency channel is
  affected, notches may appear in the CSF.

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Clinical Significance of the CSF

• Will also detect disorders that affect visual
  acuity alone.

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Contrast Sensitivity
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Spatial Frequency (cpd)
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Measurement in Infant and Toddlers

• Infants and toddlers can be assessed
  electrophysiologically using VEP
• Sine wave gratings are presented at a single
  spatial frequency while contrast is swept.
• Start at low contrast move to high contrast
• The contrast at which the VEP reaches background
  noise is a measure of contrast threshold.
• Other spatial frequencies are then presented.

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Measurement in Infants and Toddlers

• Infants and toddlers can also be assessed using
  contrast sensitivity cards following FPL.
• The cards are modeled on the Teller Acuity Cards.
• The CS cards consist of 40 cards arranged in 5
  spatial frequency sets.

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Contrast Sensitivity Cards

• The lowest contrast sine wave grating detected at
  each spatial frequency is a measure of contrast
  threshold.

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Development

• Overall contrast sensitivity increases with age.
• More combinations of SFs and contrasts can be
  detected.
• Peak CS shifts upwards and rightwards

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Development

• CS at higher SFs develops at the fastest rate.
• Continues to develop until 4 years of age.
• Improvement at low spatial frequencies continues
  until 9 years of age.
• Low frequency attenuation may not be present
  until 2 months of age.

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Development

• Overall CS increase is likely due to increase
  lengths of the photoreceptors and an increase in
  funneling capacity.
• Increase CS at higher SFs may be due to tighter
  packing of cones in the fovea and an increase in
  the number of photons caught.