Psychology 4051

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Psychology 4051

• Vernier Acuity

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Vernier Acuity

• The smallest amount of misalignment that can be
  detected within a stimulus.
• Usually measured by determining the smallest
  offset that can be detected within a single line,
  or series of lines.

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Vernier Acuity

• Human vernier acuity is exceptional.
• Often measures -0.2 to -0.4 logMAR.
• 20/12 to 20/8.
• This is superior to both optotype acuity and to
  grating acuity.
• Superior to what would be predicted based on
  photoreceptor properties.
• Referred to as a hyperacuity

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Measurement of Vernier Acuity

• In adults, its simply measured as the smallest
  offset that can be detect.
• In young children, it can be measured using sweep
  VEP or FPL techniques.
• In sweep VEPs, the participant is presented with
  a square wave grating or checkerboard pattern in
  which the amount of misalignment is swept.

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Measurement of Vernier Acuity

• VEPs are recorded as misalignment is swept
• Slope is determined for the VEP as it approaches
  background noise.
• The point where the slope intercepts background
  noise is a measure of vernier acuity

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Measurement of Vernier Acuity

• In FPL, a misaligned square wave grating can be
  paired with normal square wave grating.

The square grating is very salient. Subjects may
prefer this stimulus if they detect it first.
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Measurement of Vernier Acuity

• This can be fixed using vernier acuity cards in
  which the entire card is covered with a square
  wave grating.
• Part of the square wave grating is misaligned in
  the form of a shape that is familiar to toddlers.

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Development of Vernier Acuity

• Initially, vernier acuity was thought to be
  poorer than grating acuity until 3-4 months of
  age.
• Then it improved rapidly until it reached adult
  level at approximately 4 years of age.
• These early studies were flawed in that the
  misaligned stimulus was shifted from an aligned
  to a misaligned position to maintain the
  attention of the infants.

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Development of Vernier Acuity

• Skoczenski and Norcia (2002) used VEPs to show
  that vernier acuity is poorer than grating acuity
  until about 4 years of age.
• Vernier acuity does not reach adult level until
  adolescence.
• This course of development has been confirmed
  using FPL.

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Mechanisms Underlying Development

• Given its slow maturation compared to grating
  acuity and the fact its a hyperacuity, vernier
  acuity is not likely mediated by photoreceptor
  properties.
• Appears to cortically mediated.
• Children with cortical impairment show selective
  deficits in vernier acuity.
• Achievement of adult level corresponds with
  synaptic pruning.