Psychology 4051 - PowerPoint PPT Presentation

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Psychology 4051

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Psychology 4051 Vernier Acuity Vernier Acuity The smallest amount of misalignment that can be detected within a stimulus. Usually measured by determining the smallest ... – PowerPoint PPT presentation

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Title: Psychology 4051


1
Psychology 4051
  • Vernier Acuity

2
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.

3
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

4
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.

5
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

6
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.
7
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.

8
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.

9
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.

10
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.
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