PSY280H1 Chapter Notes - Chapter 3: Refractive Error, Lisa Lopes, Thalamus

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9 Feb 2016
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Chapter 3 – Spatial vision: from spots to
stripes
Visual acuity: oh say, can you see?
-Acuity: the smallest spatial detail that can be resolved
- 20/20 for eye doctors, but vision scientists talk about the smallest
visual angle of cycle of the grating that we can perceive
-cycle: for a grating, a pair consisting of one dark bar and one bright
bar; one repetition of black and white stripes
-visual angle: the angle that would be formed by lines going from the
top and bottom, or left and right, of a cycle on the page, through the
center of the lens, and on to the retina
oto calculate visual angle of resolution acuity, divide the size of
the cycle by the viewing distance at which you could just barely
make out the orientation of the gratings, and then take the
arctan of this ratio
usually, good vision = one cycle subtends an angle of 1
minute of arc
- resolution acuity represents one of the fundamental limits of spatial
vision
- limit is determined primarily by the spacing of photoreceptor in the
retina
osin wave grating: grating with a sinusoidal luminance pro'le
olight intensity in sine wave gratings varies smoothly and
continuously across each cycle
- however, visual system “samples” the grating discretely, through the
array of receptors at the back of the retina
oif receptors are spaced such that the whitest and blackest parts
of the grating fall on separate cones, we should be able to make
out the grating
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oif entire cycle falls on a single cone, we will see gray 'eld, or
aliasing (misperception of a grating due to undersampling)
-cones in the fovea have a center-to-center separation of about 0.5
minute of an arc, which 'ts with the observed acuity limit of 1 min of
arc (need 2 cones per cycle)
-rods and cones in periphery are packed together less tightly, but many
receptors converge on each ganglion cell
ovisual acuity is much poorer in the periphery than in the fovea
a visit to the eye doctor
- Snellen constructed a set of block letters for which the letter as a
whole was 5x as large as the strokes that formed the letter
acuity=distance at which a person can just identify the letter
distance at which a person with normal vision can just identify the letter
- At the eye doctor: usually 20 feet
Acuity for low-contrast stripes
-Spatial frequency: the number of grating cycles in a given unit of
space
o
visual angle between each pair of
1
¿stripes ¿
oex: 1/0.25 = 4 cycles per degree: the number of pairs of dark
and bright bars per degree of visual angle
- human contrast sensitivity function (CSF): a function describing
how the sensitivity to contrast) de'ned as the reciprocal of the contrast
threshold) depends on the spatial frequency (size) of the stimulus
2
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othe function is shaped like an
upside-down U
oy-axis: reciprocal of the
contrast threshold (smallest
amount of contrast required to
detect a pattern)
for a 1 cycle/deg grating
to be just distinguishable,
the dark stripes must be
about 1% darker than the
light stripes
reciprocal of this
threshold is 1/0.01 = 100
contrast of 100%
corresponds to a
sensitivity value of 1 (60 cycles/degree), corresponds to a
cycle width of 1 min of arc
retinal ganglion cells and stripes
- when spatial frequency of the grating is too low, the ganglion cell
responds weakly because part of the fat, bright bar of the grating lands
in the inhibitory surround, damping the cell’s response
- when spatial frequency is too high, the ganglion cell responds weakly
because both dark and bright stripes fall within the receptive-'eld
center, washing out response
- when spatial frequency is just right (bright in center and dark in
surround), the cell responds vigorously
3
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