Transcript Psychology 4051 - Memorial University of Newfoundland

Psychology 4051
Spatial Vision
Spatial Vision
• The ability to detect objects and patterns and
distinguish them from a background.
• Arguably, the most important single aspect of
vision.
• Assessed using tests of visual acuity and
contrast sensitivity.
Visual Acuity
• The smallest stimulus that can be detected or
recognized.
• One’s sharpness of vision.
• Resolution Acuity: The smallest stimulus that
can be resolved from a uniform background.
Resolution Acuity
• Can be measured behaviorally or
electrophysiologically.
• Stimuli include square wave gratings, sine
wave gratings, checkerboard patterns.
• AKA: grating acuity
Recognition Acuity
• The smallest stimulus that can be identified or
recognized.
• Measured using optotypes (i.e., optotype acuity).
Snellen Test
Recognition Acuity
• Measured in Snellen notation.
• Expressed in terms of test distance
(numerator) and in comparison to an adult
with normal vision (denominator).
• 20/20
• 20/50
• 20/16
Recognition Acuity
• Snellen test possess pitfalls
– Unequal number of letters
– Unequal crowding
– No systematic progression
– Unequal detection
Recognition Acuity
• LogMAR tests provide a better alternative
•
•
•
•
ETDRS
Equal number of letters
Proportionate spacing
Equivalent letter difficulty
Regular line progression
Recognition Acuity
• Use logMAR units.
• Log10 minimum angle of resolution
•
•
•
•
•
Smaller number = better vision
20/200 = 1.0 logMAR
20/20 = 0 logMAR
20/16 = -0.1 logMAR
Lines progress in 0.1 logMAR units
Recognition Acuity
• Resolution acuity overestimates “true acuity.”
• Recognition acuity should be measured as
early in life as possible.
– Preschool years
• Preschoolers are not literate and can not
complete letter acuity tests.
Recognition Acuity
• Preschoolers can be assessed with limited number of
optotypes.
• Simplified letter optotypes can be used.
Illiterate E
HOTV
Recognition Acuity
• Easy to recognize symbols can be used.
Lea Symbols
PattiPics Symbols
Development
Resolution Acuity
• Adult-like at age 5 (Skoczenski & Norcia, 2002).
Age (months)
Acuity (cpd)
Snellen
Newborns
2.0
20/300
6
6.4
20/95
12
8.1
20/75
18
9.3
20/65
24
10.9
20/60
36
17.3
20/35
48
24.4
20/25
60
30.0
20/20
Data are drawn from Salomao & Ventura (1995), Mayer et al. (1995), and
Drover et al. (2009).
Development
• Grating acuity appears to be mediated by
optical and retinal properties.
– Length of the eye, pupil, photoreceptors
• In adults, grating acuity can be predicted
based on photorecepter diameter and
spacing.
Devolopment
• To see a grating, an unstimulated
photoreceptor must lie between two
stimulated photoreceptors.
Development
• Improvements in grating acuity are probably
due to changes in cone diameter and spacing.
child
adult
Development
Recognition Acuity
• Adultlike by approximately 6 - 10 years of age
(Drover et al. 2008; Simmers et al. 1997).
Visual Acuity
Age
Number
Number
Mean
Mean
95%
Lower
Group
of
of
Visual
Visual
Tolerance
Limit
(years)
Participants
Eyes
Acuity
Acuity
Limits
(Snellen)
3
37
68*
0.08
20/24
±0.21
0.29
4
182
360*
0.08
20/24
±0.17
0.25
5
47
93*
0.03
20/21
±0.19
0.22
6
34
68
-0.03
20/19
±0.18
0.19
7
35
70
-0.02
20/19
±0.10
0.11
10
49
98
-0.06
20/17
±0.12
0.06
Adults
23
46
-0.04
20/18
±0.04
0.13
Development
• Optotype acuity can not be predicted based
on photoreceptor spacing and is likely
mediated by other mechanisms.
Contrast Sensitivity
• Measurement of visual acuity can be
problematic.
• Assesses vision at very high contrast levels
only.
• Contrast: The difference in brightness levels
between light and dark elements of a pattern.
98% contrast
Contrast Sensitivity
• In the real world, brightness and contrast vary.
• Visual disorders may affect patients at lower
contrast levels.
– May score normally on visual acuity but still
complain of blurry vision.
Contrast Sensitivity
• CS: the minimum amount of contrast required
to detect sine wave gratings at different
spatial frequencies.
• Measures one’s sensitivity to size and contrast
simultaneously.
• Measured by assessing one’s contrast
threshold to sinewave gratings at different
spatial frequencies.
Contrast Sensitivity
• Contrast threshold is measure at each spatial
frequency.
Contrast Sensitivity
• Specifically, contrast sensitivity is the
reciprocal of contrast threshold.
– Low threshold = high contrast sensitivity
– High threshold = low contrast sensitivity
• One’s contrast sensitivity can be plotted for
each spatial frequency.
• The result is the contrast sensitivity function
(CSF).
Contrast Sensitivity
• Inverted u-shaped
function.
• Contrast sensitivity is
highest at mid-spatial
frequencies.
• The CSF contains
several important
landmarks.
The Contrast Sensitivity Function
• Provides an evaluation of
real-world vision
• Everything under the CSF is
visible
• Everything above the CSF is
invisible
• It’s a window of visibility.
Landmarks
1000
Contrast Sensitivity
• 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.
100
1
2
4
8
16
32
Spatial Frequency (cpd)
Landmarks
• Can provide a
measure of visual
acuity.
• Good agreement
between this
measure and
traditional resolution
acuity (Drover et al.
2006).
Contrast Sensitivity
1000
100
1
2
4
8
16
32
Spatial Frequency (cpd)
Landmarks
• Peak CS is at 2 – 5
cpd.
• This may correspond
to average receptive
field size of retinal
ganglion cells.
Contrast Sensitivity
1000
100
1
2
4
8
16
32
Spatial Frequency (cpd)
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
+
Landmarks
• The reduction in CS
at low SF is low
frequency
attenuation.
• May be due to
lateral inhibition
throughout the
visual system
Contrast Sensitivity
1000
100
1
2
4
8
16
32
Spatial Frequency (cpd)
Landmarks
• The low SF grating illuminates both the center and
surround.
• The surround inhibits the center of the receptive
field
+
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.
Spatial Frequency Channels
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.
Clinical Significance of the CSF
Contrast Sensitivity
1000
100
1
2
4
8
16
32
Spatial Frequency (cpd)
• If a single spatial frequency channel is
affected, notches may appear in the CSF.
Clinical Significance of the CSF
• Will also detect disorders that affect visual
acuity alone.
Contrast Sensitivity
1000
100
1
2
4
8
16
32
Spatial Frequency (cpd)
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.
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.
Contrast Sensitivity Cards
• The lowest contrast sine wave grating
detected at each spatial frequency is a
measure of contrast threshold.
Development
• Overall contrast sensitivity increases with age.
• More combinations of SFs and contrasts can be detected.
• Peak CS shifts upwards and rightwards
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.
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.