psychology 4051 spatial vision. the ability to detect objects and patterns and distinguish them from...
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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
DevelopmentResolution 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
DevelopmentRecognition 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• 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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Spatial Frequency (cpd)
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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Spatial Frequency (cpd)
Landmarks
• Peak CS is at 2 – 5 cpd.
• This may correspond to average receptive field size of retinal ganglion cells.
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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
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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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Spatial Frequency (cpd)
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.
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
• If a single spatial frequency channel is affected, notches may appear in the CSF.
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Spatial Frequency (cpd)
Clinical Significance of the CSF
• Will also detect disorders that affect visual acuity alone.
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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.