Psychology 4051 Spatial Vision. The ability to detect objects and patterns and distinguish them from...

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


• 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.


• 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 threshold is measure at each spatial frequency.


• 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).


• 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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Landmarks

• Peak CS is at 2 – 5 cpd.

• This may correspond to average receptive field size of retinal ganglion cells.

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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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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.


• If a single spatial frequency channel is affected, notches may appear in the CSF.

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• Will also detect disorders that affect visual acuity alone.

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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.

Psychology 4051 Spatial Vision. The ability to detect objects and patterns and distinguish them from...

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