2Spatial VisionThe 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.
3Visual AcuityThe 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.
4Resolution AcuityCan be measured behaviorally or electrophysiologically.Stimuli include square wave gratings, sine wave gratings, checkerboard patterns.AKA: grating acuity
5Recognition AcuityThe smallest stimulus that can be identified or recognized.Measured using optotypes (i.e., optotype acuity).Snellen Test
6Recognition Acuity Measured in Snellen notation. Expressed in terms of test distance (numerator) and in comparison to an adult with normal vision (denominator).20/2020/5020/16
7Recognition Acuity Snellen test possess pitfalls Unequal number of lettersUnequal crowdingNo systematic progressionUnequal detection
8Recognition Acuity LogMAR tests provide a better alternative Equal number of lettersProportionate spacingEquivalent letter difficultyRegular line progressionETDRS
9Recognition Acuity Use logMAR units. Log10 minimum angle of resolution Smaller number = better vision20/200 = 1.0 logMAR20/20 = 0 logMAR20/16 = -0.1 logMARLines progress in 0.1 logMAR units
10Recognition Acuity Resolution acuity overestimates “true acuity.” Recognition acuity should be measured as early in life as possible.Preschool yearsPreschoolers are not literate and can not complete letter acuity tests.
11Recognition AcuityPreschoolers can be assessed with limited number of optotypes.Simplified letter optotypes can be used.Illiterate EHOTV
12Recognition Acuity Easy to recognize symbols can be used. Lea Symbols PattiPics Symbols
13Development Resolution Acuity Adult-like at age 5 (Skoczenski & Norcia, 2002).Age (months)Acuity (cpd)SnellenNewborns2.020/30066.420/95128.120/75189.320/652410.920/603617.320/354824.420/256030.020/20Data are drawn from Salomao & Ventura (1995), Mayer et al. (1995), andDrover et al. (2009).
14DevelopmentGrating acuity appears to be mediated by optical and retinal properties.Length of the eye, pupil, photoreceptorsIn adults, grating acuity can be predicted based on photorecepter diameter and spacing.
15DevolopmentTo see a grating, an unstimulated photoreceptor must lie between two stimulated photoreceptors.
16DevelopmentImprovements in grating acuity are probably due to changes in cone diameter and spacing.childadult
17Development Recognition Acuity Adultlike by approximately years of age (Drover et al. 2008; Simmers et al. 1997).Visual AcuityAgeNumberMean95%LowerGroupofVisualToleranceLimit(years)ParticipantsEyesAcuityLimits(Snellen)33768*0.0820/24±0.210.294182360*±0.170.2554793*0.0320/21±0.190.2263468-0.0320/19±0.180.1973570-0.02±0.100.11104998-0.0620/17±0.120.06Adults2346-0.0420/18±0.040.13
18DevelopmentOptotype acuity can not be predicted based on photoreceptor spacing and is likely mediated by other mechanisms.
19Contrast 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
20Contrast 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.
21Contrast SensitivityCS: 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.
22Contrast SensitivityContrast threshold is measure at each spatial frequency.
23Contrast SensitivitySpecifically, contrast sensitivity is the reciprocal of contrast threshold.Low threshold = high contrast sensitivityHigh threshold = low contrast sensitivityOne’s contrast sensitivity can be plotted for each spatial frequency.The result is the contrast sensitivity function (CSF).
24Contrast Sensitivity Inverted u-shaped function. Contrast sensitivity is highest at mid-spatial frequencies.The CSF contains several important landmarks.
25The Contrast Sensitivity Function Provides an evaluation of real-world visionEverything under the CSF is visibleEverything above the CSF is invisibleIt’s a window of visibility.
26Landmarks 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.Contrast Sensitivity110010002481632Spatial Frequency (cpd)
27Landmarks Can provide a measure of visual acuity. Contrast Sensitivity110010002481632Spatial Frequency (cpd)Can provide a measure of visual acuity.Good agreement between this measure and traditional resolution acuity (Drover et al. 2006).
28Landmarks Peak CS is at 2 – 5 cpd. Contrast Sensitivity110010002481632Spatial Frequency (cpd)Peak CS is at 2 – 5 cpd.This may correspond to average receptive field size of retinal ganglion cells.
29LandmarksThe 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+
30Landmarks The reduction in CS at low SF is low frequency attenuation. Contrast Sensitivity110010002481632Spatial Frequency (cpd)The reduction in CS at low SF is low frequency attenuation.May be due to lateral inhibition throughout the visual system
31Landmarks The low SF grating illuminates both the center and surround. The surround inhibits the center of the receptive field+
32Spatial 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.
34Clinical 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.
35Clinical Significance of the CSF Contrast Sensitivity110010002481632Spatial Frequency (cpd)If a single spatial frequency channel is affected, notches may appear in the CSF.
36Clinical Significance of the CSF Will also detect disorders that affect visual acuity alone.1000Contrast Sensitivity10012481632Spatial Frequency (cpd)
37Measurement in Infant and Toddlers Infants and toddlers can be assessed electrophysiologically using VEPSine wave gratings are presented at a single spatial frequency while contrast is swept.Start at low contrast move to high contrastThe contrast at which the VEP reaches background noise is a measure of contrast threshold.Other spatial frequencies are then presented.
38Measurement 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.
39Contrast Sensitivity Cards The lowest contrast sine wave grating detected at each spatial frequency is a measure of contrast threshold.
40Development Overall contrast sensitivity increases with age. More combinations of SFs and contrasts can be detected.Peak CS shifts upwards and rightwards
41Development 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.
42DevelopmentOverall 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.