POVS Basic Vision Science Core. Visual Acuity

Transcription

1 POVS Basic Vision Science Core Visual Acuity

2 Visual acuity Visual acuity is the smallest spatial detail that can be detected or identified. Types of acuity tasks: Detection acuity Resolution acuity Localization acuity Identification (letter or optotype) acuity

3 Detection acuity: bright targets The retinal image size of a bright point is constrained by the point spread function (PSF) of the eye. Optimal width of PSF 1 arc min.

4 Detection acuity: bright targets Detection acuity for a bright point or line does not depend on its angular size, but on the number of photons in the target s image. A bright target on a dark background (e.g., a star at night) is detected when its energy exceeds the retinal light threshold, despite an angular size < 1 sec arc.

5 Detection acuity: retinal contrast In the eye, this energy is spread into the point spread function (assumed width, p = 60 arc sec). Resulting retinal contrast, I 100/60 1.7%. L E = w * L Position Illuminance E.g., assume target width, w = 1 arc sec; contrast ( L) = 100%. Total contrast energy is proportional to w x L = 100 x 1. Luminance A bright target on light background (e.g., a star at dawn or dusk) is detected when it exceeds the w retinal contrast threshold. p I E = p * I Retinal Position

6 Detection acuity: dark targets Smallest detectable line arc sec. Smallest detectable dot 15 arc sec. Threshold size depends on retinal contrast threshold 1-2% after image spread due to retinal PSF or LSF.

7 Resolution acuity: two lines Two lines can be resolved when their images fall on nonadjacent cones with a relatively unstimulated cone in between. Cone separation (or sampling ) places a potential limit on acuity.

8 Resolution acuity: gratings High SF cut-off of CSF = grating resolution. 2.5 Subject MR log Contrast Sensitivity Cut off Spatial Frequency Spatial Frequency (c/deg) Photopic grating acuity 40 c/deg, i.e., each light or dark bar is about 0.75 arc min.

9 Localization acuity Vernier & spatial interval acuity: the precision of judging spatial location for one target with respect to one or more other targets.

10 Hyperacuities These very fine spatial thresholds are designated as hyperacuities. Vernier Threshold vs. Line Separation (Williams et al., 1984) 100 Vernier Threshold (arc sec) Under optimal conditions, localization acuities reach a few arc sec, much finer than the separation between adjacent cones Target Separation (arc min) 100

11 Identification acuity Letter targets most often used: Sloan letters (5 x 5 matrix) British letters (4 x 5 matrix) Others: Landolt Cs, tumbling Es, etc.

12 Standard acuity = 20/20 A standard observer can just read letters with stroke width = 1 arc min.

13 Normal corrected acuity is better than 20/20 Normal corrected adult acuity 20/16 (-0.09 log MAR). Based on Weymouth, 1960

14 Comparisons among acuity tasks Acuity Task Detection Typical Stimulus Bright line or spot Single dark line Single dark spot Resolution Two bright lines/spots Two dark lines/spots Grating Localization Spatial interval Vernier lines Identification Letters or numerals Optimal Threshold ca. 0 sec arc sec arc sec arc sec arc sec arc sec arc 2-4 sec arc 3-6 sec arc sec arc

15 Measurement error for acuity Measurement error for visual acuity depends on: acuity scale used slope of the psychometric function number of measurements Measurement errors are approximately independent of acuity on the logmar scale

16 Blur and visual acuity Dioptric blur degrades letter acuity much more than grating-detection acuity. Thorn & Schwartz, 1990

17 Spurious resolution permits detection of blurred high SF gratings 1 Object 0.50D defocus Smith, 1982

18 Spurious resolution results from overlapping blur circles Rectangular pupil Object = Grating B=0 B<P B=P P P = period B>P Arthur Bradley, Indiana University

19 Spurious resolution and the modulation transfer function 1 Modulation % 0.8 Defocused Image spurious resolution resolution limit % Spatial Frequency (c/deg) Arthur Bradley, Indiana University

20 Blur and localization acuity Vernier Threshold (arc sec) Localization acuity is affected more by blur for small than large separations between the component targets. 100 Vernier Threshold & Blur (Stigmar, 1971) Blur half width (min arc) 8

21 Acuity and contrast Acuity improves gradually with contrast, as predicted by the highsf limb of the CSF. Consequently, small changes in chart contrast should not affect clinical acuity measurements.

22 Letter Acuity and SF However, optimal identification of small spatially filtered letters depends on lower letter spatial frequencies (ca. 1.5 cyc/letter). Relative Contrast Sensitivity The stroke width of acuity targets corresponds to 2.5 cyc/letter. Spatial Frequency (cyc/letter) Chung, et al., 2002

23 Filtered Letters unfiltered 0.63 c/letter 0.88 c/letter 1.25 c/letter 1.77 c/letter 2.5 c/letter 3.54 c/letter 5 c/letter 7.07 c/letter 10 c/letter Chung, et al., 2002

24 Solomon & Pelli 1994

25 SF and Acuity 2.5 cycles If each spatial detail subtends an angle of 1 arc min, the whole letter subtends 5 arc min. There are 60 arc min in a degree. Therefore 2.5 cycles / 5 arc min = 30 cpd To convert to Snellen: 600/cpd = Snellen Denominator Thus 30 cpd letter = 20/20

26 More Examples

27 More Examples

28 Common Visual Acuity Scales MAR = stroke width of threshold sized acuity letter in arcmin Snellen = 20 / (20*MAR) Decimal = 1 / MAR LogMAR = log(mar)

29 Scales for Visual Acuity

30 Acuity and luminance Acuity improves with both scotopic and photopic target luminance. Clinically, acuity is assessed at about 100 cd/m2 (approximately 3 trolands here), where small luminance changes have little effect.

31 Acuity and luminance The improvement of letter acuity with luminance is consistent with luminancedependent changes in the spatial CSF. DeValois & DeValois, 1980

32 Acuity and retinal eccentricity Contrast Sensitivity Photopic acuity is best at the fovea and declines steadily with increasing retinal eccentricity. Spatial Frequency (cpd) Virsu & Rovamo, 1979

33 Acuity and retinal eccentricity Acuity changes with eccentricity at different rates for different kinds of acuity tasks. The change in letter acuity with eccentricity agrees qualitatively, not quantitatively with the CSF. Herse & Bedell, 1989

34 Visual acuity & contour interaction Visual acuity is impaired by the presence of nearby contours. Contour interaction is not optical, as it occurs also with dichoptic target presentation. Flom et al., 1963

35 Crowding in the periphery Toet & Levi (1992)

36 Myths Regarding Acuity High contrast acuity is the best way to test everyday vision. A patient who has 20/20 acuity has perfect vision.

37 Summary Different visual acuity tasks do not necessarily measure identical aspects of visual performance. Many of the factors that limit visual acuity can be appreciated from stimulus-dependent changes in the contrast sensitivity function. Visual acuity may be limited by optical and neural blur, spatial sampling, and/or the detectability of the acuity target. Geisler 1989