Psychology 4051

1. Psychology 4051 Spatial Vision

2. 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.

3. 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.

4. Resolution Acuity • Can be measured behaviorally or electrophysiologically. • Stimuli include square wave gratings, sine wave gratings, checkerboard patterns. • AKA: grating acuity

5. Recognition Acuity • The smallest stimulus that can be identified or recognized. • Measured using optotypes (i.e., optotype acuity). Snellen Test

6. 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

7. Recognition Acuity • Snellen test possess pitfalls • Unequal number of letters • Unequal crowding • No systematic progression • Unequal detection

8. Recognition Acuity • LogMAR tests provide a better alternative • Equal number of letters • Proportionate spacing • Equivalent letter difficulty • Regular line progression ETDRS

9. 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

10. 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.

11. Recognition Acuity • Preschoolers can be assessed with limited number of optotypes. • Simplified letter optotypes can be used. HOTV Illiterate E

12. Recognition Acuity • Easy to recognize symbols can be used. Lea Symbols PattiPics Symbols

13. Development Resolution Acuity • Adult-like at age 5 (Skoczenski & Norcia, 2002). Data are drawn from Salomao & Ventura (1995), Mayer et al. (1995), and Drover et al. (2009).

14. 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.

15. Devolopment • To see a grating, an unstimulated photoreceptor must lie between two stimulated photoreceptors.

16. Development • Improvements in grating acuity are probably due to changes in cone diameter and spacing. child adult

17. Development Recognition Acuity • Adultlike by approximately 6 - 10 years of age (Drover et al. 2008; Simmers et al. 1997).

18. Development • Optotype acuity can not be predicted based on photoreceptor spacing and is likely mediated by other mechanisms.

19. 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

20. 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.

21. 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.

22. Contrast Sensitivity • Contrast threshold is measure at each spatial frequency.

23. 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).

24. Contrast Sensitivity • Inverted u-shaped function. • Contrast sensitivity is highest at mid-spatial frequencies. • The CSF contains several important landmarks.

25. 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.

26. 1000 Contrast Sensitivity 100 1 4 8 16 32 2 Spatial Frequency (cpd) 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.

27. 1000 Contrast Sensitivity 100 1 4 8 16 32 2 Spatial Frequency (cpd) Landmarks • Can provide a measure of visual acuity. • Good agreement between this measure and traditional resolution acuity (Drover et al. 2006).

28. 1000 Contrast Sensitivity 100 1 4 8 16 32 2 Spatial Frequency (cpd) Landmarks • Peak CS is at 2 – 5 cpd. • This may correspond to average receptive field size of retinal ganglion cells.

29. + 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

30. 1000 Contrast Sensitivity 100 1 4 8 16 32 2 Spatial Frequency (cpd) Landmarks • The reduction in CS at low SF is low frequency attenuation. • May be due to lateral inhibition throughout the visual system

31. Landmarks • The low SF grating illuminates both the center and surround. • The surround inhibits the center of the receptive field +

32. 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.

33. Spatial Frequency Channels

34. 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.

35. 1000 Contrast Sensitivity 100 1 4 8 16 32 2 Spatial Frequency (cpd) Clinical Significance of the CSF • If a single spatial frequency channel is affected, notches may appear in the CSF.

36. Clinical Significance of the CSF • Will also detect disorders that affect visual acuity alone. 1000 Contrast Sensitivity 100 1 4 8 16 32 2 Spatial Frequency (cpd)

37. 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.

38. 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.

39. Contrast Sensitivity Cards • The lowest contrast sine wave grating detected at each spatial frequency is a measure of contrast threshold.

40. Development • Overall contrast sensitivity increases with age. • More combinations of SFs and contrasts can be detected. • Peak CS shifts upwards and rightwards

41. 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.

42. 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.