1. Effects of Transverse Chromatic Aberration (TCA) on Peripheral Reading
Shun-nan Yang, Ph.D

2. Collaborators James Sheedy (OD, PhD) Yu-chi Tai (PhD)
Hannu Laukkenan (OD)
David Glabe, Wendy Zhi, and Jennifer Ziegler (Research Assistants)

3. Outline Limits on peripheral reading and likely TCA effect
Experimental design
Effect of TCA on letter recognition
Effect of TCA on word identification
Implications on facilitating reading with ClearType (CT) rendering

4. Effective Visual Span in Reading
xxxxxx xxxxxxx xxxx Briony Lodge was in x xxxxxxxx xxxxxxxx
effective visual span
Numerous studies have shown that readers can identify word/letter in both fovea and parafovea.
Masking the task in periphery slows down reading.
Recent studies on reading has revealed that readers extract information from multiple words in fovea and parafovea.
Currently, it is known that retinal neural density, cortical magnification, and attentional modulation all play a role in such limit in periphery processing; however, optical aberration has also been shown to significantly affect visual acuity in periphery.
increasing TCA
increasing TCA
McConkie & Rayner (1975)

5. Limiting Factors on Peripheral Processing
Eccentricity (Low, 1970; Berkeley et al., 1975):
Retinal neuronal density (Croner & Kaplan, 1995)
Cortical magnification (Cowey & Rolls, 1970)
Visual attention allocation (Bahcall & Kowler, 1999)
Optical aberration (Marcos et al., 1999).

6. Limiting Factors on Text Processing
Lateral interference:
Spacing (Dakin and Hess, 1997)
Neighbor interference (Bouma, 1973)
Contextual facilitation
Top-down predictability (McClelland & Johnson, 1979)
Orthographic facilitation (Jonathan et al., 1992)

7. TCA Effect in Periphery
Refraction of different-wavelength Light induces natural TCA (Thibos et al., 1990).
TCA in periphery can be theoretically computed, but it is not clear whether TCA in the near periphery surpasses the spatial resolution and actually impedes letter recognition (Thibos, 1987). B
right periphery
left periphery

8. Effect of Color Fringe on TCA
RGB
BGR
TCA effect can be indirectly assessed using ClearType text:
It generates vertical color bands in red, green and blue (RGB) or reverse order (BGR).
RGB-rendered image should increase TCA when it is to the left of the fovea, and reduce it when it is to the right.
BGR-rendered image should have the opposite effect.
Perceived image
Magnified image

9. BGR
RGB
Perceived image
Magnified image

10. Effect of Color Fringe on TCA
RGB
BGR
TCA effect can be indirectly assessed using ClearType text:
It generates vertical color bands in red, green and blue (RGB) or reverse order (BGR).
RGB-rendered image should increase TCA when it is to the left of the fovea, and reduce it when it is to the right.
BGR-rendered image should have the opposite effect.
Perceived image
Magnified image

11. Expected Change in Visual Span
xxxxxx xxxxxxx xxxx Briony Lodge was in x xxxxxxxx xxxxxxxx
effective visual span
Numerous studies have shown that readers can identify word/letter in both fovea and parafovea.
Masking the task in periphery slows down reading.
Recent studies on reading has revealed that readers extract information from multiple words in fovea and parafovea.
Currently, it is known that retinal neural density, cortical magnification, and attentional modulation all play a role in such limit in periphery processing; however, optical aberration has also been shown to significantly affect visual acuity in periphery.
significant TCA
significant TCA

12. Hypotheses
RGB- and BGR-rendered letters should reduce and exacerbate TCA effect in the periphery, and affect the accuracy of letter recognition there.
TCA on letter recognition might be affected by the amount of lateral interference (spacing and neighbor similarity).
TCA effect on word identification should be affected by word familiarity.

13. Experiment I: Letter Recognition
Subjects
31 college students
Native English speaker
20/20 or better visual acuity
Normal or contact lenses correction
Task
Recognizing the middle of five horizontally placed letters
One-hour testing
Equipment: LCD monitor with RGB subpixels

14. Text Stimulus Font (Consolas, 11 point, equal spacing)
Font type (b&w, GS, RGB-CT, BGR-CT)
Types of neighboring distractor (group A [aceos], B [bdhk], G [gpqy], F [fijlt], M [mnur], and W [wvxz])
Spacing (normal [2 pixels, 5.4 min of arc] vs. wide [4 pixels or 10.8 min of arc]).
distracting neighbor
target

15. Trial Sequence

16. Percentages of Correct Responses
84% interval

17. Choice Probability (narrow spacing)

18. Effect of Neighboring Distractors

19. TCA effect on word identification
Word identification relies on both bottom-up feature analysis and top-down contextual facilitation.
TCA effect on word identification might be reduced with greater word familiarity.

20. Experiment II: Word Identification
Subjects:
30 subjects required, 21 included in the data.
Same recruitment criteria as in exp. 1.
Task:
480 high- and low-frequency words (Frances & Kucera, 1982)
6- to 7-letter length
6, 4, 2, 1 degrees to the left or right of fixation
2 degree fixation zone to prevent foveation (eye-tracking enforced)
Measure: oral naming latency and accuracy.

21. Examples

22. Trial sequence

23. Response Accuracy

24. Response Accuracy

25. Latency for Accurate Response

26. Latency for Accurate Response

27. Frequency Effect on RT

28. Subjective Preference

29. Subjective Preference

30. Summary
As expected, RGB-CT facilitated letter recognition in the right and impeded it in the left; opposite effects were found for BGR-CT.
Spacing but not neighbor similarity exacerbated TCA effect.
RGB-CT had an effect on response latency but not accuracy, with low-frequency word identification facilitated in the right.
Reduced TCA helped attenuate frequency effect, and increased TCA enhanced it.

31. Implications
With the typical RGB LCD display, RGB-CT text appeared to minimize TCA and extend visual span in the right periphery, where text previewing in English reading takes place.
Insignificant impedance was found in the left periphery with RGB-CT text.
RGB-CT rendering was especially useful for reading narrowly spaced and unfamiliar text.
RGB-CT text was overall rated as easier to read, consistent with its effect on peripheral reading.