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

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

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

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)

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).

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)

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

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

BGR RGB Perceived image Magnified image

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

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

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.

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

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

Trial Sequence

Percentages of Correct Responses 84% interval

Choice Probability (narrow spacing)

Effect of Neighboring Distractors

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.

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.

Examples

Trial sequence

Response Accuracy

Response Accuracy

Latency for Accurate Response

Latency for Accurate Response

Frequency Effect on RT

Subjective Preference

Subjective Preference

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.

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.