Yu-Chi Tai

Objective measurements of lower-level visual stress.

Purpose. To determine the sensitivity of the electromyography (EMG) response of the orbicularis oculi muscle to selected lower-level visually stressful conditions to establish the extent to which it can be used as a measure of visual discomfort. Methods. Thirty-one subjects (18 years or older) with 20/20 vision, without history of ocular pathology, oculomotor limitation, or cognitive deficits participated in the study. Subjects read on a computer display for 27 trials of 5 min duration under different low-level asthenopic conditions. The conditions were graded levels of font size, font type, contrast, refractive error, and glare. Orbicularis oculi activity was recorded using surface EMG. Blink-free epochs of EMG data were analyzed for power for all the conditions. Blink rate for all the trials was also measured. At the end of each trial, subjects rated the severity of visual discomfort experienced while reading. Results. Conditions that benefit from squint (refractive error and glare) showed increased EMG power (p < 0.001) from the orbicularis and increased blink rate (p = 0.002), whereas those that do not benefit from squint (small font and low contrast) showed no significant EMG response and a significant decrease in blink rate (p = 0.003 and p = 0.01). All conditions resulted in significant visual discomfort; the p value for font type was 0.039 and p < 0.001 for the other conditions. Conclusions. The results suggest that the squint-beneficial conditions are operated by a local mechanism involving contraction of the orbicularis and increase in reflex blinking, whereas those that do not benefit from squint do not engage the orbicularis and decrease blink, possibly through central inhibition of spontaneous blinking. The EMG response is a sensitive objective measure for the squint-beneficial conditions. However, for the non-squint-beneficial conditions, blink rate may be a more sensitive objective measure, although EMG with longer trial durations should be tested.

ClearType sub-pixel text rendering: Preference, legibility and reading performance

ClearType is an onscreen text rendering technology in which the red, green, and blue sub-pixels are separately addressed to increase text legibility. However, it results in colored borders on characters that can be bothersome. This paper describes five experiments measuring subject preference, text legibility, reading performance, and discomfort symptoms for five implementation levels of ClearType rendered text. The results show that, while ClearType rendering does not improve text legibility, reading speed or comfort compared to perceptually-tuned grayscale rendering, subjects prefer text with moderate ClearType rendering to text with grayscale or higher-level ClearType contrast. Reasons for subject preference and for lack of performance improvement are discussed.

Comparative effect of lens care solutions on blink rate, ocular discomfort and visual performance

Citation information: Yang N, Tai C, Sheedy JE, Kinoshita B, Lampa M & Kern JR. Comparative effect of lens care solutions on blink rate, ocular discomfort and visual performance. Ophthalmic Physiol Opt 2012, 32, 412–420. doi: 10.1111/j.1475‐1313.2012.00922.x

Depth perception from stationary and moving stereoscopic three-dimensional images

Purpose: The study evaluated the accuracy of depth perception afforded by static and dynamic stereoscopic three-dimensional (S3D) images with proportional (scaled to disparity) and constant size cues. Methods: Sixty adult participants, 18 to 40 years (mean, 24.8 years), with good binocular vision participated in the study. For static S3D trials, participants were asked to indicate the depth of stationary S3D images rendered with 36, 48 and 60 pixels of crossed disparity, and with either proportional or a constant size. For dynamic S3D trials, participants were asked to indicate the time when S3D images, moving at 27, 32 and 40 pixels/sec, matched the depth of a reference image which was presented with 36, 48 and 60 pixels of crossed image disparity. Results: Results show that viewers perceived S3D images as being closer than would be predicted by the magnitude of image disparity, and correspondingly they overestimated the depth in moving S3D images. The resultant depth perception and estimate of motion speed were more accurate for conditions with proportional and larger image size, slower motion-in-depth and larger image disparity. Conclusion: These findings possibly explain why effects such as looming are over stimulating in S3D viewing. To increase the accuracy of depth perception, S3D content should match image size to its disparity level, utilize larger depth separation (without inducing excessive discomfort) and render slower motion in depth.

Effects of ocular transverse chromatic aberration on near foveal letter recognition

Transverse chromatic aberration (TCA) smears retinal images of peripheral stimuli. In reading, text information is extracted from both foveal and near fovea, where TCA magnitude is relatively small and variable. The present study investigated whether TCA significantly affects near foveal letter identification. Subjects were briefly presented a string of five letters centered one degree of visual angle to the left or right of fixation. They indicated whether the middle letter was the same as a comparison letter subsequently presented. Letter strings were rendered with a reddish fringe on the left edge of each letter and a bluish fringe on the right edge, consistent with expected left periphery TCA, or with the opposite fringe consistent with expected right periphery TCA. Effect of the color fringing on letter recognition was measured by comparing the response accuracy for fringed and non-fringed stimuli. Effects of lateral interference were examined by manipulating inter-letter spacing and similarity of neighboring letters. Results demonstrated significantly improved response accuracy with the color fringe opposite to the expected TCA, but decreased accuracy when consistent with it. Narrower letter spacing exacerbated the effect of the color fringe, whereas letter similarity did not. Our results suggest that TCA significantly reduces the ability to recognize letters in the near fovea by impeding recognition of individual letters and by enhancing lateral interference between letters.

Effects of ocular transverse chromatic aberration on peripheral word identification

Transverse chromatic aberration (TCA) smears the retinal image of peripheral stimuli. We previously found that TCA significantly reduces the ability to recognize letters presented in the near fovea by degrading image quality and exacerbating crowding effect from adjacent letters. The present study examined whether TCA has a significant effect on near foveal and peripheral word identification, and whether within-word orthographic facilitation interacts with TCA effect to affect word identification. Subjects were briefly presented a 6- to 7-letter word of high or low frequency in each trial. Target words were generated with weak or strong horizontal color fringe to attenuate the TCA in the right periphery and exacerbate it in the left. The center of the target word was 1°, 2°, 4°, and 6° to the left or right of a fixation point. Subjects eye position was monitored with an eye-tracker to ensure proper fixation before target presentation. They were required to report the identity of the target word as soon and accurately as possible. Results show significant effect of color fringe on the latency and accuracy of word recognition, indicating existing TCA effect. Observed TCA effect was more salient in the right periphery, and was affected by word frequency more there. Individuals subjective preference of color-fringed text was correlated to the TCA effect in the near periphery. Our results suggest that TCA significantly affects peripheral word identification, especially when it is located in the right periphery. Contextual facilitation such as word frequency interacts with TCA to influence the accuracy and latency of word recognition.