Sowjanya Gowrisankaran

Eyelid squint response to asthenopia-inducing conditions.

Purpose. To study the orbicularis oculi muscle response to asthenopia-inducing conditions. Methods. Twenty subjects (18–36 years) screened for 20/20 vision in each eye participated in the study. Subjects read passages under different asthenopia-inducing conditions. The inducing conditions were glare, low contrast, small font size, refractive error, up gaze, accommodative stress and convergence stress. Surface electromyography (EMG) was used to study the orbicularis oculi response from the right eye. Palpebral fissure height was measured from recorded video images of the right eye. At the end of each condition subjects were asked to rate the severity and type of visual discomfort experienced. Results. Outcome measures for the asthenopia-inducing conditions were compared with their respective nonstress controls. Repeated measures analysis of variance was used to analyze the data. Refractive error (p = 0.0001), glare (p = 0.0001), low contrast (p = 0.007), small font (p = 0.034), and up gaze (p = 0.001) resulted in a significant increase in EMG power. Refractive error (p = 0.0001) and glare (p = 0.0001) also caused significant reduction in aperture size. Reading a low contrast text caused a reduction in blink rate (p = 0.035), whereas refractive error (p = 0.005) and glare (p = 0.01) caused an increase in blink rate. All conditions induced significant visual discomfort (p < 0.001). Conclusion. Refractive error and glare, which reduce image quality and benefit from eyelid squint, caused increased EMG power, eyelid squint response and increased blink rate. Low contrast and small font, which reduce image quality but do not benefit from eyelid squint, resulted in increased EMG power without changes in aperture size and reduced blink rate (for low contrast). Accommodative and convergence stress (in subjects with normal accommodative and vergence abilities) did not cause changes in EMG power, aperture size or blink rate. These results suggest that contraction of the orbicularis oculi is a part of the asthenopia mechanism related to compromised image quality.

Asthenopia and Blink Rate Under Visual and Cognitive Loads

Purpose. Asthenopia has been associated with reading under visually stressful conditions. However, it is not known whether increased cognitive load contributes to asthenopic symptoms. The purpose of this study was to evaluate the contribution of increased cognitive load (with or without visual stress inducing conditions) to asthenopic symptoms associated with prolonged near work. Methods. Thirty-three visually normal subjects, aged 18 to 30 years, participated in the study. Subjects read texts or watched videos under different visual stress and cognitive loads. Visual stress conditions were good visual quality, low contrast, and induced refractive error. The cognitive load levels were watching video, reading fairy tales, and reading technical articles. As an additional task, subjects also listened to technical articles. At the end of each condition, subjects rated the magnitude of any asthenopic symptoms, visual discomfort, and cognitive discomfort they experienced during the task. Electromyography potentials recorded from the lower orbicularis oculi muscle were used to obtain blink rate. Results. Subjects reported greater internal symptoms for the refractive error condition coupled with higher cognitive load compared to good visual and low contrast conditions (p < 0.01). For the low contrast condition coupled with higher cognitive load, greater external symptoms were reported compared to good visual and refractive error conditions (p < 0.05). However, asthenopic symptoms were not reported for cognitively demanding tasks when the visual condition was good. Blink rates were not significantly different between the good visual and low contrast conditions within each cognitive load level. For the cognitively difficult reading conditions, blink rate was significantly decreased for the low contrast and good visual conditions compared to the refractive error condition. Conclusions. An interaction between cognitive and visual demands was observed. Greater cognitive loads accentuate the same differentiated symptoms normally caused by visual stressors.

Blink rate decreases with eyelid squint

Purpose. We hypothesize that eyelid squint inhibits blink rate. This is part of a larger hypothesis that, because eyelid squint improves vision under conditions of optical defocus and/or glare, and reduced blink rate is assumed to contribute to dry eye symptoms, eyelid squint is part of the mechanism resulting in asthenopia. This study investigates the effect of voluntary eyelid squint on blink activity and on electromyography (EMG) measures from the orbicularis oculi. Methods. Ten subjects (18 to 38 years of age) performed 3 1-minute trials each (Latin Square order) of voluntary target squint levels of 5%, 20%, 35%, and 50% with respect to previously demonstrated 0% (relaxed) and 100% (maximum) squint levels. EMG recordings using surface electrodes were obtained from the orbicularis muscle. Vertical dimension of the palpebral fissure and eye lid blinks were measured with an ISCAN eye tracker and video recorder. Results. Each target squint level produced significant changes (p < 0.0001) in ocular aperture size, EMG power, and EMG amplitude. For target voluntary squint levels of 5%, 20%, 35%, and 50%, the mean squint responses were 24%, 35%, 42%, and 53%, respectively. Blink rate was inversely related to both target squint level and squint response (p < 0.0001), decreasing from 15 blinks per minute at 0% squint to 7.5 blinks per minute at 5% target voluntary squint and to 4 blinks per minute at 50% target voluntary squint. Conclusions. Voluntary eyelid squint significantly reduces blink rate by an average of 50% or more dependent on attempted level. Further study is required to determine if involuntary squint causes the same. All tested levels of voluntary squint resulted in an EMG signal from the orbicularis muscle that is measurably different from resting state. This indicates that EMG can be used as a reliable indicator of eyelid squint.

The influence of vergence adaptation on open-loop vergence dynamics

The objectives of this experiment were to measure the effect of sustained convergence on the open-loop vergence peak velocity and open-loop vergence amplitude, and to assess the correlation between changes in the phoria and changes in open-loop vergence peak velocity induced by sustained convergence. Subjects sustained convergence on a target that required 12 degrees of convergence for 5 minutes. Convergence and divergence movements of 4 degrees from the 12 degrees convergent position were measured before and after sustained convergence. Following sustained convergence, the open-loop vergence peak velocity and vergence amplitude both increased for convergence (regression slope=3.68, r=0.47). Vergence velocity and vergence amplitude both decreased for divergence (regression slope=1.76, r=0.36). After sustained convergence, a convergent shift in the phoria was noted in most cases. This shift correlated with changes in open-loop peak vergence velocity more for convergence (regression slope=1.1, r=0.33) than for divergence (regression slope=0.71, r=0.22). The results might be due to shifts in disparity detection brought about by the period of sustained convergence.

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.

Interactions of visual and cognitive stress

BACKGROUND The objective of this research is to assess the ocular and muscular response to long-duration reading under different visual and cognitive difficulty levels. METHODS Thirty-five subjects, with 20/20 vision and without history of ocular pathology or cognitive deficits, participated in the study. Subjects read under different visual and cognitive difficulty levels for 6 (30-minute) conditions. Upper and lower orbicularis oculi, frontalis, and trapezius muscle activities were recorded using surface electromyography (EMG). Aperture size, pupil diameter, and pulse rate of the subjects were recorded with a video camera, pulse meter, and ISCAN eye tracker (ISCAN Inc.), respectively. RESULTS The results show that the texts read with a refractive error caused increased orbicularis oculi EMG power and reduced aperture size (P < 0.001). There was no statistically significant difference between the conditions for pulse rate, pupil diameter, or EMG activity of the frontalis and trapezius muscles with either visual or cognitive stress presented in this experiment. CONCLUSION Visual stress experienced due to reading under an induced refractive error is potentially mediated by a local mechanism, different from the mechanism underlying reading under low contrast or high cognitive demand.