Eyelid squint response to asthenopia-inducing conditions
ABSTRACT To study the orbicularis oculi muscle response to asthenopia-inducing conditions.
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.
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).
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.
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ABSTRACT: 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. 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. 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. 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.Optometry and Vision Science 08/2007; 84(7):620-9. DOI:10.1097/OPX.0b013e3180dc99a8 · 2.04 Impact Factor
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ABSTRACT: Augmented-vision devices that we are developing to aid people with low vision (impaired vision) employ vision multiplexing- the simultaneous presentation of two different views to one or both eyes. This approach enables compensation for vision deficits without depriving the wearers of their normal views of the scene. Ideally, wearers would make use of the simultaneous views to alert them to potential mobility hazards, without a need to divide attention consciously. Inattentional blindness, the frequent inability to notice otherwise-obvious events in one scene while paying attention to another, overlapping, scene, works against that sort of augmentation, so we are investigating ways to mitigate it. In this study, we filtered the augmented view, creating cartoon-like representations, to make it easier to detect significant features in that view and to minimise interference with the normal view. We reproduced a classic inattentional blindness experiment to evaluate the effect, and found that, surprisingly, edge filtering had no detectable effect - positive or negative - on the noticing of unexpected events in the unattended scene. We then modified the experiment to determine if the inattentional blindness was because of the confusion of overlaid views or simply a matter of attention, and found the latter to be the case.Ophthalmic and Physiological Optics 06/2008; 28(3):204-17. DOI:10.1111/j.1475-1313.2008.00537.x · 2.66 Impact Factor