The forced vergence cover test and phoria adaptation

Toole, Andrew J.; Fogt, Nick

doi:10.1111/j.1475-1313.2007.00498.x

Cited by 6 publications

(4 citation statements)

References 38 publications

(88 reference statements)

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“…Variability in Purkinje image displacements which can arise from minimal variance in orientation and/or placement of the prisms before the infrared-occluded eye during repeat measurements will influence the results. 26 Furthermore, variability in a subject's phoria adaptation at different measurement times, 27 could lead to the high variability observed with the prism-based technique. Finally, potential conflicts in fixation between the target presented to the non-occluded eye and the image of the infrared LED's in the occluded eye could lead to additional variability in this technique.…”

Section: Discussionmentioning

confidence: 99%

Comparison of Three Gaze-position Calibration Techniques in First Purkinje Image–based Eye Trackers

et al. 2019

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This study highlights potential differences that can arise in gaze position estimates from 1st Purkinje image-based eye trackers based on how individual Hirschberg ratios are calculated. Purpose: To evaluate the accuracy and repeatability of eccentric-viewing, prism-based and theoretical techniques that are routinely used to calibrate Hirschberg Ratio (HR) in 1 st Purkinje image-based eye trackers. Methods: Hirschberg ratios of 28 UK participants (18-40 years) were obtained using the PlusOptix PowerRef3 photorefractor and eye tracker. In the gold-standard eccentric viewing technique, participants viewed eccentric targets (±12°, 4° steps) at 2m. In the prism-based technique, 4-16∆D base-out and base-in prisms were placed in 4∆D steps before an eye occluded with an infrared filter; the fellow eye fixated a target at 1m. Each participant's HR was calculated as the slope of the linear regression of the shift in Purkinje image, relative to the pupil centre, for each target eccentricity or induced prism power. Theoretical HR was calculated from the participant's corneal curvature and anterior chamber depth measures. Data collection was repeated on another visit using all three techniques to assess repeatability. Data were also obtained from an Indian cohort (n=30,18-40 years) using similar protocols. Results: HR ranged from 10.61-14.63°/mm (median: 11.90°/mm) in the eccentric viewing technique. The prism-based and theoretical techniques demonstrated inaccuracies of 12% and 4% relative to the eccentric viewing technique. The 95% limits of agreement of intra-subject variability ranged from ±2.00°/mm, ±0.40°/mm, and ±0.30°/mm for the prism-based, eccentric viewing and theoretical techniques (P>.05). Intraclass correlation coefficients (95%CI) were 0.99 (0.98-1.00) for eccentric, 0.99 (0.99-1.00) for theoretical and 0.88 (0.74-0.94) for prism-based techniques. Similar results were found for the Indian cohort. Conclusion: The prism-based and theoretical techniques both demonstrated relative inaccuracies in measures of Hirschberg ratio, compared to the eccentric viewing technique. The prism-based technique exhibited the poorest repeatability.

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Section: Discussionmentioning

confidence: 99%

Comparison of Three Gaze-position Calibration Techniques in First Purkinje Image–based Eye Trackers

et al. 2019

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“…Another problem with our model is that acute prism induced phoria may be compensated over time to result in orthophoria again. This effect is well known as “prism adaptation,” “vergence adaptation,” or “phoria adaptation” (Toole and Fogt, 2007). As we have not measured the actual phoria during the experiment, we cannot conclude that we truly measured reading with phoria.…”

Section: Discussionmentioning

confidence: 99%

Reading performance is not affected by a prism induced increase of horizontal and vertical vergence demand

Dysli¹,

Vogel²,

Abegg³

2014

Front. Hum. Neurosci.

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Purpose: Dyslexia is the most common developmental reading disorder that affects language skills. Latent strabismus (heterophoria) has been suspected to be causally involved. Even though phoria correction in dyslexic children is commonly applied, the evidence in support of a benefit is poor. In order to provide experimental evidence on this issue, we simulated phoria in healthy readers by modifying the vergence tone required to maintain binocular alignment.Methods: Vergence tone was altered with prisms that were placed in front of one eye in 16 healthy subjects to induce exophoria, esophoria, or vertical phoria. Subjects were to read one paragraph for each condition, from which reading speed was determined. Text comprehension was tested with a forced multiple choice test. Eye movements were recorded during reading and subsequently analyzed for saccadic amplitudes, saccades per 10 letters, percentage of regressive (backward) saccades, average fixation duration, first fixation duration on a word, and gaze duration.Results: Acute change of horizontal and vertical vergence tone does neither significantly affect reading performance nor reading associated eye movements.Conclusion: Prisms in healthy subjects fail to induce a significant change of reading performance. This finding is not compatible with a role of phoria in dyslexia. Our results contrast the proposal for correcting small angle heterophorias in dyslexic children.

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“…If an individual's phoria deviates greatly from the current vergence demand, it is thought to put strain on the oculomotor system ( Brodsky, 2020 ; Carter, 1965 ). To reduce this strain, phorias quickly adapt in a matter of seconds to minutes and can even adapt to non-concomitant vergence demands similar to those produced by monocular minification ( Brodsky, 2020 ; Erkelens, Thompson, & Bobier, 2016 ; Henson & Dharamshi, 1982 ; Leigh & Zee, 2015 ; Toole & Fogt, 2007 ; Ying & Zee, 2006 ). A modified Thorington chart and a Maddox rod were used to measure vertical and horizontal phoria.…”

Section: Methodsmentioning

confidence: 99%

The contribution of image minification to discomfort experienced in wearable optics

McLean,

Erkelens,

Sherbak

et al. 2023

Journal of Vision

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Wearable optics have a broad range of uses, for example, in refractive spectacles and augmented/virtual reality devices. Despite the long-standing and widespread use of wearable optics in vision care and technology, user discomfort remains an enduring mystery. Some of this discomfort is thought to derive from optical image minification and magnification. However, there is limited scientific data characterizing the full range of physical and perceptual symptoms caused by minification or magnification during daily life. In this study, we aimed to evaluate sensitivity to changes in retinal image size introduced by wearable optics. Forty participants wore 0%, 2%, and 4% radially symmetric optical minifying lenses binocularly (over both eyes) and monocularly (over just one eye). Physical and perceptual symptoms were measured during tasks that required head movement, visual search, and judgment of world motion. All lens pairs except the controls (0% binocular) were consistently associated with increased discomfort along some dimension. Greater minification tended to be associated with greater discomfort, and monocular minification was often—but not always—associated with greater symptoms than binocular minification. Furthermore, our results suggest that dizziness and visual motion were the most reported physical and perceptual symptoms during naturalistic tasks. This work establishes preliminary guidelines for tolerances to binocular and monocular image size distortion in wearable optics.

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The forced vergence cover test and phoria adaptation

Cited by 6 publications

References 38 publications

Comparison of Three Gaze-position Calibration Techniques in First Purkinje Image–based Eye Trackers

Comparison of Three Gaze-position Calibration Techniques in First Purkinje Image–based Eye Trackers

Reading performance is not affected by a prism induced increase of horizontal and vertical vergence demand

The contribution of image minification to discomfort experienced in wearable optics

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