The effect of stimulus duration on tonic accommodation and tonic vergence

Digital Eyestrain and the Critical Fusion Frequency

SIGNIFICANCE

Although the high prevalence of digital eyestrain has been well established, to date, there is no objective measurement of this condition. Previous studies have suggested that digital eyestrain may be associated with decreased critical fusion frequency, but the published evidence to support this association is sparse.

PURPOSE

This study sought to determine whether symptoms of digital eyestrain after a sustained computer task are indeed associated with changes in critical fusion frequency.

METHODS

The experiment was performed on 30 young visual-normal subjects. They attended two sessions, during which they undertook a 20-minute reading task. This comprised either reading random words from a tablet computer or a story from a printed children's book. Critical fusion frequency was measured both before and immediately after each of the reading tasks. In addition, the level of digital eyestrain was assessed by subjects completing a questionnaire regarding ocular and visual symptoms experienced during each of the reading trials.

RESULTS

The mean increase in digital eyestrain symptoms after the digital and printed conditions was 11.37 (standard error of the mean [SEM], 2.23) and 4.40 (SEM, 1.34), respectively. Both the post-task symptom change (P < .001) and the difference between the two reading conditions (P = .004) were significant. The mean change in critical fusion frequency after the digital and printed conditions was -0.42 (SEM, 0.25) and -0.72 (SEM, 0.26), respectively. Neither the post-task change nor the difference between the two reading conditions was significant. In addition, when considering the digital condition only, no significant correlation was observed between the changes in critical fusion frequency and reported symptoms.

CONCLUSIONS

These results do not support the proposal that changes in critical fusion frequency can be used as an objective measure of eyestrain or other symptoms of digital eyestrain.

A differential role for the posterior cerebellum in the adaptive control of convergence eye movements

INTRODUCTION

The vergence oculomotor system possesses two robust adaptive mechanisms; a fast "dynamic" and a slow "tonic" system that are both vital for single, clear and comfortable binocular vision. The neural substrates underlying these vergence adaptive mechanisms in humans is unclear.

METHODS

We investigated the role of the posterior cerebellum in convergence adaptation using inhibitory continuous theta-burst repetitive transcranial magnetic stimulation (cTBS) within a double-blind, sham controlled design while eye movements were recorded at 250hz via infrared oculography.

RESULTS

In a preliminary experiment we validated our stimulation protocols by reproducing results from previous work on saccadic adaptation during the classic double-step adaptive shortening paradigm. Following this, across a series of three separate experiments we observed a clear dissociation in the effect of cTBS on convergence adaptation. Dynamic adaptation was substantially reduced while tonic adaptation was unaffected. Baseline dynamic fusional vergence response were also unaffected by stimulation.

CONCLUSIONS

These results indicate a differential role for the posterior cerebellum in the adaptive control of convergence eye movements and provide initial evidence that repetitive transcranial magnetic stimulation is a viable tool to investigate the neurophysiology of vergence control. The results are discussed in the context of the current models of implicit motor adaptation of vergence and their application to clinical populations and technology design in virtual and augmented head mounted display architectures.

SIGNIFICANCE STATEMENT

The cerebellum plays a critical role in the adaptive control of motor systems. Vergence eye movements shift our gaze in depth allowing us to see in 3D and exhibit two distinct adaptive mechanisms that are engaged under a range of conditions including reading, wearing head-mounted displays and using a new spectacle prescription. It is unclear what role the cerebellum plays in these adaptive mechanisms. To answer this, we temporarily disrupted the function of the posterior cerebellum using non-invasive brain stimulation and report impairment of only one adaptive mechanism, providing evidence for neural compartmentalization. The results have implications for vergence control models and applications to comfort and experience studies in head-mounted displays and the rehabilitation of clinical populations exhibiting vergence dysfunctions.

The forced vergence cover test and phoria adaptation

Phoria adaptation (PA) provides innervation to help maintain the correct vergence posture for binocularly viewing objects. Once fusion is disrupted, such as is required for measuring a (dissociated) phoria, this innervation is slowly depleted. Thus, extended periods of monocular occlusion can be required to dissipate PA and reveal the full extent of the phoria. Two versions of a forced vergence modification of the alternating cover test (CT) have been proposed to rapidly show the full phoria. We evaluated the ability of these forced vergence CTs (FVCT-1 and FVCT-2) to deplete the PA induced by a 15 min adaptation period to base out prism. In both experiments, the CT followed by the FVCT was performed before and after the adaptation period. In Experiment 1 (n = 13), the FVCT-1 was evaluated at 40 cm. Experiment 2 (n = 32) evaluated the FVCT-2 at 4 m. In Experiments 1 and 2, the difference between pre- and post-adaptation CTs showed significant PA occurred during the adaptation period (7.8(Delta), p < 0.0005, and 5.4(Delta), p < 0.0005 respectively). In Experiment 1, the FVCT-1 did not reveal a larger phoria than the CT before (mean difference: 0.4(Delta), p = 0.34) or after (no difference for all subjects) the adaptation period. Thus, the FVCT-1 did not alter PA. In Experiment 2, the FVCT-2 did show a shift in the phoria compared to the CT. However, this shift was found to be equivalent before and after the adaptation period (mean difference in shift: 0.22(Delta), 95% CI: -0.52 to 0.96(Delta)). Thus, the FVCT-2 shifts the phoria a constant amount independent of the amount of PA present. We conclude that neither FVCT's behaviour is dependent on the PA present. Thus, these procedures are unlikely to be effective clinical procedures for revealing the full magnitude of the phoria.

The effect of a beta-adrenoceptor antagonist on accommodative adaptation in Hong Kong children

PURPOSE

Increased susceptibility to nearwork-induced accommodative adaptation has been suggested as a risk factor for myopia development. We investigated whether accommodative adaptation may explain in part the high prevalence of myopia in Hong Kong children and examined the effect of beta-antagonism with topical timolol maleate on accommodative adaptation.

METHODS

Thirty children (10 emmetropes and 20 myopes) aged between 8 and 12 years were recruited. Tonic accommodation was measured before and after 5 min of video game-playing using an open-field Shin-Nippon autorefractor. Measurements were repeated 30 min after timolol instillation.

RESULTS

Children with progressing myopia demonstrated accommodative adaptation following the near task, whereas stable myopes showed counter-adaptive, hyperopic accommodative changes. Timolol increased the magnitude of accommodative adaptation in the stable myopes but had little effect on responses of the progressing myopes or emmetropes.

CONCLUSIONS

Neuropharmacological modulation of the accommodative system may have a possible etiological role in the progression of myopia.

Tonic accommodation: a review. II. Accommodative adaptation and clinical aspects

Part I of this review considered basic aspects of tonic accommodation (TA), i.e. the accommodative response observed under degraded stimulus conditions. Part II considers accommodative adaptation, i.e. the apparent change in TA following periods of sustained fixation, and clinical aspects of both baseline TA and accommodative adaptation. It is suggested that the apparent post-task shift in TA reflects the slow rate of decay of the stimulus-mediated adaptive accommodative response, while the actual level of tonic innervation to the ciliary muscle remains relatively constant. The clinical implications of both TA and accommodative adaptation are discussed with regard to night, space and instrument myopia and refractive error development, notably nearwork-induced myopia. It is concluded that the evidence for any association between this form of myopia and either TA or accommodative adaptation is equivocal, and furthermore it seems likely that TA plays only a minor role in influencing the closed-loop steady-state accommodative response.

Accommodative adaptation and age of onset of myopia

Tonic accommodation in an empty field and accommodative adaptation, defined as accommodation shift after 2 min viewing of a 4 D near task, were measured using an infrared optometer. Subjects were classified into three types: emmetropes (n = 18), early onset myopes (n = 18) and late onset myopes (n = 15). The relationship between accommodative adaptation and the onset age of myopia were compared. There was no statistical difference in the pre-task tonic accommodation among the three subject groups. However, the accommodative adaptation differed in the three groups during the 5 min post-task period. The average accommodative adaptation of emmetropia, early onset myopia and late onset myopia were 0.66, 0.27 and 1.19 D, respectively. Accommodative adaptation in the late onset myopia group was significantly higher than in the emmetropia group. Furthermore, the accommodative adaptation in the early onset myopia group was significantly lower than in the emmetropia group. Two possible aetiologies of myopia are discussed: one for the group which shows low accommodative adaptation that has no relationship with the age of onset and another for the group which shows high accommodative adaptation that has a relationship to an onset age of 15 years or later. The aetiology is also considered with regard to a potentiating chemical effect on the synapses of the ciliary muscle.

Temporal characteristics of proximally-induced accommodation

This study sought to determine whether proximally-induced accommodation (PIA) is capable of sustaining an accommodative response over time. Accordingly, the steady-state PIA response was measured during the course of a continuous 5 min stimulus period for targets located at distances of 0.2 or 6 m. The vergence and accommodation loops were opened by subjects (n = 8) monocularly viewing the targets through a 0.5 mm pinhole, while accommodation was assessed using an objective, open-field, infrared optometer. The mean PIA response did not change significantly during the fixation period. This finding suggests that PIA is indeed capable of maintaining a sustained response. This observation is consistent with the constancy of apparent target distance perception during extended fixation of a stationary object of regard under these conditions.