Anomalous myopias and the intermediate dark focus of accommodation

2022 Glenn A. Fry Award lecture: Enhancing clinical assessment for improved ophthalmic management

ABSTRACT

Detailed clinical assessment is critical to allow sensitive evaluation of the eye and its management. As technology advances, these assessment techniques can be adapted and refined to improve the detection of pathological changes of ocular tissue and their impact on visual function. Enhancements in optical medical devices including spectacle, contact, and intraocular lenses have allowed for a better understanding of the mechanism and amelioration of presbyopia and myopia control. Advancements in imaging technology have enabled improved quantification of the tear film and ocular surface, informing diagnosis and treatment strategies. Miniaturized electronics, large processing power, and in-built sensors in smartphones and tablets capacitate more portable assessment tools for clinicians, facilitate self-monitoring and treatment compliance, and aid communication with patients. This article gives an overview of how technology has been used in many areas of eye care to improve assessments and treatment and provides a snapshot of some of my studies validating and using technology to inform better evidence-based patient management.

The Effect of Retinal Illuminance on the Subjective Amplitude of Accommodation

SIGNIFICANCE

We show that the amplitude of accommodation decreases with retinal illumination even under photopic reading conditions and a constant pupil size. This result provides a basis for clinical approaches that are not based on an optical explanation.

PURPOSE

We investigated the effect of retinal illuminance on the amplitude of accommodation while the pupil of the eye remained constant.

METHODS

The amplitudes of accommodation of 10 young subjects (from 20 to 38 years of age) and that of 10 presbyopic subjects (from 45 to 54 years of age) were measured subjectively through an artificial pupil of 5 mm using a Badal optometer and for four values of retinal illuminance: 222, 821, 2138, and 5074 trolands. Phenylephrine was instilled to all the subjects to ensure that their natural pupil was greater than the artificial one in all experimental runs. Linear mixed-effects model for repeated measures with age and log luminance as covariates were used to check whether changes in amplitude of accommodation with retinal illumination were statistically significant.

RESULTS

In the range of illuminances tested, the amplitude of accommodation decreased on average from 6.34 to 4.35 D in the young subjects and from 1.69 to 1.04 D in the presbyopic subjects. Illuminance was associated with the amplitude of accommodation in both young and presbyopic groups, with P < .01.

CONCLUSIONS

The reduction in the amplitude of accommodation with target illumination (a phenomenon named night presbyopia) under photopic light conditions is not only due to a reduction in the depth of focus as a consequence of pupil dilation; it is strongly affected by the decrease of retinal illumination.

The effect of spherical aberration on visual performance and refractive state for stimuli and tasks typical of night viewing

PURPOSE

The aim of this work was to examine the impact of Seidel spherical aberration (SA) on optimum refractive state for detecting and discriminating small bright lights on a dark background.

METHODS

An adaptive-optics system was used to correct ocular aberrations of cyclopleged eyes and then systematically introduce five levels of Seidel SA for a 7-mm diameter pupil: 0,±0.18, and±0.36diopters (D)mm-2. For each level of SA, subjects were required to detect one or resolve two points of light (0.54 arc min diameter) on a dark background. Refractive error was measured by adjusting stimulus vergence to minimize detection and resolution thresholds. Two other novel focusing tasks for single points of light required maximizing the perceived intensity of a bright point's core and minimizing its overall perceived size (i.e. minimize starburst artifacts). Except for the detection task, luminance of the point of light was 1000cdm-2 on a black background lower than 0.5cdm-2.

RESULTS

Positive SA introduced myopic shifts relative to the best subjective focus for dark letters on a bright background when there was no SA, whereas negative SA introduced hyperopic shifts in optimal focus. The changes in optimal focus were -1.7, -2.4, -2.0, and -9.2D of focus per Dmm-2 of SA for the detection task, resolution task, and maximization of core's intensity and minimization of size, respectively.

CONCLUSION

Ocular SA can be a significant contributor to changes in refractive state when viewing high-contrast point sources typically encountered in nighttime environments.

Surround Propinquity and Tonic Accommodation

PURPOSE

Previous studies of dark focus have found evidence of a propinquity response, in which subjects accommodate to the perceived distance of their surroundings despite being in absolute darkness. Their distance perception in a lighted room allows a spatial representation to persist within the darkened room. This study sought to establish the significance and magnitude of the propinquity response in a large sample of young adults, and to determine whether the propinquity response can be predicted by a subject's initial dark focus in an unknown dark room.

METHODS

Dark focus was measured with a dynamic infrared optometer (Plusoptix PowerRef 3) in 30 young adult subjects, 26 of whom were naive to the testing room and its dimensions. Dark focus was then measured at varying dioptric wall distances of 0.25-4D. The subject viewed the lighted room before each measurement. For each individual, the dark focus was plotted as a function of dioptric wall distance. The slope of this function was used as a measure of the propinquity response.

RESULTS

The mean initial dark focus value was 1.05D (SD 1.04D) for the 26 naive subjects. The mean slope of the plot of dark focus as a function of dioptric wall distance was small (+0.097), yet highly statistically significant (P = .0002). The propinquity response function showed no statistically significant quadratic or cubic trends, and so may be considered linear. No statistically significant correlation was found between naive dark focus and propinquity response (r = +0.246, P = .226).

CONCLUSIONS

Propinquity seems to be a small but statistically significant factor in dark focus measurements. Though it is unlikely to contaminate tonic accommodation measurements in large samples under normal laboratory conditions, a minority of individuals exhibit large propinquity responses equal to that of proximal accommodation in lighted conditions.

Effect of Phenylephrine on the Accommodative System

Accommodation is controlled by the action of the ciliary muscle and mediated primarily by parasympathetic input through postganglionic fibers that originate from neurons in the ciliary and pterygopalatine ganglia. During accommodation the pupil constricts to increase the depth of focus of the eye and improve retinal image quality. Researchers have traditionally faced the challenge of measuring the accommodative properties of the eye through a small pupil and thus have relied on pharmacological agents to dilate the pupil. Achieving pupil dilation (mydriasis) without affecting the accommodative ability of the eye (cycloplegia) could be useful in many clinical and research contexts. Phenylephrine hydrochloride (PHCl) is a sympathomimetic agent that is used clinically to dilate the pupil. Nevertheless, first investigations suggested some loss of functional accommodation in the human eye after PHCl instillation. Subsequent studies, based on different measurement procedures, obtained contradictory conclusions, causing therefore an unexpected controversy that has been spread almost to the present days. This manuscript reviews and summarizes the main research studies that have been performed to analyze the effect of PHCl on the accommodative system and provides clear conclusions that could help clinicians know the real effects of PHCl on the accommodative system of the human eye.

The Effects of Age and Ametropia on the Dark Focus of Visual Accommodation

The effects of age and ametropia on the dark focus of visual accommodation have gone largely unmeasured. Subject populations of young people are generally screened and their vision “standardized” by testing them while they wear corrective lenses. In this study, 301 participants of both sexes, aged 17 to 67, both nearsighted and farsighted were measured for their near points, far points, and dark focuses. It was found that the more nearsighted the eye the larger the dark focus shift. This increase in shifts, however, is relatively small over the commonly found range of ametropia. Age was also found to have a small effect on the dark focus. Older individuals tended to have a smaller dark focus shift, and the dark focus was found to recede with age at roughly the same rate as the far point.

Tonic accommodation predicts closed-loop accommodation responses

The purpose of this study is to examine the potential relationship between tonic accommodation (TA), near work induced TA-adaptation and the steady state closed-loop accommodation response (AR). Forty-two graduate students participated in the study. Various aspects of their accommodation system were objectively measured using an open-field infrared auto-refractor (Grand Seiko WAM-5500). Tonic accommodation was assessed in a completely dark environment. The association between TA and closed-loop AR was assessed using linear regression correlations and t-test comparisons. Initial mean baseline TA was 1.84diopter (D) (SD±1.29D) with a wide distribution range (-0.43D to 5.14D). For monocular visual tasks, baseline TA was significantly correlated with the closed-loop AR. The slope of the best fit line indicated that closed-loop AR varied by approximately 0.3D for every 1D change in TA. This ratio was consistent across a variety of viewing distances and different near work tasks, including both static targets and continuous reading. Binocular reading conditions weakened the correlation between baseline TA and AR, although results remained statistically significant. The 10min near reading task with a 3D demand did not reveal significant near work induced TA-adaptation for either monocular or binocular conditions. Consistently, the TA-adaptation did not show any correlation with AR during reading. This study found a strong association between open-loop TA and closed-loop AR across a variety of viewing distances and different near work tasks. Difference between the correlations under monocular and binocular reading condition suggests a potential role for vergence compensation during binocular closed-loop AR.

Effects of Head-Mounted Display on the Oculomotor System and Refractive Error in Normal Adolescents

PURPOSE

To investigate the clinical effects of head-mounted display on the refractive error and oculomotor system in normal adolescents.

METHODS

Sixty volunteers (age: 13 to 18 years) watched a three-dimensional movie and virtual reality application of head-mounted display for 30 minutes. The refractive error (diopters [D]), angle of deviation (prism diopters [PD]) at distance (6 m) and near (33 cm), near point of accommodation, and stereoacuity were measured before, immediately after, and 10 minutes after watching the head-mounted display. The refractive error was presented as spherical equivalent (SE). Refractive error was measured repeatedly after every 10 minutes when a myopic shift greater than 0.15 D was observed after watching the head-mounted display.

RESULTS

The mean age of the participants was 14.7 ± 1.3 years and the mean SE before watching head-mounted display was -3.1 ± 2.6 D. One participant in the virtual reality application group was excluded due to motion sickness and nausea. After 30 minutes of watching the head-mounted display, the SE, near point of accommodation, and stereoacuity in both eyes did not change significantly (all P > .05). Immediately after watching the head-mounted display, esophoric shift was observed (0.6 ± 1.5 to 0.2 ± 1.5 PD), although it was not significant (P = .06). Transient myopic shifts of 17.2% to 30% were observed immediately after watching the head-mounted display in both groups, but recovered fully within 40 minutes after watching the head-mounted display.

CONCLUSIONS

There were no significant clinical effects of watching head-mounted display for 30 minutes on the normal adolescent eye. Transient changes in refractive error and binocular alignment were noted, but were not significant. [J Pediatr Ophthalmol Strabismus. 2016;53(4):238-245.].

Space perception of strabismic observers in the real world environment

PURPOSE

Space perception beyond the near distance range (>2 m) is important for target localization, and for directing and guiding a variety of daily activities, including driving and walking. However, it is unclear whether the absolute (egocentric) localization of a single target in the intermediate distance range requires binocular vision, and if so, whether having subnormal stereopsis in strabismus impairs one's ability to localize the target.

METHODS

We investigated this by measuring the perceived absolute location of a target by observers with normal binocular vision (n = 8; mean age, 24.5 years) and observers with strabismus (n = 8; mean age, 24.9 years) under monocular and binocular conditions. The observers used the blind walking-gesturing task to indicate the judged location of a target located at various viewing distances (2.73-6.93 m) and heights (0, 30, and 90 cm) above the floor. Near stereopsis was assessed with the Randot Stereotest.

RESULTS

Both groups of observers accurately judged the absolute distance of the target on the ground (height = 0 cm) either with monocular or binocular viewing. However, when the target was suspended in midair, the normal observers accurately judged target location with binocular viewing, but not with monocular viewing (mean slant angle, 0.8° ± 0.5° vs. 7.4° ± 1.4°; P < 0.001, with a slant angle of 0° representing accurate localization). In contrast, the strabismic observers with poorer stereo acuity made larger errors in target localization in both viewing conditions, though with fewer errors during binocular viewing (mean slant angle, 2.7° ± 0.4° vs. 9.2° ± 1.3°; P < 0.0025). Further analysis reveals the localization error, that is, slant angle, correlates positively with stereo threshold during binocular viewing (r(2) = 0.479, P < 0.005), but not during monocular viewing (r(2) = 0.0002, P = 0.963).

CONCLUSIONS

Locating a single target on the ground is sufficient with monocular depth information, but binocular depth information is required when the target is suspended in midair. Since the absolute binocular disparity information of the single target is weak beyond 2 m, we suggest the visual system localizes the single target using the relative binocular disparity information between the midair target and the visible ground surface. Consequently, strabismic observers with residual stereopsis localize a target more accurately than their counterparts without stereo ability.

The visible ground surface as a reference frame for scaling binocular depth of a target in midair

The natural ground surface carries texture information that extends continuously from one's feet to the horizon, providing a rich depth resource for accurately locating an object resting on it. Here, we showed that the ground surface's role as a reference frame also aids in locating a target suspended in midair based on relative binocular disparity. Using real world setup in our experiments, we first found that a suspended target is more accurately localized when the ground surface is visible and the observer views the scene binocularly. In addition, the increased accuracy occurs only when the scene is viewed for 5 s rather than 0.15 s, suggesting that the binocular depth process takes time. Second, we found that manipulation of the configurations of the texture-gradient and/or linear-perspective cues on the visible ground surface affects the perceived distance of the suspended target in midair. Third, we found that a suspended target is more accurately localized against a ground texture surface than a ceiling texture surface. This suggests that our visual system uses the ground surface as the preferred reference frame to scale the distance of a suspended target according to its relative binocular disparity.

Night myopia studied with an adaptive optics visual analyzer

PURPOSE

Eyes with distant objects in focus in daylight are thought to become myopic in dim light. This phenomenon, often called "night myopia" has been studied extensively for several decades. However, despite its general acceptance, its magnitude and causes are still controversial. A series of experiments were performed to understand night myopia in greater detail.

METHODS

We used an adaptive optics instrument operating in invisible infrared light to elucidate the actual magnitude of night myopia and its main causes. The experimental setup allowed the manipulation of the eye's aberrations (and particularly spherical aberration) as well as the use of monochromatic and polychromatic stimuli. Eight subjects with normal vision monocularly determined their best focus position subjectively for a Maltese cross stimulus at different levels of luminance, from the baseline condition of 20 cd/m(2) to the lowest luminance of 22 × 10(-6) cd/m(2). While subjects performed the focusing tasks, their eye's defocus and aberrations were continuously measured with the 1050-nm Hartmann-Shack sensor incorporated in the adaptive optics instrument. The experiment was repeated for a variety of controlled conditions incorporating specific aberrations of the eye and chromatic content of the stimuli.

RESULTS

We found large inter-subject variability and an average of -0.8 D myopic shift for low light conditions. The main cause responsible for night myopia was the accommodation shift occurring at low light levels. Other factors, traditionally suggested to explain night myopia, such as chromatic and spherical aberrations, have a much smaller effect in this mechanism.

CONCLUSIONS

An adaptive optics visual analyzer was applied to study the phenomenon of night myopia. We found that the defocus shift occurring in dim light is mainly due to accommodation errors.

Snow blindness and other eye problems during the heroic age of Antarctic exploration

During the heroic age of Antarctic exploration, snow blindness was a common problem, but not all the descriptions of it fit the modern view of the disease, and some of the explorers complained of long-term problems. This article describes the snow blindness and other eye problems that occurred during this era. It also describes how snow blindness was prevented and treated.

The use of cues to convergence and accommodation in naïve, uninstructed participants

A remote haploscopic video refractor was used to assess vergence and accommodation responses in a group of 32 emmetropic, orthophoric, symptom free, young adults naïve to vision experiments in a minimally instructed setting. Picture targets were presented at four positions between 2 m and 33 cm. Blur, disparity and looming cues were presented in combination or separately to asses their contributions to the total near response in a within-subjects design. Response gain for both vergence and accommodation reduced markedly whenever disparity was excluded, with much smaller effects when blur and proximity were excluded. Despite the clinical homogeneity of the participant group there were also some individual differences.

Depth of focus and visual recognition of imagery presented on simultaneously viewed displays: implications for head-mounted displays

OBJECTIVE

We sought to determine the optimal focal distance for a semitransparent monocular head-mounted display (HMD) integrated with a flight simulator display and to investigate whether observers experienced visual discomfort or impaired target recognition when using an HMD set at the optimal distance.

BACKGROUND

When an observer wears a monocular HMD and views a simulator display, focal distances of both displays must be within the observers' depth of focus to prevent blurred imagery. Because focal distance can vary by as much as 0.5 m in U.S. Air Force multifaceted simulator displays, we determined whether a monocular HMD could be integrated with a simulator display without blurred imagery or discomfort.

METHOD

Depth of focus and visual recognition were measured with a staircase procedure, and visual discomfort was measured with a questionnaire.

RESULTS

Depth of focus was 0.64 diopters in one condition tested, but it was affected by luminance level and display resolution. It was recommended that HMD focal distance equal the optical midpoint of the range of viewing distances encountered in the simulator. Moreover, wearing an HMD produced a decline in recognition performance for targets presented on the simulator display despite both displays being within observers' depth of focus and producing no visual discomfort.

CONCLUSION

Monocular HMDs can be integrated with multifaceted simulator displays without blurred imagery or visual discomfort, provided that the correct focal distance is adopted.

APPLICATION

For situations involving simultaneously viewed visual displays.

Objective real-time measurement of instrument myopia in microscopists under different viewing conditions

While instrument myopia is known to occur when microscopes are used, little is known about the accommodation response during microscopy, or about the factors which may alter the magnitude of instrument myopia. In addition, there has been no real-time objective measurement of instrument myopia during the microscopy task. Twenty inexperienced subjects and 10 experienced microscopists (average work experience of 4.8 years (SD 3.2 yr)) with mean age of 24.1 years (SD 2.9 yr) and 31.2 years (SD 2.9 yr) respectively were recruited to the study. Instrument myopia was measured using an infrared photorefractor (PowerRefractor) under different viewing conditions and microscope settings (with different forms of refractive error correction, changes in target quality, changes in eyepiece power settings, changes in magnification and changes in illumination of the target). Instrument myopia was greater in inexperienced (1.98 D (SD 0.91 D)) than in experienced (1.38 D (SD 0.75 D)) microscope users. There was no statistically significant change in the level of instrument myopia under the different viewing conditions or different microscope settings, and there were large individual variations. Other factors may play more of a role in determining the degree of instrument myopia during microscopy than the task variables altered here.

Initial destination of the disaccommodation step response

Peak velocity and peak acceleration of disaccommodation step responses remain invariant of response magnitude for a constant starting position and they increase linearly with proximity of starting position. This suggests that disaccommodation response is initiated towards an initial (default) destination and is switched mid-flight to attain the desired final destination. The dioptric location of initial destination was estimated from the x-intercept of regression of peak velocity on response starting position. The x-intercept correlated well with subject's cycloplegic refractive state and poorly with their dark focus of accommodation. Altering the dark focus by inducing fatigue in the accommodative system did not alter the x-intercept. These observations suggest that cycloplegic refractive state is a good behavioral correlate of initial destination of disaccommodation step responses.

Perceptual issues in the use of head-mounted visual displays

OBJECTIVE

We provide a review and analysis of much of the published literature on visual perception issues that impact the design and use of head-mounted displays (HMDs).

BACKGROUND

Unlike the previous literature on HMDs, this review draws heavily from the basic vision literature in order to help provide insight for future design solutions for HMDs.

METHOD

Included in this review are articles and books found cited in other works as well as articles and books obtained from an Internet search.

RESULTS

Issues discussed include the effect of brightness and contrast on depth of field, dark focus, dark vergence, and perceptual constancy; the effect of accommodation-vergence synergy on perceptual constancy, eyestrain, and discomfort; the relationship of field of view to the functioning of different visual pathways and the types of visual-motor tasks mediated by them; the relationship of binocular input to visual suppression; and the importance of head movements, head tracking, and display update lag.

CONCLUSION

This paper offers a set of recommendations for the design and use of HMDs.

APPLICATION

Consideration of the basic vision literature will provide insight for future design solutions for HMDs.

Accommodation functions: co-dependency and relationship to refractive error

We assessed the extent to which different accommodative functions are correlated and whether accommodative functions predict the refractive error or the progression of myopia over a 12 month period in 64 young adults (30 myopes and 34 non-myopes). The functions were: amplitude of accommodation; monocular and binocular accommodative facility (6 m and 40 cm); monocular and binocular accommodative response to target distance; AC/A and CA/C ratios, tonic accommodation (dark focus and pinhole), accommodative hysteresis, and nearwork-induced transient myopia. Within groups of related accommodative functions (such as facility measures or open-loop measures) measurements on individuals were generally significantly correlated, however correlations between functions from different groups were generally not significant. Although accommodative amplitude and pinhole (open loop) accommodation were significantly different in myopes than in non-myopes, these functions were unrelated to myopia progression. Facility of accommodation and accommodative lag was independent predictors of myopia progression.

Accommodative state of young adults using reading spectacles

We examined the accommodative state of young adults wearing +2D and +3D reading spectacles under normal conditions and with the elimination of accommodative cues. Subjects' refractions were measured with an infrared PowerRefractor. Power of the vertical meridian was recorded for subjects viewing far and near targets in free space and through a Badal lens apparatus with and without reading spectacles. Additionally, refractive measurements were taken after subjects wore +2D reading spectacles for 30 min (post-adaptation). In free viewing and viewing through the Badal lens, subjects uniformly over-accommodated relative to the target while wearing reading spectacles (i.e., with the spectacles, they focused at a plane in front of the target). Subjects in the first post-adaptation test showed no significant difference in accommodation between viewing a near target with and without +2D spectacles after having read with them for 30 min, though they had without post-adaptation. Subjects in the second post-adaptation test were not significantly differently accommodated before and after reading when binocularly viewing a near target with +2D reading spectacles. The results imply that no adaptation of the subjects' accommodative postures while viewing visual targets occurred as a result of a 1/2 h near work task with the spectacles. The over-accommodation of subjects using reading spectacles while they are performing visual tasks shows the necessity of measurement if their true accommodative posture is to be determined.

Visual acuity in 5-7-year-old children: individual variability and dependence on observation distance

Accurate measurements of uncorrected binocular and monocular visual acuity were performed in 65 children aged 5-7 years at five viewing distances in the range 0.5-5.0 m by means of the test charts containing widely spaced E stimuli in four orientations. It was found that, in most children of this age, visual acuity (V) changed with test distance, as had been reported previously with older subjects. Visual acuity could be considered as practically independent of observation distance (Vmax-Vmin<or=0.2 decimal units) in <50% of children. The remaining children demonstrated evident distance dependence of visual acuity but the shape of the acuity-distance curve varied. The acuity-distance curve of most children peaked at an intermediate distance (1-2 m), the typical difference Vmax-Vmin being 0.4 decimal units. To explain the existence of an optimal observation distance in the majority of children, a number of developmental and environmental factors could be proposed that adjust the parameters of the visual system to the parameters of operational visual space and prevailing activity.

Effects of viewing distance on accommodative and pupillary responses following a three-dimensional task

We have investigated accommodation and pupil responses after viewing stereoscopic images related to the resting position of accommodation. After a 10-min visual task viewing stereoscopic three-dimensional images, measurements of dynamic accommodation and pupil response were made using an infrared optometer and a pupil analyser. Four conditions were given for the viewing distance: 0.4 m (closer than the subjects' resting position), 1 m (approximating to their resting state), and 1.5 and 3 m (farther than the resting position). For the 0.4- and 1-m groups, a delay in the accommodation response for near-to-far movement was shown after the task, from 817 to 1120 ms and from 830 to 898 ms, respectively, but there was no change in the pupillary response. In the 1.5- and 3-m groups, on the other hand, a significant delay in the accommodation response for far-to-near movement was shown, from 827 to 912 ms and from 857 to 1150 ms, respectively, and greater miosis was found, from 7.2 to 9.2 mm2 and from 8.0 to 10.4 mm2. The data reveal that there are different after-effects on accommodation and pupillary functions when subjects perform the task within their accommodative resting position, than when the task is performed outside the resting position.

Senescent changes in scotopic contrast sensitivity

Scotopic contrast sensitivity functions (CSFs) were measured for 50 observers between the ages of 20 and 88 years. Using a maximum-likelihood, 2-alternative, temporal forced-choice threshold-estimation algorithm, scotopic CSFs were measured at 7 spatial frequencies ranging from 0.2 to 3.0 cpd, with mean retinal illuminance equated for observers at -0.85 log scotopic Trolands. For each stimulus condition, eight cycles of a horizontal sinusoidal grating were presented within +/- 1 S.D. of a 2-D Gaussian-spatial envelope and within a 1-s Gaussian-temporal envelope. Stimuli were centered on the nasal retina along the horizontal meridian 6 degrees from the fovea. Scotopic CSFs were found to be low-pass. Statistically significant age-related declines in contrast sensitivities were found for spatial frequencies at or below 1.2 cpd. There was also a statistically significant decrease in the high frequency cut-off with age (P < 0.01). An explanation of these results in terms of optical factors is rejected, while the results are consistent with age-related changes in the magnocellular pathway.

Predicting optimal accommodative performance from measures of the dark focus of accommodation

Leibowitz and his colleagues found that accommodation rests at an intermediate distance that shows wide interindividual variation. They proposed that this intermediate dark focus is useful for correcting anomalous refractive errors, but this proposal was later questioned when different measurement techniques yielded discrepant dark focus values. The present study measured dark focus under two levels of visual attentiveness: (a) when performing an open-loop, active viewing task (aDF); and (b) when looking passively into darkness (pDF). These dark focus measures were then compared with an optimal accommodation distance that was derived from accommodative response functions in bright and dim luminance. The aDF measures were found to be more myopic (nearer) than the pDF measures and highly correlated with the optical accommodation distance. No significant relationship was found between pDF and optical accommodation distance. These findings confirm that measures of dark focus are affected by nonoptical aspects of the measurement technique; they also suggest that techniques that demand visual attention (aDF) yield dark focus values that are more useful for optimizing accommodation and potentially reducing fatigue in difficult situations.

Static aspects of accommodation: age and presbyopia

Although the progressive reduction in accommodative amplitude with increased age is well documented, little is known about several other aspects of static or steady-state accommodation to provide a comprehensive assessment of changes related to age and presbyopia. Static components of accommodation (tonic accommodation, depth-of-focus, slope of the stimulus/response function, and accommodative controller gain) were assessed objectively using an infrared (IR) optometer in 30 human subjects aged 21-50 years; depth-of-focus was also determined psychophysically as was accommodative amplitude. Tonic accommodation and the amplitude of accommodation decreased with increased age, whereas the subjective depth-of-focus increased; the other parameters remained unchanged. The decrease in tonic accommodation and amplitude of accommodation was attributed to biomechanical factors, whereas the increase in subjective depth-of-focus was believed to result from increased tolerance to defocus related to the gradual onset of presbyopia. Constancy of the objective depth-of-focus suggested absence of age effects on the neurologic control of reflex accommodation, whereas the lack of systematic change in slope and controller gain provided support for the Hess-Gullstrand theory of accommodation and presbyopia.

Effect of an intervening screen on accommodation to a distant object

Background: An intervening screen has been suggested to induce an inward shift of accommodation when viewing a distant object. This is an example of the Mandelbaum effect. However, there have been no objective measures of the magnitude of this effect in this particular situation. Methods: Accommodation was recorded with an infra-red optometer, while subjects (n = 16) viewed a distant letter target with or without an intervening screen. Screens were placed near the individual dark focus distance or at 50 cm. In a second experiment the contrast of the distant target was varied and subjects (n = 5) viewed the target directly or through a screen placed near the individual dark focus distance. Results: In the main experiment, the Mandelbaum effect was not significantly different from zero and was less than 0.5 D in every subject. In addition, accommodation was not more variable when viewing through the screen. However, it may be that some subjects do demonstrate a Mandelbaum effect while others do not. The individual dark focus level did not predict susceptibility to the Mandelbaum effect for a screen at the dark focus. Subjects reported their perceptions of the tasks and some noted changes in the perceived distances of objects when viewing through a screen. In the second experiment, the Mandelbaum effect (< 0.6 D) did not vary with distant target contrast. Conclusions: When viewing a distant object through a screen there is a small (< 0.6 D) or negligible inward shift of accommodation.

Multiaccommodative stimuli in VR systems: problems & solutions

Virtual reality environments can introduce multiple and sometimes conflicting accommodative stimuli. For instance, with the high-powered lenses commonly used in head-mounted displays, small discrepancies in screen lens placement, caused by manufacturer error or user adjustment focus error, can change the focal depths of the image by a couple of diopters. This can introduce a binocular accommodative stimulus or, if the displacement between the two screens is unequal, an unequal (anisometropic) accommodative stimulus for the two eyes. Systems that allow simultaneous viewing of virtual and real images can also introduce a conflict in accommodative stimuli: When real and virtual images are at different focal planes, both cannot be in focus at the same time, though they may appear to be in similar locations in space. In this paper four unique designs are described that minimize the range of accommodative stimuli and maximize the visual system's ability to cope efficiently with the focus conflicts that remain: pinhole optics, monocular lens addition combined with aniso-accommodation, chromatic bifocal, and bifocal lens system. The advantages and disadvantages of each design are described and recommendation for design choice is given after consideration of the end use of the virtual reality system (e.g., low or high end, entertainment, technical, or medical use). The appropriate design modifications should allow greater user comfort and better performance.

Emmetropia approach dynamics with diurnal dual-phase cycling

Numerical experiments are performed on a first order exponential response function subjected to a diurnal square wave visual environment with variable duty cycle. The model is directly applicable to exponential drift of focal status. A two-state square wave is employed as the forcing function with high B for time H and low A for time L. Duty cycles of (1/3), (1/2) and (2/3) are calculated in detail. Results show the following standard linear system response: (1) Unless the system runs into the stops, the ready state equilibrium settling level is always between A and B. The level is linearly proportional to a time-weighted average of the high and low states. (2) The effective time constant t(eff) varies hyperbolically with duty cycle. For DC = (1/3) and t1 = 100 days, the effective time constant is lengthened to 300 days. An asymptote is encountered under certain circumstances where t(eff) approaches infinity. (3) Effective time constants and steady state equilibria are independent of square wave frequency f, animal time constant t1, magnitude and sign of A & B, and diurnal sequencing of the highs and lows. By presenting results on dimensionless coordinates, we can predict the drift rates of some animal experiments. Agreement between theory and experiments has a correlation coefficient r = 0.97 for 12 Macaca nemestrina eyes.

Relations between individual differences in oculomotor resting states and visual inspection performance

Individual differences in the oculomotor resting states (dark vergence and dark focus) have previously been linked to subjective and visual consequences of near visual work. The present experiment investigated whether these resting states are related also to performance on a near visual inspection task. Dark vergence and dark focus were measured in 38 students before and after they spent 40 min searching for a target letter among distractor letters on a video display terminal at a distance of 20 cm. Subjects with relatively near dark vergence positions performed the inspection task significantly more quickly than subjects with relatively far dark vergence positions. Also, subjects who showed a relatively large inward shift in dark vergence tended to perform quickly. Inspection performance was not related to individual differences in dark focus. These results extend existing oculomotor theory and suggest that the performance of visual inspectors is maximized when the mismatch between the task distance and their dark vergence posture is minimized.

Oculomotor responses with aviator helmet-mounted displays and their relation to in-flight symptoms

Previous experiments have raised the possibility that abnormal oculomotor functioning could be the substrate for visual problems among aviators who use helmet-mounted displays. However, until the present investigation, no direct evidence linked the two. In our experiments, experienced Apache helicopter pilots, using the same helmet-mounted display that they use in flight, viewed Apache symbology and sensor imagery while we measured their accommodation and vergence eye movements in the laboratory. We found three circumstances that frequently occur in flight in which aviators with visual symptoms had different oculomotor responses than did asymptomatic aviators. Further work needs to be done to determine why symptomatic aviators respond differently from asymptomatic ones.

Visual accommodation as a cue for size

Accommodation micropsia is examined in the general context of ocular accommodation as a cue for object size. The nature and limits of accommodation micropsia and arguments dealing with the possible contribution of accommodation to the perception of size are reviewed. Literature on the anomalous myopias, the intermediate-resting hypothesis, and theories of ciliary muscle innervation is examined critically in so far as it bears on the accommodation-micropsia hypothesis. The anomalous myopias and evidence for the intermediate-resting hypothesis are well documented, but without a mechanism for proprioceptive feedback from the ciliary complex about the state of accommodation it can only be concluded that such feedback would have to be indirect, either via the reflex link with vergence, or possibly through the agency of efference-copy neurones.

A behavioral link between the oculomotor and cardiovascular systems

Although the eyes and the heart serve very different purposes, each receives autonomic innervation. Capitalizing on recent theoretical and technological innovations in the understanding and assessment of oculomotor and cardiovascular behavior, three experiments measured behavioral covariation between the oculomotor and cardiovascular systems. Measures of dark focus and dark vergence indexed oculomotor tone, and the spectral decomposition of variations in heart rate indexed cardiovascular control mechanisms. In Experiment 1, individual differences in cardiovascular parameters could predict individuals' dark vergence (R2 = .806) but not their dark focus (R2 = .404). In Experiment 2, the same parameters were measured from subjects who experience either panic attacks (n = 11) or blood phobia (n = 9). Heart rate was positively correlated with dark vergence and the two subject groups were separable based on both oculomotor and cardiovascular variables. Using a within-subjects approach, Experiment 3 found that both dark vergence and dark focus tended to be nearer during sympathetic dominance of the heart than during parasympathetic dominance, within-subjects variations in cardiovascular parameters could predict dark focus, and between-subjects variations in interbeat intervals could predict dark vergence. Shared patterns of autonomic activation may be responsible for this eye-heart link.

Clinical and laboratory investigations of the relationship of accommodation and convergence function with refractive error. A literature review

Studies of the relationship of clinical and laboratory measures of accommodation and convergence function with refractive error are reviewed. There are inconsistencies in results from study to study presumably due, in part, to methodological differences. However, some basic trends can be outlined. In studies in young adults, accommodation in darkness (dark focus), optical reflex accommodation, and proximally induced accommodation are less in myopes than in emmetropes and hyperopes. It also appears that nearpoint esophoria is associated with higher rates of myopia progression in children. Implications for myopia etiology are discussed.

Spatiotemporal transfer function of human accommodation

We have investigated the spatiotemporal transfer function of human "reflex" accommodation. An accommodative mechanism that is sensitive to an intermediate temporal rate of retinal image contrast change is proposed as the basis of the fine focus control hypothesis. To test the proposed mechanism accommodative responses were monitored by a dynamic infrared optometer while the subject focused on sinusoidal gratings (0.98-10.5 c/deg) which were moving sinusoidally at temporal frequencies in the range of 0.05-0.80 Hz over a 0.50 or 2.00 D peak-to-peak amplitude. The accommodative responses were best at 3 and 5 c/deg at both amplitudes of target motion. This result does not support the proposed mechanism or the fine focus control hypothesis for "reflex" accommodation. Fitting the data with first-order response functions showed little evidence of prediction. In addition, a second experiment found that the profile of the accommodative gain function is not altered by instruction at spatial frequencies above 5 c/deg in this type of dynamic accommodation experiment. The use of sinusoidally moving accommodative blur targets, particularly with careful instruction, seems to discourage voluntary accommodation in investigations of "reflex" control mechanisms of accommodation.

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.

The effect of knowledge of object distance on accommodation during instrument viewing

We present data in which instrument accommodation was measured while knowledge of object distance was varied. The accommodative feedback loop was 'semiopen'--an intermediate state between the closed-loop and open-loop conditions of previous experiments. The semiopen-loop situation mimicked the degraded-image conditions which are frequently encountered during instrument viewing. The results show that for some subjects knowledge of object distance is a more powerful cue for instrument accommodation than is the optical distance of the object; however, for the majority of subjects this is not the case. We also found that subjects whose accommodation is influenced by knowledge of object distance tend to have a more proximal dark focus than those whose accommodation is independent of knowledge of object distance. We propose that the Mandelbaum effect, in which involuntary accommodation occurs when a transparency is superimposed between the observer and the object of regard, could account for the accommodative behavior of all subjects. However, the Mandelbaum effect would have to be interpreted more broadly than before. In the broader interpretation, the transparency could be cognitive (ie known distance) rather than physical.

Repeatability of post-task regression of accommodation in emmetropia and late-onset myopia

The characteristics of post-task regression of accommodation to pre-task tonic accommodation (TA) levels have been examined in a number of studies to clarify the nature of the within-task facility for accommodative adaptation. Of special interest is the recent observation that significant attenuation of post-task regression occurs in late-onset myopes (LOMs) when compared with emmetropes (EMMs). These findings have led to speculation that such attenuation may reflect a deficit in inhibitory sympathetic innervation to ciliary smooth muscle in late-onset myopia and hence a predisposition to sustained accommodative adaptation which then acts as a precursor to the induced myopia. A consequence of this study was that post-task regression may have some value in predicting those individuals who may be susceptible to post-task accommodative hysteresis. A pre-requisite for such a predictive value is that for a given individual the variation in inter-trial regression patterns is not significant. The aim of this study is principally to investigate the inter-trial variability of post-task regression for individual subjects following a sustained near vision task, and to confirm further differences that have been reported between EMMs and LOMs with respect to the time course of post-task regressions. A modified Canon R1 infrared optometer was used to measure accommodation objectively throughout a near task and for 2 min post-task. Accommodative level was measured following 3 min fixation of a high contrast photopic Maltese cross target placed 3 D above the subject's baseline TA. Repeatability of post-task regression in 10 EMMs and 10 LOMs was assessed by taking measurements on three separate occasions.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship among accommodation, focus, and resolution with optical instruments

We studied instrument myopia as an overall effect consisting of interactions among the observer's accommodation and resolution during instrument viewing and the manner in which the instrument is focused. Previous investigations of instrument myopia generally have been limited to only one of these variables. Our results are consistent with the dark-focus-bias theory of accommodation, which holds that accommodation tends to seek its resting point, or dark focus, whether or not the observer is viewing through an optical instrument. The amount of accommodation measured during instrument viewing was found to be dependent on instrument design features that modulate the bias of accommodation for the dark focus.

Apparent minification in an imaging display under reduced viewing conditions

When extended outdoor scenes are imaged with magnification of 1 in optical, electronic, or computer-generated displays, scene features appear smaller and farther than in direct view. This has been shown to occur in various periscopic and camera-viewfinder displays outdoors in daylight. In four experiments it was found that apparent minification of the size of a planar object at a distance of 3-9 m indoors occurs in the viewfinder display of an SLR camera both in good light and in darkness with only the luminous object visible. The effect is robust and survives changes in the relationship between object luminance in the display and in direct view and occurs in the dark when subjects have no prior knowledge of room dimensions, object size or object distance. The results of a fifth experiment suggest that the effect is an instance of reduced visual size constancy consequent on elimination of cues for size, which include those for distance.

Effect of target luminance on microfluctuations of accommodation

It is feasible that small temporal variations in steady-state accommodation may provide feedback to the accommodation control system through changes in retinal image contrast and that this feedback may be used to maintain an optimal accommodation response. The complex waveform of microfluctuations is dominated by two distinct regions of activity; a low frequency component (LFC < 0.6 Hz) and a high frequency component (1.0 < or = HFC < or = 2.3 Hz). Whereas the HFCs appear to be correlated with some intraocular manifestation of arterial pulse the contribution of the LFCs to the control of steady-state accommodation is unclear. The present study investigates the effect of target luminance on the waveform of accommodative microfluctuations. Three young emmetropic observers viewed monocularly a high contrast (90%) Maltese cross target placed at a vergence equal to their dark-focus level of accommodation in a Badal stimulus system. The luminance of the target was varied from 0.002 to 11.63 cd m-2 in nine equal logarithmic steps. Five continuous accommodation signals were collected for each viewing condition at a sampling rate of 102.4 Hz, and average power spectra subsequently calculated with a frequency resolution of 0.1 Hz. One-way ANOVA revealed a significant variation in the root-mean-square (r.m.s.) value of the microfluctuations (F = 19.795, d.f. 124, P = 0.0001) which could be attributed mainly to increases in the r.m.s. value for the two lowest luminances (0.002 and 0.004 cd m-2). Power spectrum analysis revealed that these changes in the microfluctuations could be attributed to increases of power in the LFC.(ABSTRACT TRUNCATED AT 250 WORDS)

Tonic accommodation: a review. I. Basic aspects

In the absence of an adequate visual stimulus, accommodation adopts an intermediate position of approximately 1 D. Since this position was believed to reflect the level of tonic innervation to the ciliary muscle, this response has been termed tonic accommodation (TA). Part I of this review will consider various aspects of this parameter, including its reference to closed-loop accommodative function and autonomic physiology. In addition, both the methods of measurement and appropriate terminology for this function will be discussed. It is concluded that the response, which becomes apparent under so-called 'stimulus-free' conditions, in fact probably represents an aggregate response resulting from multiple, non-optical stimuli. Thus the designation tonic accommodation may not be appropriate, since it fails to describe accurately the heterogeneous composition of the stimulus-free accommodative response. An associated paper (to be published as part II of this review) will examine accommodative adaptation and both clinical aspects of TA and adaptation of TA.

Effects of a visual task with cognitive demand on dynamic and steady-state accommodation

Dynamic accommodative response before and after, and steady-state accommodation during, a visual task were measured using an infrared optometer to investigate the effects of psychological stress on accommodative facility. Increase in the post-task dynamic accommodation response time (for a near-to-far condition) was greater for a group of subjects given a mental task at a higher presentation speed. In addition the steady-state accommodation showed an inward shift under a closed-loop condition which was enhanced when computation time was decreased. However, an inward shift was not evident when the performance level of subjects increased following training. The relationship between psychological stress, cognitive demand and accommodative function is discussed.

Losses of flicker sensitivity during dark adaptation: effects of test size and wavelength

The losses of flicker sensitivity that occur during dark adaptation following extinction of a bleach depend greatly on the test stimuli that are used to measure those losses. Generally, the larger the test size is for a given test wavelength, the later is the loss of flicker sensitivity; for sufficiently large tests, losses become progressively more gradual. Similarly, the shorter the test wavelength is (between about 660 and 580 nm) for a given test size, the later is the loss of flicker sensitivity; for sufficiently short test wavelengths, losses become progressively more gradual. Although the pattern of flicker sensitivity loss as a function of test size is the same both foveally and parafoveally, foveal losses occur later and/or more gradually for a given test stimulus. The results suggest that losses of flicker sensitivity occur when spectrally antagonistic response(s) to the test are no longer adequate to maintain the flicker-response to that test. The results suggest also that the adequacy of such spectral antagonism depends on the local adaptation state of the retina, which becomes increasingly more light-adapted with increasingly large test size.