Accommodation System Research Articles

Temporal integration of astigmatic blur information

Purpose: The accommodation response has recently been shown to improve visual acuity (VA) in the presence of astigmatism in some individuals. This improvement could potentially result from the accommodation system exhibiting a cyclic tracking response. This type of response would allow the integration of target features over time, resulting in an improvement in VA. In this study we measure VA in simulated astigmatic blur conditions, with the magnitude of astigmatic blur being temporally modulated over a range of astigmatic blur intervals. Improvement of VA with increased range of astigmatic blur would support the integration of visual information over time hypothesis.Methods: Three fully corrected subjects participated in a 3‐Alternative Force Choice method of constant stimuli paradigm where they were asked to identify one of three randomly chosen letters (K, R or X). The stimuli (Arial, font size 18) were subjected to varying amounts of Gaussian blur along the horizontal and vertical meridian and presented at a distance of 1.3 m. On any trial, the stimuli were randomly presented to the subjects at one of six astigmatic blur intervals. Each blur interval revolved at a temporal frequency of 2 Hz around the circle of least confusion with the smallest interval presenting the stimuli at the circle of least confusion and the largest oscillating the stimuli across the full interval, i.e. from the most horizontally to vertically blurred meridian.Results: From the resulting psychometric functions it was shown that all subjects exhibit a progressive improvement in letter recognition with increasing astigmatic blur interval.Conclusions: The improvement of VA with increased astigmatic blur interval strengthens the hypothesis that visual information can be integrated over a period of time.

항공기 제어면/구동장치 고장에 대한 진단규명 및 보완 제어시스템 설계에 관한 연구

본 논문에서는 항공기의 제어면 혹은 구동장치가 고장이 났을 때 이를 신속하게 진단규명하고 안정한 상태로 되돌리기 위한 고장보완제어시스템을 설계한다. 제어시스템 설계에는 수렴 속도가 빠른 확장 역전파 알고리즘을 적용한 신경회로망을 이용한다. 제어대상으로는 비선형 운동방정식으로 표현된 F-4 항공기이며 수평미익이나 에일러런의 고장이 발생할 경우에 대하여 제어시스템을 설계하고 시뮬레이션을 통하여 성능을 검증한다. In this study a control surface/actuator fault detection, identification, and accommodation system for aircraft is designed. This fault tolerant control system tries to return aircraft to its stable trim condition in a short time. The control system is designed using neural networks with Extended Back Propagation Algorithm which shows fast convergence. F-4 aircraft with possible stabilator or aileron failure/stuck is simulated with the proposed scheme.

Accommodation to simulations of defocus and chromatic aberration in the presence of chromatic misalignment

Previous studies have demonstrated that accommodation will respond to sine gratings in which the relative modulations of red, green and blue image components have been altered to simulate the effects of defocus and longitudinal chromatic aberration. The present study aimed to determine the tolerance of the accommodative system to relative phase shifts in those components induced by chromatic misalignment. It was found that accommodation can tolerate moderate amounts of chromatic misalignment (6 ′), but responds adversely when misalignments are large. Applications to visual display terminals and spectacle lens and instrument design are discussed.

A Model-Based Fault Accommodation System

Fault accommodation, the task of using the results from fault diagnosis to accommodate faults, is often performed by an operator through table-look-up strategies, which are often incomplete, time-consuming, and prone to errors. In this paper, we proposed an analytical model-based control redesign for fault accommodation. The whole fault accommodation system includes a dynamic process model, fault detection and isolation, fault identification and state estimation, and a dynamic optimization module for control redesign. One of the advantages of this method lies in its ability to simultaneously achieve economic optimality and operation safety because of its capability to handle both hard equipment constraints and operational constraints. Strategies for accommodating various types of abnormal situations are proposed. The details of each module are discussed, followed by a case study in which the methodology is applied to the Amoco model IV fluid catalytic cracking unit.

A Dual-Mode Dynamic Model of the Human Accommodation System

The function of the accommodation system is to provide a clear retinal image of objects in the visual scene. The system was previously thought to be under simple continuous (i.e., single mode of operation) feedback control, but recent research has shown that it is under discontinuous (i.e., two stimulus-dependent modes of operation) feedback control by means of fast and slow processes. A model using MATLAB/SIMULINK was developed to simulate this dual-mode behavior. It consists of fast and slow components in a feedback loop. The fast component responds to step target disparity with an open-loop movement to nearly reach the desired level, and then the slow component uses closed-loop feedback to reduce the residual error to an acceptable small level. For slow ramps, the slow component provides smooth tracking of the stimulus, whereas for fast ramps, the fast component provides accurate staircase-like step responses. Simulation of this model using a variety of stimuli, including pulse, step, ramp, and sinusoid, showed good agreement with experimental results. Thus, this represents the first dynamic model of accommodation that can accurately simulate the complex dual-mode behavior seen experimentally. The biological significance of this model is that it can be used to quantitatively analyze clinical deficits such as amblyopia and accommodative insufficiency.

The role of deformation in controlling depositional patterns in the south-central Niger Delta, West Africa

The Niger Delta has a distinctive structural and stratigraphic zonation. Regional and counter-regional growth faults, developed in an outer-shelf and upper-slope setting, are linked, via a translational zone containing shale diapirs, to a contractional zone defined by a fold-thrust belt that developed in a toe-of-slope setting. Structural and depositional systems have migrated with the progradation of the delta. A paleo fold belt is buried under the modern upper/middle slope. The structural system in this paleo fold belt is complex and comprises a series of en échelon thrust-cored folds and associated ponded slope-basins, shale diapirs, and extensional growth faults. Analysis of the growth sections filling the ponded slope-basins provides a record of how this accommodation was created and subsequently filled and how the individual structural elements interact to create and modify the available space. The depositional systems initially exploit primary accommodation on the slope created by structural movement—the synchronous growth of the fold, the extensional faults and the shale diapir. As the pond is progressively filled, the previously deposited strata modify the accommodation and subsequent depositional systems compensate accordingly.

Anatomical and ultrastructural comparison of the eyes of two species of aquatic, pulmonate gastropods: The bioluminescent Latia neritoides and the non‐luminescent Ancylus fluviatilis

Light‐adapted eyes of two freshwater gastropods with similar habitat preferences (Latia neritoides (Gray 1850) and Ancylus fluviatilis (Müller 1774)) were compared with each other in relation to optics, anatomy, and ultrastructure. Individuals of L. neritoides not only possess significantly larger eyes than similarly sized A. fluviatilis, they also have a more voluminous layer of photorecep‐tive membranes and appear capable of discerning at least some crude shapes. The eye of A. fluviatilis, on the other hand, has a very shallow retina which is so closely apposed to the lens that it can be little more than an indicator of light and darkness. Both types of eye are generously endowed with screening pigment granules and exhibit no signs of light‐induced damage. Although the lenticular F‐values for L. neritoides (1.78) and A. fluviatilis (1.74) alone are hardly indicative of an adaptation to a dark environment, the greater amount of photic vesicles and the substantial development of muscle fibres in the eye of L. neritoides suggest that the latter can adjust its vision more rapidly to changing light levels than that of A. fluviatilis. Since L. neritoides (but not A. fluviatilis) has the ability to secrete a bright green luminescent mucus, a faster accommodation system would serve the species well. There is, however, no proof that in Latia bioluminescence alone has been responsible for the development of a more capable eye. Most of the differences between the eyes of L. neritoides and A. fluviatilis can be explained by the distinctly greater nocturnal activity of L. neritoides in combination with the ability to produce bright light in the latter species.

Emmetropization in chicks uses optical vergence and relative distance cues to decode defocus

When visual information is confined to one object plane, the emmetropization end-point is adjusted in accord with the corresponding incident optical vergence at the eye [Proceedings of the 7th International Conference on Myopia (2000) 113]. We now report the effect of adding extra visual information beyond the target plane. Visual conditions were controlled using a cone–lens system: black Maltese cross targets on white opaque backgrounds (OMX) were attached to the open faces of 2.5 cm translucent cones fitted with either 0, +25 or +40 D imaging lenses. An alternative target (TMX) was made by substituting the opaque target background for a transparent background, which allowed access to visual information beyond the target plane. The imaging devices were applied to 7-day-old chicks and worn for 4 days. Prior to this treatment, on day 2, some chicks underwent ciliary nerve section (CNS) to preclude accommodation. All treatments were monocular. Refractive errors and axial ocular dimensions were measured using retinoscopy and A-scan ultrasonography under halothane anesthesia. Treatment effects were specified as mean ( ±S.D.) interocular differences. Eyes with the OMX/+40 D lens combination remained emmetropic (+0.73 ±3.57 D), consistent with the target plane being approximately conjugate with the retina. Switching to the TMX caused a hyperopic shift in refractive error (+3.78 ±3.41 D). This relative shift towards hyperopia in switching from the OMX to the TMX target also occurred for the other two lens powers. Thus, the OMX/+25 D lens induced myopia (−7.00 ±5.88 D), corresponding to the imposed hyperopic defocus (target plane now imaged behind the retina), and switching to the TMX resulted in a reduction in myopia (−1.73 ±5.36 D). The OMX/0 D lens combination produced the largest myopic shift, and here, switching to the TMX condition almost eliminated the myopic response (−15.50 ±6.62 D cf. −0.56 ±1.24 D). This relative hyperopic shift associated with switching from the OMX to the TMX target was eliminated by CNS surgery. Thus, the two CNS/TMX groups were both more myopic than the equivalent no CNS/TMX groups (+40 D lens: −2.66 ±2.34 D; +25 D lens: −7.97 ±6.87 D). When the visual information is restricted to one plane, incident optical vergence appears to direct emmetropization. Adding visual information at other distances produces a shift in the end-point of emmetropization in the direction of the added information. That these effects are dependent on the integrity of the accommodation system implies that accommodation plays a role in emmetropization and represents the first reported evidence of this kind.

Age changes in the interactions between the accommodation and vergence systems.

Experiments are described in which static and dynamic accommodation (Ar), accommodative convergence (AC), vergence (C) and convergence accommodation (CA) responses to small stimuli were measured dynamically in 13 subjects with ages in the range 16 to 48 years. Analysis showed that the amplitudes of both blur and disparity-driven accommodation declined significantly with age, whereas the two types of vergence response did not. As a result, the AC/Ar ratio rose significantly with age, whereas the CA/C ratio fell. No significant change with age was found in response latencies and durations.

Fixation disparity and accommodation for stimuli closer and more distant than oculomotor tonic positions

Both the vergence and the accommodative system have individual tonic positions (also referred to as dark vergence and dark focus, respectively) where the static response may be expected to be most accurate. This was confirmed by measuring fixation disparity with nonius lines and accommodation with an autorefractometer for foveal stimuli at viewing distances of 460, 100, 60, 40, and 30 cm. Multiple regression analysis was used at each viewing distance to predict fixation disparity from dark vergence, dark focus, accommodative gain and accommodative convergence: these accommodative measures had little effect on the inter-individual variability of near fixation disparity nor on the linear slope of fixation disparity as a function viewing distance.

Fatigue reduces tonic accommodation.

Ocular accommodation adopts a mean baseline response level of approximately 1.0 D in the absence of blur feedback (open-loop state). This baseline or tonic accommodation (TA) can be elevated following a sustained monocular accommodative response to a dioptric stimulus (lens adaptation) that exceeds the baseline open-loop level of TA. The accommodative response to the lens persists in the open-loop state (accommodative hysteresis), and eventually decays to a stable end-point. Interestingly, if the baseline TA is high, the monocularly adapted accommodative state can decay to an end-point that is below the initial pre-adapted baseline level of the TA (counter-adaptive response) (McBrien, N.A. and Millodot, M., (1988). Differences in adaptation of TA with refractive state. Invest. Ophthalmol. Vis. Sci., 29, 460-469). We have investigated the possible contribution of accommodation fatigue to the counter-adaptive change in baseline TA following sustained accommodation. Two fatigue procedures were used while viewing a target at 66 or 33 cm. In a monocular condition, accommodation was stimulated for 3 min with lens values alternating from -1.5 to +1.5 D at a rate of 0.25 Hz. In the binocular condition, convergence was stimulated with alternating prism values from 9 prism diopters (PD) base-out to 9 PD base-in. Both monocular and binocular tasks resulted in a significant reduction of TA. These results suggest that previously reported reductions of baseline TA following sustained monocular accommodation or binocular convergence could have resulted from fatigue of the accommodative system. Accommodative fatigue could be responsible for the lower values of TA observed in counter-adaptive responses to sustained accommodative or convergence effort.

Fatigue reduces tonic accommodation

Ocular accommodation adopts a mean baseline response level of approximately 1.0D in the absence of blur feedback (open-loop state). This baseline or tonic accommodation (TA) can be elevated following a sustained monocular accommodative response to a dioptric stimulus (lens adaptation) that exceeds the baseline open-loop level of TA. The accommodative response to the lens persists in the open-loop state (accommodative hysteresis), and eventually decays to a stable end-point. Interestingly, if the baseline TA is high, the monocularly adapted accommodative state can decay to an end-point that is below the initial pre-adapted baseline level of the TA (counter-adaptive response) (McBrien, N.A. and Millodot, M., (1988). Differences in adaptation of TA with refractive state. Invest. Ophthalmol. Vis. Sci., 29, 460–469).We have investigated the possible contribution of accommodation fatigue to the counter-adaptive change in baseline TA following sustained accommodation. Two fatigue procedures were used while viewing a target at 66 or 33cm. In a monocular condition, accommodation was stimulated for 3min with lens values alternating from −1.5 to +1.5D at a rate of 0.25Hz. In the binocular condition, convergence was stimulated with alternating prism values from 9prism diopters (PD) base-out to 9PD base-in. Both monocular and binocular tasks resulted in a significant reduction of TA.These results suggest that previously reported reductions of baseline TA following sustained monocular accommodation or binocular convergence could have resulted from fatigue of the accommodative system. Accommodative fatigue could be responsible for the lower values of TA observed in counter-adaptive responses to sustained accommodative or convergence effort.

Changes in the static accommodation response with age.

A transverse study of the monocular, steady-state accommodation response/stimulus curves for 49 normal subjects with ages between 17 and 49 years shows that the slope of the curve reduces only slowly with age up to about 40 years and then declines rapidly as complete presbyopia is approached. Such behaviour appears to result from the accommodation system changing its characteristics with age to make optimal use of the available objective amplitude of accommodation and the depth-of-focus of the eye.

Changes in the static accommodation response with age

A transverse study of the monocular, steady-state accommodation response/stimulus curves for 49 normal subjects with ages between 17 and 49years shows that the slope of the curve reduces only slowly with age up to about 40years and then declines rapidly as complete presbyopia is approached. Such behaviour appears to result from the accommodation system changing its characteristics with age to make optimal use of the available objective amplitude of accommodation and the depth-of-focus of the eye.

Guanine and its retinal distribution in the tapetum of the bigeye tuna, Thunnus obesus

The eye of the bigeye tuna (Thunnus obesus) contains a retinal tapetum composed of guanine. The total amount of the guanine in one eye of the fish (SL=120 cm) was about 88.6 mg. The mean guanine content of the tapetum was approximately 1.25 mg/cm2 of the retinal surface. The highest content of guanine (2.15 mg/cm2) was observed only in the ventro-temporal part of the retina. To distinguish this area from the rest of the eye, we suggested the term ‘locus tapetalis’ for it. The visual accommodation system clearly indicated that the visual axis of the fish is upper-forward and the resulting retinal area for acute vision was suggested to be in the ventro-temporal retina. We discussed that the area centralis of the bigeye tuna may have two functions: to guarantee high visual acuity and to allow for high photo-sensitivity in dim light vision.

Dynamic optimization based fault accommodation

Fault diagnosis is an important aspect of process operations. There is an abundance of literature on fault diagnosis for process industries. So far most of the work is focussed on fault detection, isolation, and identification. The use of results from fault diagnosis often performed by the operator through table-looking strategies, are often incomplete, time-consuming, and prone to errors. In this study, a dynamic optimization based control redesign for fault accommodation is proposed. The whole fault accommodation system includes a dynamic process model, fault detection and isolation, fault identification and state estimation, and dynamic optimization block for control redesign. This method simultaneously achieves the economic optimality and operation safety due to its capability to handle both hard equipment constraints and operational constraints.

Modelling adaptation effects in vergence and accommodation after exposure to a simulated Virtual Reality stimulus

SummaryNatural viewing conditions place equal demands on the vergence and accommodation systems. The two responses are co‐ordinated via the interactive components which couple the two systems: accommodative vergence and vergence accommodation. The crosslink components are usually quantified by the AC/A and CA/C ratios. Whether these ratios are stable entities, genetically determined, or modifiable by experience has been the subject of some debate. A ‘Virtual Reality’ stimulus was used to place unequal demands on the vergence and accommodation systems. Pre‐task and post‐task measures of AC/A and CA/C were objectively determined. The changes in the post‐task measures are discussed with reference to a dual interactive model of vergence and accommodation. Model simulations suggest that adaptation of the vergence and accommodative controllers (tonic adaptation) may also play a part by altering the open loop bias. The results therefore support the idea that vergence accommodation and accommodative vergence are capable of adaptation.

Modelling adaptation effects in vergence and accommodation after exposure to a simulated Virtual Reality stimulus

Natural viewing conditions place equal demands on the vergence and accommodation systems. The two responses are co-ordinated via the interactive components which couple the two systems: accommodative vergence and vergence accommodation. The crosslink components are usually quantified by the AC/A and CA/C ratios. Whether these ratios are stable entities, genetically determined, or modifiable by experience has been the subject of some debate. A ‘Virtual Reality’ stimulus was used to place unequal demands on the vergence and accommodation systems. Pre-task and post-task measures of AC/A and CA/C were objectively determined. The changes in the post-task measures are discussed with reference to a dual interactive model of vergence and accommodation. Model simulations suggest that adaptation of the vergence and accommodative controllers (tonic adaptation) may also play a part by altering the open loop bias. The results therefore support the idea that vergence accommodation and accommodative vergence are capable of adaptation.

Modelling Unequal Stimuli to the Human Ocular Accommodation System

Abstract A control theory model of binocular accommodation has been constructed to illustrate the control strategies adopted by the accommodation system during unequal that is, anisometropic stimulation. The model includes a component in which the retinal information from each eye is integrated to produce the same effort of accommodation to each eye. The integration of retinal information is dependent on the level of differential stimulation and whether monocular or binocular information is present.

Community Besieged: The Anglophone Minority and the Politics of Quebec

In Community Besieged Garth Stevenson describes the unusual circumstances that allowed English-speaking Quebecers to live in virtual isolation from their francophone neighbours for almost a century after Confederation. He describes their relations with Maurice Duplessis and the Union Nationale and their ambivalent response to the Quiet Revolution. New political issues - language policy, educational reform, sovereignty, and the constitution - undermined the old system of elite accommodation in Quebec, causing conflicts between anglophones and francophones and creating a new sense of anglophone identity that transcends religious differences. The changing relations of Quebec anglophones with the major political parties, as well as the role of newer entities such as Alliance Quebec and the Equality Party, are also examined. Stevenson concludes with a look at the future of anglophones in Quebec. Based in part on interviews with more than sixty English-speaking Quebecers who have played prominent parts in Quebec's political life, Community Besieged is a comprehensive and up-to-date description of the political life of this unique minority at both the federal and provincial level.