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PubMed1970

Visual adaptation in the retina of the skate.

The electroretinogram (ERG) and single-unit ganglion cell activity were recorded from the eyecup of the skate (Raja erinacea and R. oscellata), and the adaptation properties of both types of response compared with in situ rhodopsin measurements obtained by fundus reflectometry. Under all conditions tested, the b-wave of the ERG and the ganglion cell discharge showed identical adaptation properties. For example, after flash adaptation that bleached 80% of the rhodopsin, neither ganglion cell nor b-wave activity could be elicited for 10-15 min. Following this unresponsive period, thresholds fell rapidly; by 20 min after the flash, sensitivity was within 3 log units of the dark-adapted level. Further recovery of threshold was slow, requiring an additional 70-90 min to reach absolute threshold. Measurements of rhodopsin levels showed a close correlation with the slow recovery of threshold that occurred between 20 and 120 min of dark adaptation; there is a linear relation between rhodopsin concentration and log threshold. Other experiments dealt with the initial unresponsive period induced by light adaptation. The duration of this unresponsive period depended on the brightness of the adapting field; with bright backgrounds, suppression of retinal activity lasted 20-25 min, but sensitivity subsequently returned and thresholds fell to a steady-state value. At all background levels tested, increment thresholds were linearly related to background luminance.

Adaptation, Ocular↗
PubMed2004

Increased accommodation following adaptation to image blur in myopes.

Prolonged exposure to blurred images produces perceptual adaptation (M. A. Webster, M. A., Georgeson, & S. M. Webster, 2002). The purpose of this study is to test whether in addition to the reported change in perceived blur there is also a change in accommodation. Young adult (aged 18 to 31 years) myopic (n = 23) and emmetropic (n = 17) subjects participated in the study. Myopes were tested with contact lenses and had corrected monocular visual acuity of 20/20 or better. Accommodation was measured binocularly with a PowerRefractor, an eccentric infrared photorefractor. Accommodation for a near target (high-contrast text at 0.33 m) was measured for 2 min before and immediately after 3 min of blur exposure. Blur was induced using 0.2 Bangerter diffusing filters in front of both eyes. In addition, accommodation was measured for a far target (high-contrast letters at 4.0 m) before and after the near measurements, with each subject's initial far readings used as a baseline for calculating the accommodative responses at near. Compared to the pre-adaptation level, myopes showed a significant (p < .01) increase in the near accommodative response after 3 min of blur adaptation, while accommodation to the near target in emmetropes did not change. In a second experiment using monocular viewing, the increase of accommodation found in myopes was shown to occur during the period of blur exposure. The refractive group differences in the accommodative response may be related to differences in the habitual response to image clarity between myopes and emmetropes under normal viewing conditions.

Accommodation, Ocular↗
PubMed1991

Model of tonic accommodation after sustained near focus.

The effect of short-term near focus on tonic accommodation (TA) was previously investigated in 48 visually normal subjects. After sustained focus upon a near target for 10 min, all illumination was extinguished and the decay of accommodation was monitored for 20 min. We calculated from these results that accommodation decayed rapidly with a time constant of about 0.4 min, followed by a slower decay with a time constant of about 32 min. However, the asymptotic level remained slightly above the preadaptation level after 20 min in the dark. A feedback model with two parallel dynamic components (transient and decay), which summed with a "bias" element in the forward loop, was developed to account for the results. The gains of the dynamic elements were varied until the simulation curves matched the experimental adaptation and decay data. During the closed-loop adaptation phase, the outputs of the dynamic elements moved in opposite directions but combined to yield the overall accommodation step response. Moreover, when the model was subsequently open-looped in the dark, the transient component declined rapidly, whereas the decay component remained above the preadaptation level. When the 48 subjects were grouped equally into 4 refractive categories, somewhat different decay time courses could be simulated with changes in model parameter values.

Accommodation, Ocular↗
PubMed1991

Adaptation of tonic accommodation to sustained visual tasks in emmetropia and late-onset myopia.

The study complements and extends our previous investigation and reports that late-onset myopes (LOM's, N = 15) exhibit differences in adaptation of tonic accommodation (TA) to sustained visual tasks when compared to emmetropes (EMM's, N = 15). Pre- and post-task TA was measured with a modified Canon R-1 objective infra-red optometer under darkroom conditions. Post-task TA was measured immediately after a 10-min counting task and at 1-s intervals over a period of 90 s; the task was located at distances equivalent to 1, 3, and 5 D. The pattern of regression back to the pre-task TA level was used to assess the degree of adaptation generated by the task. The data indicate substantial differences between regression patterns for EMM's and for LOM's which were enhanced as accommodation stimulus levels increased. Using a two-factor split-plot analysis of variance, statistically significant differences between EMM's and LOM's could be attributed to the rates of change of post-task accommodative levels to pre-task TA levels for both the 3 and 5 D tasks.

Accommodation, Ocular↗
PubMed2003

A longitudinal study of vergence adaptation in incipient presbyopia.

BACKGROUND: To investigate vergence adaptation during the incipient phase of presbyopia, when the amplitude of accommodation approaches the level where the first reading addition is required. The study aimed to assess the ability of the vergence system to counteract changes in the component contributions to the overall vergence response with the decline in the amplitude of accommodation in presbyopia, although previous reports on the nature of changes in accommodative, tonic and proximal vergence are equivocal. METHODS: Using a 'flashed' Maddox rod technique, an assessment of vergence adaptation to 6delta base-out and 6delta base-in prism was made for 28 subjects (aged 35-45 years at the commencement of the study). The measurements were taken four times over a 2-year period. RESULTS: Using a repeated measures analysis of variance, the results show that with the decline in amplitude of accommodation, there is a statistically significant reduction in the magnitude of vergence adaptation to both base-out (p < 0.05) and base-in prism (p < 0.01). CONCLUSIONS: This study shows that with ageing, there is a decrease in the ability of the slow vergence mechanism to overcome a change in fusional vergence demand and would suggest that either the fast component of fusional vergence must cope with any change in fusional vergence demand or that the sum of the accommodative, tonic and proximal vergence responses are virtually stable with age.

Accommodation, Ocular↗
PubMed1987

Optically induced changes in the couplings between vergence and accommodation.

Vergence-induced accommodation (V-A) and accommodation-induced vergence (A-V) were measured in human subjects before and after they had worn various optical devices for 30 min. Laterally displacing periscopic spectacles, which increase the required change in vergence per unit change in accommodation, caused decreases and increases in the gain of V-A and A-V responses, respectively. These observations are consistent with the view that the gain of the neural cross-linkages between vergence and accommodation are subject to adaptive regulation. However, there were strong asymmetries: Medially displacing periscopes (cyclopean spectacles), which reduce the required change in vergence per unit change in accommodation to zero, were almost totally without effect. Base-out prisms, which increase the required convergence by an amount that is constant for all viewing distances, caused downward and upward shifts in the V-A and A-V response curves, respectively. Base-in prisms, which reduce the required convergence by a constant amount for all viewing distances, caused downward shifts of A-V curves but had no significant effect on V-A curves. These effects of prisms are in essential agreement with the work of others and confirm the existence of adaptive elements that regulate the bias in the vergence and/or accommodation control systems. Secondary effects of wearing periscopes and prisms indicated a certain lack of specificity in the sensing of gain and bias errors: vertical shifts of V-A and A-V curves (resembling those seen with base-out prisms) often occurred with the laterally displacing periscopes, and gain changes (generally resembling those seen with laterally displacing periscopes) often occurred with the base-out prisms.

Accommodation, Ocular↗
PubMed1989

Spatial organization of sensitivity regulation in rod vision.

To investigate whether scotopic sensitivity is set locally or in neural "pools", we have tested the spatial variation in sensitivity after bleaching with gratings using 3 different methods. One experiment circumvented the influence of involuntary eye movements by deliberately randomizing the horizontal position of a fine test line on the area bleached by the vertical gratings. The spatial variation of threshold across the bleached area is reflected in the width of the frequency-of-seeing curve. A clear difference between the probability-of-seeing curves following a grating bleach and a uniform bleach was seen only up to between 4.2 and 6.3 c/deg, suggesting that adaptation signals are pooled so as to almost obliterate the contrast in finer gratings than this. In a second experiment the lowest bleaching-grating contrast (for a space-averaged initial rhodopsin bleach of 10%) that produced a patterned afterimage stayed close to the scotopic threshold contrast for frequencies from 1 to 6.4 c/deg, but it rose above the contrast threshold at high spatial frequencies. This slight loss of sensitivity at the high frequencies is more evidence for pooling in adaptation. A third experiment assessed the sensitivity profile at the adapting site without any influence of later stages of neural integration. Bleaching and test gratings of slightly different spatial frequency were flashed successively. If the effect of bleaching is restricted to the bleached rods, the observer will effectively be looking at the test grating through a grid of sensitive and insensitive stripes in his own retina. The two gratings come in and out of register at the difference frequency, and a corresponding low-frequency grating should be visible even when the test and bleaching gratings are not themselves resolved by the later stages. We could not see the difference frequency unless the test and bleach gratings were themselves coarse enough to be resolvable in rod vision. This is very strong evidence against any model in which each rod has its own sensitivity-regulating mechanism, and instead supports (for these conditions) Rushton's view that adaptation is entirely the work of a neural pool. The estimated pool size is about 10 min arc of visual angle.

Adaptation, Ocular↗
PubMed1976

Behavioral control of visual fixation of the rhesus monkey.

A paradigm for the control of visual fixation of the macaque monkey in vision experiments was described. Using a Maxwellian view, the procedure permits the placement of discrete test-light stimuli in a specific area of the retina as the monkey fixates a primary target. This procedure holds foveal fixation as other behaviorally significant visual stimuli are presented to the visual field. By a methods-of-limits procedure, the sensitivity of the monkey eye was measured at different retinal locations under both photopic and scotopic visual adaptation.

Acoustic Stimulation↗
PubMed1980

[Sensory binocular relationships in divergent strabismus and convergent strabismus].

The adaptive processes mentioned here (which include inhibitory phenomena and modifications in spatial localization) were observed, using a stereoprojector which polarized light. Comparing the adaptive processes in convergent and divergent strabismus has led to the following observations: (1) there are differences between the two types of strabismus; these differences appear not only at the level of the binocular sensory parameter but also at the level of motor and accommodative parameters; adaptive processes are less deep and less serious in divergent and strabismus; these differences are also manifest in therapy: divergent strabismus is curable whatever the age, treatment is much shorter. These observations were the starting point for using prismatic overcorrection to induce a state of 'sensory divergence' as treatment in cases of convergent strabismus with anomalous retinal correspondence. The excellent results obtained using this method of treatment are not only due to the action on sensory binocular relations, but also to the fact that prismatic overcorrection also modifies abnormal motor patterns. Treating strabismus must involve the entire disturbed optico-sensorimotor complex.

Accommodation, Ocular↗
PubMed1982

Graphical analysis of prism adaptation, convergence accommodation, and accommodative convergence.

A new form of graphical case analysis is described which quantifies static interactions between accommodative convergence, convergence accommodation, and prism adaptation. A clinical gradient measure of the CA/C ratio is compared to haploscopic measures to demonstrate the validity of the new clinical technique for measuring convergence accommodation. Graphical and computational methods are illustrated which predict the quantitative interactions between accommodation and convergence that occur after the phoria is partially corrected by lenses or prisms. Complete correction of convergence accommodation errors with spherical lenses or elimination of the stimulus to convergence accommodation by full correction of the associated phoria with prism will simplify the correction of binocular disorders by preventing the complex interactions of accommodative convergence with convergence accommodation. The new form of graphical analysis quantifies the predicted stress upon binocular vision induced by convergence accommodation.

Accommodation, Ocular↗
PubMed1987

Near vision stress: vergence adaptation and accommodative fatigue.

Changes in the visual system following prolonged near work were investigated. Fifteen young, normal subjects undertook a severe, two hour long, binocular near visual task at 20 cm without any breaks. Fusional stress was assessed by near 'phoria change. Refractive change was measured with an autorefractometer to investigate whether transient myopia occurred and its subsequent recovery. The near task caused vergence adaptation which was primarily due to the fusional stress of the task (accounting for 67% of its variance). The 'phoria change was to a lesser extent (40%) dependent on the accommodative "stress" of the task. Fatigue of the accommodative system resulted in increased accommodative innervation to maintain the same accurate response. Increased innervation can continue after the task on subsequent distance viewing, resulting in transient myopia (mean 0.29 DS). This transient myopia was found to be due to a transient regression of the far point towards the subject's tonic accommodation level. This can be accounted for by a shift of the tonic level as well as an increased bias towards the pre-task tonic level.

Accommodation, Ocular↗
PubMed1989

The measurement of adaptation of tonic accommodation under two open-loop conditions.

Pre- and post-task tonic accommodation (TA) was measured using an objective infra-red optometer under two open-loop conditions: a darkroom and a bright empty-field (60 cd m-2). The task was of 10 minutes' duration and was located at -1, -3 and -5 D. Post-task TA was measured immediately after the task and over a period of 90 s. Mean data for the group of 10 young men subjects showed that the open-loop condition adopted had no significant influence on the post-task regression patterns of TA towards pre-task values. Three subjects did, however, show significantly more sustained increases in post-task TA for the bright-field condition; one markedly so. Conversely, two subjects showed more pronounced post-task shifts for the darkroom condition.

Accommodation, Ocular↗
PubMed2005

The adaptive effect of narrowing the interocular separation on the AC/A ratio.

The purpose of this study was to determine whether the response AC/A ratio could be altered when the subject's interpupillary distance (IPD) was optically halved. We measured the changes in the AC/A ratio for 10 subjects after using the optical device for 30 min. Accommodative response was measured using a Canon R-1 optometer, and vergence response was measured with an ASL 210 Eye Movement Monitor. The average AC/A ratios were 1.20+/-0.35 (SD) (MA/D) and 0.84+/-0.39 (MA/D) before and after wearing the device, respectively. The decrease in AC/A ratio was statistically significant (p=0.01). This was mainly caused by a reduction in the slope of the accommodative vergence. The results of this study suggest that the AC/A ratio can be decreased if an IPD-narrowing device is used. A possible application of this mechanism in the study of myopia is discussed.

Accommodation, Ocular↗
PubMed2000

Effect of 11-cis 13-demethylretinal on phototransduction in bleach-adapted rod and cone photoreceptors.

We used 11-cis 13-demethylretinal to examine the physiological consequences of retinal's noncovalent interaction with opsin in intact rod and cone photoreceptors during visual pigment regeneration. 11-Cis 13-demethylretinal is an analog of 11-cis retinal in which the 13 position methyl group has been removed. Biochemical experiments have shown that it is capable of binding in the chromophore pocket of opsin, forming a Schiff-base linkage with the protein to produce a pigment, but at a much slower rate than the native 11-cis retinal (Nelson, R., J. Kim deReil, and A. Kropf. 1970. Proc. Nat. Acad. Sci. USA. 66:531-538). Experimentally, this slow rate of pigment formation should allow separate physiological examination of the effects of the initial binding of retinal in the pocket and the subsequent formation of the protonated Schiff-base linkage. Currents from solitary rods and cones from the tiger salamander were recorded in darkness before and after bleaching and then after exposure to 11-cis 13-demethylretinal. In bleach-adapted rods, 11-cis 13-demethylretinal caused transient activation of phototransduction, as evidenced by a decrease of the dark current and sensitivity, acceleration of the dim flash responses, and activation of cGMP phosphodiesterase and guanylyl cyclase. The steady state of phototransduction activity was still higher than that of the bleach-adapted rod. In contrast, exposure of bleach-adapted cones to 11-cis 13-demethylretinal resulted in an immediate deactivation of transduction as measured by the same parameters. These results extend the validity of a model for the effects of the noncovalent binding of a retinoid in the chromophore pockets of rod and cone opsins to analogs capable of forming a Schiff-base and imply that the noncovalent binding by itself may play a role for the dark adaptation of photoreceptors.

1-Methyl-3-isobutylxanthine↗
PubMed1989

Temporal aspects of accommodative adaptation.

Pre- and post-task measures of dark focus (DF) were used to assess accommodative adaptation induced by brief near-vision tasks in 10 late-onset myopes (LOM's) and 10 emmetropes. The degree of adaptation was examined after tasks of 15, 30, and 45 s duration. Accommodation was measured after the task at 1 s intervals over a 90 s period using an objective infrared optometer. Emmetropic subjects showed a significant increase in DF during the first 10 s of post-task recording, which regressed to the pre-task value within a further 20 to 30 s. No significant difference in post-task accommodative regression was observed after the three task durations. Similar findings were observed for LOM's after the 15 and 30 s tasks. However, after the 45 s task, LOM's exhibited a marked negative shift in DF during the 10 to 50 s post-task period. Instillation of a beta adrenergic antagonist attenuated this negative shift, suggesting that it was mediated via beta adrenergic innervation to the ciliary muscle.

Accommodation, Ocular↗
PubMed1997

Induced pupillary hippus following near vision: increased occurrence in visual display unit workers.

Using a quasistatic method, it has been found previously that one-third of normal subjects retained pupillary constriction under empty-field conditions for a substantial period following a near-vision task. This pupil after-effect was dissociated from adaptation of tonic accommodation. In this paper, essentially the same method was used to investigate further the effects of near vision on pupil response in VDU (Visual Display Unit) workers with complaint of general fatigue and/or asthenopia. In some cases, large and slow pupillary oscillation accompanied by miosis was noted after near vision. Pupillary oscillation is a well-established phenomenon that has a broadly spread frequency spectrum with an occasional component 'hippus' at approximately 0.2 Hz. A proportion of subjects (30 out of 100) showed hippus, a consistent oscillation of 0.22 Hz (SD = 0.03 Hz) in VDU workers, while only one out of 24 normal subjects showed this. Occurrence of pupillary hippus following near vision increased in VDU workers. It was suggested that this hippus is concerned with cumulative asthenopia and/or general fatigue although the origin of hippus is still unknown.

Accommodation, Ocular↗
PubMed1970

Invariant features of spatial summation with changing locus in the visual field.

1. Spatial summation curves have been determined under photopic conditions for loci between 5 and 55 degrees from fixation. At each locus the area and Weber fraction for the largest stimulus showing complete summation (Sc) has been estimated. While the area of Sc increases progressively towards the periphery, its Weber fraction remains constant.2. The invariance of the Weber fraction for Sc, coupled with changes in its area, have been compared with similar findings observed during changes in adaptation level, in impaired fields, and during accommodative and convergence changes. It is suggested that the Weber fraction for Sc may be an important parameter of visual function which is maintained under changing conditions by alterations in the area over which background energy is summated.3. It has been shown that, for stimulus areas up to at least 1 log unit greater than Sc, all the summation curves from 5 to 55 degrees from fixation can be superimposed by simple displacement along the log area axis. For loci closer together, the superimposition has been shown for larger ranges of areas beyond Sc. This suggests that, so far as spatial summation is concerned, changes in locus serve only to change the spatial scale of the visual system.4. Comparisons have been made between the area of Sc at various loci and the sizes of the central regions of the receptive fields of retinal ganglion cells determined electrophysiologically in the primate.

Accommodation, Ocular↗