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PubMedDate not supplied

Adrenoceptors affect accommodation by modulating cholinergic activity.

In an attempt to clarify the functional role of adrenoceptors in accommodation, the effects of various adrenergic agents on the state of accommodation were studied. Fifty-two emmetropic, visually normal subjects (24.6 +/- 0.42 years old) participated in this study. Using an infrared optometer, the far and near points of accommodation were measured by a quasistatic method. Tonic accommodation and accommodative adaptation were also investigated. All these parameters were measured before and after topical application of various adrenergic agents. Both bunazosin hydrochloride (0.1%) and phenylephrine hydrochloride (5%) had no effect on tonic accommodation and accommodative adaptation. However, bunazosin hydrochloride increased the near point of accommodation. Timolol maleate (0.5%) and isoproterenol hydrochloride (3.0%) did not affect tonic accommodation. Isoproterenol hydrochloride evoked a hyperopic shift of the far point of accommodation by 0.23 +/- 0.42 diopters (D). Additionally, accommodative adaptation was increased by timolol maleate (0.36 +/- 0.62 D) and decreased by isoproterenol hydrochloride (0.18 +/- 0.48 D). These results indicate that both alpha and beta adrenoceptors affect accommodation. Activation of alpha adrenoceptors increased the near point of accommodation and activation of beta adrenoceptors decreased accommodative adaptation, which suggests that activation of adrenoceptors may modify parasympathetic activity; hence, affecting the state of accommodation.

Accommodation, Ocular↗
PubMed1992

Accommodative hysteresis as a function of target--dark focus separation.

The concept of dark focus (DF) as fulcrum for accommodative effort was examined by inducing adaptation of tonic accommodation with the target at varying dioptric distances from the DF. Mean pre- to post-changes in DF at the initial post fixation test essentially were linear (r2 = 0.975), with zero intercept, with respect to the target-DF separation, as were the data of most individual subjects. Target-DF distance thus is a major determinant of adaptation of tonic accommodation. These results are consistent with the interpretation of DF as a zero reference point or fulcrum about which accommodative effort varies.

Accommodation, Ocular↗
PubMed1992

Pharmacological effects on accommodative adaptation.

We review the research which has attempted to determine whether the characteristics of autonomic innervation of ciliary smooth muscle are relevant to the process of accommodative adaptation. The effect of various topical autonomic drugs on the three phases of adaptation were analyzed: pre-task tonic accommodation (open-loop); within-task accommodative response (closed-loop); and post-task regression of accommodation to pre-task tonic levels. Although it is clear that parasympathetic innervation predominates, there is evidence that some individuals utilize supplementary inhibitory sympathetic innervation. When sympathetic innervation is augmented by substantial levels of concurrent parasympathetic accommodative activity, it may serve to attenuate the magnitude and duration of post-task shifts in tonic accommodation. It is proposed that individuals with a deficit in sympathetic inhibition may therefore be predisposed to anomalies of accommodative adaptation. However, the mechanism by which the oculomotor system responds to such predisposition is at present obscure.

Accommodation, Ocular↗
PubMed1997

[Effects of topical application of adrenergic agents on refraction and accommodation].

In an attempt to clarify the functional role of adrenergic receptors on accommodation, effects of various adrenergic agents on the state of accommodation were studied. Fifty-two emmetropic normal subjects (age: 19-28 y) participated in this study. We examined the effect of the alpha agonist phenylephrine hydrochloride (5%), the alpha 1 antagonist adrenergic bunazosin hydrochloride (0.1%), the beta antagonist timolol maleate (0.5%), the beta 2 antagonist betaxolal hydrochloride (0.5%), and the beta agonist isoproterenol hydrochloride (3%). Using an infrared optometer, the refraction, far point and near point of accommodation, resting point of accommodation (empty-field accommodation and dark focus of accommodation), and accommodative adaptation were measured before and after topical application of various adrenergic agents. Application of bunazosin slightly but significantly decreased the near point of accommodation. Isoproterenol evoked a hyperopic shift on the far point of accommodation. None of the agents had any effect on the resting point of accommodation. Accommodative adaptation was increased by timolol and betaxolol, and decreased by isoproterenol. We surmised that beta adrenergic agents exerted an indirect influence on the parasympathetic nervous system. The activation of beta receptors produced a hyperopic shift on the far point of accommodation and decreased accommodative adaptation, which suggests that activation of adrenergic receptors may reduce parasympathetic activity, hence affecting the state of accommodation.

Accommodation, Ocular↗
PubMed1988

Influence of accommodative and vergence adaptation on binocular motor disorders.

Tait described four categories of binocular disorders including convergence excess, convergence insufficiency, divergence excess, and divergence insufficiency. These disorders are defined by the distance where the largest heterophoria occurs (distance or near), and the amplitude of the accommodative vergence ratio (AC/A). Insufficiency corresponds to a low AC/A ratio, whereas excess corresponds to a high AC/A. The magnitude of the AC/A ratio, which may be influenced by the adaptability of the accommodation and vergence systems, has been shown to be reciprocally related to adaptability of accommodation. Likewise, the degree of vergence accommodation has been shown to be related reciprocally to adaptability of vergence to prism. An imbalance of adaptability of accommodation and vergence systems produces abnormal cross-coupling between the two motor systems. When accommodation is more adaptable than vergence, the AC/A ratio is low and the CA/C ratio is high. Conversely, when vergence is more adaptable than accommodation, the AC/A ratio is high and the CA/C ratio is low. A method is reported for temporarily restoring moderate amplitudes of abnormal AC/A and CA/C ratios by reducing excessive adaptation with fatigue. Finally, new clinical procedures for measuring adaptation of accommodation and the CA/C ratio are presented. Taken together with current measures of vergence adaptation and AC/A ratio these procedures will permit a more complete evaluation of mutual interactions between accommodation and vergence in patients diagnosed as having excessive and insufficient vergence.

Accommodation, Ocular↗
PubMed1993

Decreased uncorrected vision after a period of distance fixation with spectacle wear.

Myopes of low degree commonly report that their vision seems poorer upon removal of their spectacles compared to that after a period without spectacle wear. Notably, this difference in vision can be appreciated after distance fixation. In this paper, we propose and test several alternative hypotheses to explain the phenomenon: an accommodative response to spectacles, sensory adaptation, or altered criteria for blur of psychological origin. We measured visual acuity (VA), refractive error, and lens thickness on 10 subjects with less than 2.00 D of myopia. Testing was performed after two 90-min sessions viewing at distance. At one session, the subjects wore their current spectacle correction and, at the other session, no correction was worn. VA underwent a slight but significant decrease (0.4 of a line) after the session in which spectacles were worn, but no difference in refractive error or lens thickness was found. The change in acuity in the absence of a refractive shift suggests sensory adaptation to blur. However, the demonstrated change in VA appears to be less than that which is subjectively reported; accordingly, psychological input cannot be ruled out.

Accommodation, Ocular↗
PubMed1980

Functional analysis of vision in patients after retinal detachment repair.

A number of tests of vision were performed in patients after surgical repair of retinal detachment. The major tests performed were Snellen visual acuity, interferometric acuity, Stiles-Crawford function, and increment-threshold analysis. In two patients it was possible to evaluate all functions preoperatively. Althrough the sample it modest (ten eyes in nine patients), trends in visual recovery are identified and discussed.

Adaptation, Ocular↗
PubMed1978

Functional relationship between myopia, accommodative stress and against-the-rule astigmia: a hypothesis.

A model is proposed to explain the well-established tendency for against-the-rule astigmatism to precede and be predictive of the later development of myopia. It is hypothesized that the against-the-rule astigmatism is a functional adaptation to reduce near point stress by permitting reduced accommodative output (increased lag of accommodation) while maintaining resolution for the primarily vertically-oriented characters of the English language.

Accommodation, Ocular↗
PubMed1983

Effects of vestibulocerebellar lesions upon dynamic characteristics and adaptation of vestibulo-ocular and optokinetic responses in pigmented rabbits.

The vestibulo-ocular reflex (VOR) and optokinetic response (OKR) in the horizontal plane were examined in pigmented rabbits, using sinusoidal whole-body rotation and sinusoidal rotation of a striped screen. Sustained rotation of the animal (5 degrees peak-to-peak, 0.1 Hz) for 4 h, under different optokinetic stimulus conditions, induced the following adaptive changes in the VOR: (1) outphase rotation of the screen (5 degrees) increased the VOR gain by 0.3 (on average); (2) with the screen fixed in space, VOR gain increased by 0.2. (3) in-phase rotation of the screen (5 degrees) decreased the VOR gain by 0.16. However, (4) in-phase rotation of the screen at twice (10 degrees) the amplitude of whole-body rotation did not affect the gain. Instead, it induced a significant phase lead (23 degrees) in the VOR, which did not occur in other stimulus conditions. Adaptive increases of the OKR gain occurred under sustained rotation of the screen alone (2.5 degrees, 0.33 Hz). After bilateral destruction of floccular Purkinje cells with kainic acid the VOR gain and phase were affected only very slightly, but adaptive changes in the VOR were abolished. By contrast, the OKR gain was reduced and the OKR phase delayed. OKR adaptation was also affected in such a way that a gain increase initially produced could not be maintained during sustained screen rotation. Ablation of nodulus-uvala caused a gain increase and phase lead in both VOR and OKR, and its only effect on adaptability of the VOR or the OKR was observed for the VOR under stimulus condition (4).

Acclimatization↗
PubMed1988

Accommodative adaptation to monocular and binocular stimuli.

Pre- and post-task measures of dark-focus (DF) were used to assess accommodative adaptation induced by a 45-s near-vision task at 33 cm. Adaptation was measured under monocular and binocular conditions for a group of 10 young emmetropic subjects (mean age 21.6 years). The accommodative response was measured objectively using an infrared optometer (Canon Autoref R-1). Post-task DF was sampled immediately after the task at 1-s intervals over a 90-s period. No significant difference in accommodative adaptation was observed between the monocular and binocular near-vision tasks. The implications of this finding are discussed with regard to the oculomotor constituents of the closed-loop accommodative response.

Accommodation, Ocular↗
PubMed1994

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.

Accommodation, Ocular↗
PubMed1965

VISUAL ACUITY OF ESSENTIAL FATTY ACID-DEFICIENT RATS.

1. Rats were maintained on a diet deficient in fat and on a normal diet of rat cubes. 2. Rats were trained to discriminate between vertical and horizontal striations. The minimal stripe width that could be used for discrimination was determined. 3. In bright illumination (0.7 or 4.5 ft.lamberts) both deficient and normal rats had the same ability to discriminate between black and white stripes. 4. With an illuminance of 0.002 ft.lambert, supplemented rats could discriminate as efficiently as at 0.7 ft.lambert, but deficient animals were unable to discriminate at 0.002 ft.lambert. 5. Control rats had 14% of docosahexaenoic acid in their retinal fats but the deficient rats had only 1%. 6. Deficient animals had no vitamin A stores in the liver whereas the control animals had about 190 i.u./g. 7. The visual acuity of the rats used was about 45' of arc.

Adaptation, Ocular↗
PubMed1999

Nonlinear alteration of transient vergence dynamics after sustained convergence.

BACKGROUND: Adaptation models of the horizontal disparity vergence system assume a nonadaptable transient component. They also predict identical postadaptation dynamics during convergence and divergence movements. METHOD: To test the adaptation property of the transient component, a set of experiments were performed in which closed-loop vergence dynamics measured before and after sustained convergence were compared, primarily by comparing the peak vergence velocity, occurrence time of peak vergence velocity, and steady-state vergence posture. Vergence dynamics after durations of 30, 60, and 90 s of sustained convergence were compared with those after a control duration of 5 s. RESULTS: The peak divergence velocity was reduced by about 25% within 30 s of sustained vergence. However, the peak convergence velocity was unchanged for all the exposure durations. Additionally, for all durations, the peak divergence velocity was significantly higher than peak convergence velocity. In contrast to peak velocities, the occurrence time of peak convergence and divergence velocity did not differ significantly and remained unchanged for all durations. CONCLUSIONS: The transient component is adaptable. Furthermore, the adaptation is direction dependent and affects divergence and convergence dynamics differently, thereby suggesting involvement of separate pathways for convergence and divergence in the vergence sensorimotor control.

Accommodation, Ocular↗
PubMed1986

Floccular lesions abolish adaptive control of post-saccadic ocular drift in primates.

After several days of exposure to optically-imposed post-saccadic retinal slip, the saccades of normal monkeys acquire an exponential ocular drift. This drift is in the direction of the imposed image motion, and persists in the dark. It has been argued that these changes result from the operation of a visually mediated adaptive mechanism that normally functions to minimize post-saccadic ocular drift. Adaptation to persistent post-saccadic retinal slip was assessed in two rhesus monkeys before and after bilateral ablations of the flocculi and portions of the paraflocculi ("flocculectomy"). After flocculectomy, both monkeys showed some post-saccadic ocular drift. Flocculectomized animals also failed to adapt to optically-imposed post-saccadic slip. We infer from this that the flocculi and/or paraflocculi are necessary for the successful suppression of post-saccadic ocular drift.

Adaptation, Ocular↗