准分子激光角膜切削术性光学离焦对幼猴正视化影响的研究

目的了解视觉反馈对幼猴正视化过程的调控作用。方法年龄2～3个月的健康恒河猴8只,每只猴一只眼接受准分子激光角膜屈光手术(PRK),使术后产生3．0O D相对近视或远视性离焦(各4只);另一只眼作为对照。术后不同时间用角膜地形图、散瞳验光、A超和裂隙灯显微镜动态观察猴眼的变化。结果术眼角膜上皮均在术后3d之内完全愈合,在观察期内角膜保持透明。实验眼均表现为代偿生长的过程,玻璃体腔增长速率随术后屈光状态不同而变化。术后远视性离焦,玻璃体腔增长速率较对照眼快(t=3．656,P=0．0354);术后近视性离焦,玻璃体腔增长速率较对照眼慢(t=3．576,P=0．0374)。结论PRK造成光学离焦,可以调控幼猴的玻璃体腔生长和正视化过程,表明幼猴的正视化过程受视觉反馈的调节,提示临床上婴幼儿屈光不正的光学矫正应慎重。PRK造成猴眼光学离焦动物模型可用于视觉科学的研究。     

Dark-rearing interference with emmetropization in the rhesus monkey

Dark rearing has been shown to protect against the development of lid-suture myopia in monkeys and tree shrews. Dark-reared monkeys and cats, with or without lid suture, are significantly hyperopic in comparison to light-reared controls. The time course of refractive change during dark rearing has only been systematically investigated in chicks, with hyperopia increasing from 14 to 42 days after hatching. Longitudinal refractions of dark-reared monkeys have not been reported previously. Five infant rhesus monkeys were dark reared with their mothers from the first day of life until 58 to 161 days of age. Cycloplegic retinoscopies were performed at 2-week intervals and were compared with cross-sectional data from 18 normal monkeys at ages 1 to 81 days. The normal monkeys typically had hyperopic refractions from +4 to +8 diopters at birth with an average refraction of +2.8 diopters between 30 and 81 days of age, compared with an average refraction of +5.3 diopters between 30 and 81 days of age for the monkeys raised in darkness (difference significant at P less than 0.05). Three of the dark-reared animals retained an average of 7.0 diopters of hyperopia. Darkness thus slowed or interrupted the normal loss of hyperopia in three of the five experimental subjects, and may be useful for creating model hyperopic animals on the order of +5 to +8 diopters.     

Modification of visually guided saccades by a nonvisual afferent feedback signal

PURPOSE: To investigate the role of extraocular muscle afferent signals in the control of saccadic eye movements. METHODS: A suction scleral contact lens was used to impede the movements of the right eye while subjects executed visually guided saccades to briefly presented targets. Movements of the left eye were measured using infrared oculography. Saccade amplitude, peak velocity, and duration were analyzed trial by trial and compared before, during, and after the right eye was impeded. RESULTS: When the right eye was impeded, the amplitudes of saccades executed by the left eye were reduced. There was no alteration in the main sequence relationships. The amplitude effect had a rapid onset and offset. There was no evidence that the effects built up over a number of trials, nor was there evidence that individual saccades were modified on-line. CONCLUSIONS: These results are consistent with the hypothesis that extraocular muscle afferent signals provide a feedback signal of the movements of the eyes that is used to produce rapid adjustments of oculomotor output when required.     

Modification of smooth pursuit initiation by a nonvisual, afferent feedback signal

PURPOSE: To investigate the role of extraocular muscle afferent signals in the initiation and early maintenance of smooth-pursuit eye movements. METHODS: A suction scleral contact lens was used to impede the movements of the right eye while subjects tracked small targets in a step-ramp pursuit paradigm. Movements of the left eye were measured by infrared oculography. Pursuit latency, eye acceleration, and velocity were analyzed trial-by-trial and compared before, while, and after the right eye was impeded. RESULTS: When the right eye was impeded, initial acceleration and eye velocity were reduced. Pursuit latency was unchanged. The velocity effect had a rapid onset and offset; there was no evidence that the effects built up over a number of trials. Detailed analysis suggested that the reduction in velocity occurred approximately 40 msec after pursuit was initiated. CONCLUSIONS: These results are consistent with the hypothesis that extraocular muscle afferent signals provide a feedback signal of the movements of the eyes that may be used to modify the initiation and early maintenance of smooth pursuit on-line. It appears that for pursuit, as with saccades, the priority in these conditions is to maintain conjugacy.     

Developmental visual system anomalies and the limits of emmetropization

Optical defocus can within certain limits predictably alter ocular growth and refractive development in infant monkeys. However defocus, particularly unilateral defocus associated with anisometropia, can also promote abnormal sensory and motor development. We investigated the relationship between the effective operating range for emmetropization in infant monkeys and the refractive errors that produced amblyopia. Specifically, we examined the refractive-error histories of monkeys that did not demonstrate compensating ocular growth for imposed refractive errors and used operant psychophysical methods to measure contrast sensitivity functions for 17 infant monkeys that were reared with varying degrees of optically imposed anisometropia. Imposed anisometropias that were within the operating range of the monkey's emmetropization process were eliminated by differential interocular growth and did not produce amblyopia. On the other hand imposed anisometropias that failed to initiate compensating growth consistently produced amblyopia; the depth of the amblyopia varied directly with the magnitude of the imposed anisometropia. These results indicate that amblyopia and anisometropia are frequently associated because persistent anisometropia causes amblyopia. However, the failure of emmetropization in infants with refractive conditions that are known to promote sensory and motor anomalies indicates that factors other than optical defocus, presumably factors associated with the development of amblyopia and/or strabismus, can also influence early refractive development and in some cases cause anisometropia.     

人眼和动物眼视觉系统发育的正视化过程研究进展

在视觉环境的刺激下，眼部各屈光成分互相协调发展，最终发育成正视眼，此过程被称为“正视化”。正视化过程是一个动态的连续变化的过程，已经在很多动物体上，包括人在内得到验证。本研究对几种常用于近视研究的动物正视化过程做一综述并与人做了简单的对比。     

Individual set-point and gain of emmetropization in chickens

During the developmental process of emmetropization evidence shows that visual feedback guides the eye as it approaches a refractive state close to zero, or slightly hyperopic. How this “set-point” is internally defined, in the presence of continuous shifts of the focal plane with different viewing distances and accommodation, remains unclear. Minimizing defocus blur over time should produce similar end-point refractions in different individuals. However, we found that individual chickens display considerable variability in their set-point refractive states, despite that they all had the same visual experience. This variability is not random since the refractions in both eyes were highly correlated - even though it is known that they can emmetropize independently. Furthermore, if chicks underwent a period of experimentally induced ametropia, they returned to their individual set-point refractions during recovery (correlation of the refractions before treatment versus after recovery: n = 19 chicks, 38 eyes, left eyes: slope 1.01, R = 0.860; right eyes: slope 0.85, R = 0.610, p < 0.001, linear regression). Also, the induced deprivation myopia was correlated in both eyes (n = 18 chicks, 36 eyes, p < 0.01, orthogonal regression). If chicks were treated with spectacle lenses, the compensatory changes in refraction were, on average, appropriate but individual chicks displayed variable responses. Again, the refractions of both eyes remained correlated (negative lenses, n = 18 chicks, 36 eyes, slope 0.89, R = 0.504, p < 0.01, positive lenses: n = 21 chicks, 42 eyes, slope 1.14, R = 0.791, p < 0.001). The amount of deprivation myopia that developed in two successive treatment cycles, with an intermittent period of recovery, was not correlated; only vitreous chamber growth was almost significantly correlated in both cycles (n = 7 chicks, 14 eyes; p < 0.05). The amounts of ametropia and vitreous chamber changes induced in two successive cycles of treatment, first with lenses and then with diffusers, were also not correlated, suggesting that the “gains of lens compensation” are different from those in deprivation myopia. In summary, (1) there appears to be an endogenous, possibly genetic, definition of the set-point of emmetropization in each individual, which is similar in both eyes, (2) visual conditions that induce ametropia produce variable changes in refractions, with high correlations between both eyes, (3) overall, the “gain of emmetropization” appears only weakly controlled by endogenous factors.     

A model for emmetropization: predicting the progression of ametropia

The response of a second-order feedback system is used to describe the controlling process that regulates the refraction of the human eye to achieve optimal visual acuity over the years (emmetropization). From data collected in past refractions, the equation of the feedback system has been derived for individual eyes and used to predict changes in their refraction with age with an accuracy of 0.50 diopters for 89.1% of the eyes tested. If the model is applied to other populations, the root mean square error of prediction will be between 0.20 and 0.36 diopters with a 95% degree of confidence. The model indicates that corrective lenses applied to the eyes, especially in the early years of life, will cheat the servo system and defeat the emmetropization process.     

Localization in the frontal plane is not susceptible to manipulation of afferent feedback via the Jendrassik Maneuver

We have previously shown that registered vergence eye position is altered while participants perform the Jendrassik Maneuver (JM). We proposed that the altered eye position signal registration is due to the effect of the JM which changes the gain of the sensory feedback from the eye muscles, possibly via the activity of non-twitch motoneurons. We conducted two studies to further extend and clarify one of our previous findings by examining whether the JM also affects registered eye position during localization in the frontal plane. Since the non-twitch motoneurons do not receive premotor input from areas involved in the programming of saccades, we hypothesized that localization responses associated with the saccadic system should not be affected by the JM. The data confirmed our prediction. We propose that the non-twitch motoneurons are involved in parametric adjustment of the proprioceptive feedback loops of the vergence but not the version eye movements.     

Effects of foveal ablation on emmetropization and form-deprivation myopia

PURPOSE: Because of the prominence of central vision in primates, it has generally been assumed that signals from the fovea dominate refractive development. To test this assumption, the authors determined whether an intact fovea was essential for either normal emmetropization or the vision-induced myopic errors produced by form deprivation. METHODS: In 13 rhesus monkeys at 3 weeks of age, the fovea and most of the perifovea in one eye were ablated by laser photocoagulation. Five of these animals were subsequently allowed unrestricted vision. For the other eight monkeys with foveal ablations, a diffuser lens was secured in front of the treated eyes to produce form deprivation. Refractive development was assessed along the pupillary axis by retinoscopy, keratometry, and A-scan ultrasonography. Control data were obtained from 21 normal monkeys and three infants reared with plano lenses in front of both eyes. RESULTS: Foveal ablations had no apparent effect on emmetropization. Refractive errors for both eyes of the treated infants allowed unrestricted vision were within the control range throughout the observation period, and there were no systematic interocular differences in refractive error or axial length. In addition, foveal ablation did not prevent form deprivation myopia; six of the eight infants that experienced monocular form deprivation developed myopic axial anisometropias outside the control range. CONCLUSIONS: Visual signals from the fovea are not essential for normal refractive development or the vision-induced alterations in ocular growth produced by form deprivation. Conversely, the peripheral retina, in isolation, can regulate emmetropizing responses and produce anomalous refractive errors in response to abnormal visual experience. These results indicate that peripheral vision should be considered when assessing the effects of visual experience on refractive development.