Visual sensitivity to temporal change in focus and its relevance to the accommodation response

Measurements were made of the ability of observers, whose accommodation had been paralysed, to detect the small changes in contrast resulting from sinusoidal, oscillatory changes in the dioptric focus of test objects, as a function of the mean position of focus. Variables studied included the form and wavelength composition of the test object, the diameter of the entrance pupil of the eye and the temporal frequency of the focus change. The dioptric amplitudes of the thresholds found (approximately 0.1 D) were comparable with the normally observed amplitudes of the microfluctuations of the accommodation system of the eye. A maximum for the threshold change of focus was found to occur about a mean position of focus corresponding to the nominal "best-focus". Two threshold minima occurred symmetrically on either side of this position of optimal mean focus; the positions of these minima depended on the spatial frequency content of the test object and the pupil diameter. The results are interpreted in terms of the corresponding through-focus changes in optical modulation transfer of the eye and data on the sensitivity of the visual system to spatial and temporal modulation. For sinusoidal grating test objects, it appears that the results can qualitatively be explained if it is assumed that the detectable focus change is such that the ratio of the modulation change in the retinal image, produced by the focus change, to the modulation at the mean position of focus is constant. Results for targets of broad spatial bandwidth suggest that sensitivity to focus change may be dominated by spatial frequency components approximately 5 c/deg. The implications of the data for the understanding of the accommodation control system are briefly considered.     

Role of the sclera in the development and pathological complications of myopia

Myopia is one of the most prevalent ocular conditions and is the result of a mismatch between the power of the eye and axial length of the eye. As a result images of distant objects are brought to a focus in front of the retina resulting in blurred vision. In the vast majority of cases the structural cause of myopia is an excessive axial length of the eye, or more specifically the vitreous chamber depth. In about 2% of the general population, the degree of myopia is above 6 dioptres (D) and is termed high myopia. The prevalence of sight-threatening ocular pathology is markedly increased in eyes with high degrees of myopia ( > -6 D). This results from the excessive axial elongation of the eye which, by necessity, must involve the outer coat of the eye, the sclera. Consequently, high myopia is reported as a leading cause of registered blindness and partial sight. Current theories of refractive development acknowledge the pivotal role of the sclera in the control of eye size and the development of myopia. This review considers the major biochemical mechanisms that underlie the normal development of the mammalian sclera and how the scleral structure influences the rate of eye growth during development. The review will characterise the aberrant mechanisms of scleral remodelling which underlie the development of myopia. In describing these mechanisms we highlight how certain critical events in both the early and later stages of myopia development lead to scleral thinning, the loss of scleral tissue, the weakening of the scleral mechanical properties and, ultimately, to the development of posterior staphyloma. This review aims to build on existing models to illustrate that the prevention of aberrant scleral remodelling must be the goal of any long-term therapy for the amelioration of the permanent vision loss associated with high myopia.     

Retinal detachment after perforating injuries of the eye. I. Analysis of anamnestic data related to the injury, analysis of deficiencies in the primary care of the eye injury and principles of prevention of post-traumatic retinal detachment

In 1977-1987 the authors operated by the cryosurgical method 69 patients on account of detachment of the retina. The detachment occurred in 37 patients after simple perforation of the eye and in 32 patients after perforation with a foreign intraocular body. In all patients the perforation penetrated as far as the vitreous body. In the first part of the paper the authors analyze in detail anamnestic data related to the injury: age, mechanism of injury, rapidity and method of surgical treatment, chemical nature of foreign body and its size, rapidity and method of extraction of the foreign body. In the subsequent part the authors define the principles of prevention of post-traumatic detachment of the retina during surgical treatment of the injury. Finally the basic shortcomings of primary treatment of the injury from the aspect of detachment of the retina are outlined, as recorded in the group treated by the authors.     

Early changes in the bioelectrical activity of retina after eyeball contusion

Characteristic signs of changes in the bioelectrical activity of the retina soon after contusion of the eyeball supplement the functional criteria used for accurate diagnosis of the severity of contusion (classification of the injury). The most sensitive indicator of electroretinography is the amplitude of the electroretinogram (ERG) a-wave, which is sharply suppressed during the first three days in injury of any severity. Recovery of the bioelectrical activity of photoreceptors is delayed in comparison with the time course of the function of the second-order neurons (more rapid recovery of the b-wave in comparison with the a-wave). The reactivity of Müller cells increases in severe contusion, which is shown by a more pronounced decrease in the ERG b-wave amplitude and less expressed changes in low-frequency ERG. Analysis of the time course of biopotentials during repeated examinations of the patients helps control the efficacy of treatment. The results indicate that electroretinographic examinations are the most desirable 5-7 days after the contusion, because at this period the differences in the bioelectrical activity of the retina are the most expressed. The most informative parameters of electroretinography are the time course of total ERG a- and b-waves and the low-frequency rhythmic ERG.     

Etiopathogenetic problems in primary open-angle glaucoma]

The authoress discusses the contemporary opinions on the genetically conditioned predisposition to primary open angle glaucoma, histopathological changes in the structures responsible for the increase of the resistance of the outflow and on the mechanism of their formation. The pathogenesis of glaucomatous neuropathy is considered in the view of the damage caused by a direct compression of the nervous fibres and their chronic ischaemia. Both these mechanisms have an influence on the transport of axoplasma, the obstruction of which leads to atrophic changes of the nervous fibres and of the ganglion cells of the retina. The place of the primary lesion of these elements is considered.     

Age-related changes in carbonic anhydrase of toad lens

Carbonic anhydrase in the toad lens is immunologically identical to the (high activity) enzyme form present in the erythrocyte. As with the erythrocyte carbonic anhydrase, electrofocusing of the freshly-extracted lens enzyme separated three variants with isoelectric points of 6.1, 5.7 and 5.4, all of which exhibited inhibition properties characteristic of the high activity type of carbonic anhydrase. No evidence of the low activity form of the enzyme was found. In contrast to the erythrocyte isoelectric variants, which are stable in vitro, the three variants of lens carbonic anhydrase exhibited progressive anodization in the course of purification and during storage, and could not be isolated in the original form. Acidification of lens carbonic anhydrase appears to be part of the in vivo aging process of the enzyme protein: the still-active enzyme from the oldest lens region--the nucleus--exhibited a considerably lower isoelectric point than the enzyme extracted from the younger, soft fibers of the lens. Although the specific activity of the carbonic anhydrase from the fibers of lens nucleus was considerably lower than the activity of the enzyme from the younger fibers, the observed modification (acidification) of the aged enzyme did not appear to affect substantially its binding properties towards acetazolamide nor the heat lability of the active protein.     

Studies on visual pattern recognition in man and animals

The investigation of the time characteristics of visual recognition in man and animals has revealed three types of recognition performance. The first and the third types are characterized by independence of the amount of information received of the time of its processing; the second type involves the linear relationship between these quantities. The first type of recognition has been shown to be controlled by the innate mechanisms of the visual system while the second and the third types result from learning. With extirpations of various regions of the animal cortex it has been demonstrated that the mechanisms responsible for different types of recognition have different localization in the cerebral cortex.Visual images in man are invariant under size transformation and rotation around the vertical axis, the invariance to other types of rotation is formed as a result of training. The invariant recognition of the visual images is determined by the existence in the man visual system of separate channels to process the information concerning the shape of an object and its spatial characteristics. Progressive development of the capability of invariant recognizing of visual images in the course of evolution has been demonstrated.     

Experimental studies of the role of the adrenergic system in the development of bioelectric response of the retina and visual cortex. III. Effect of adrenaline on the ERG and VEP in rabbits

The influence of adrenalin on the electroretinogram and visual evoked potentials in rabbit were checked. After application of adrenalin the ERG showed a flattening of the a-wave and an increase of the b-wave amplitude and of oscillating potentials. One observed also a reduction of the A and B peaks and an increase of the C-wave in visual evoked potentials. On the basis of the results obtained the author discusses the role of the adrenergic system in the formation of the bioelectric response of the retina and visual cortex.     

A model of inhomogeneous expansion of the cornea and stability of its focus

A simple derivation of an analytical condition for expanding the cornea to get the stable position of (he corneal focus is given. The corneal thickness increases from the apex till the limbus and the corneal shell expands inhomogeneously as a result of changes in intraocular pressure conditions. It expands more in the centre of the cornea and less at its apex. As a result the expansion changes the paraxial radius of the corneal curvature by the value of δr as well as displaces the position of the centre of the central corneal curvature by the value of s. Presented paraxial calculations show that the position of the corneal focus is stable and insensitive on the expansion of the cornea if the ratio of both values amounts to about 0.34, Comparison of analytical results with published results of experiments on expanding intact cornea in vitro indicates their high accordance. This suggests a new. important role of the thickness distribution of the cornea.     

Clinical characteristics of anomalous correspondence

The correspondence status of 68 constant, early-age onset strabismics was evaluated with three common clinical tests to determine the prevalence of anomalous retinal correspondence (ARC) and which clinical features of the strabismus were most highly associated with the ARC. Ninety-six per cent, 71%, and 51% of the patients manifested ARC with the striated lens test, the synoptophore, and the afterimage test, respectively. The clinical features of the strabismus that were correlated, although weakly, with the diagnosis of ARC were: (1) the magnitude of the deviation at near for the striated lens test; (2) the age of the patient at the time of testing and the laterality of the deviation on the synoptophore; and (3) the age of the patient at the time of testing, the change in the deviation from distance to near, and the spherical equivalent refractive error for the afterimage test. A low correlation also existed between the age of the patient and the depth of ARC (superficial or deep-rooted). We conclude that the clinical features of strabismus cannot be utilized effectively to predict the status of retinal correspondence.     

The slope of the psychometric function at different wavelengths

Many current models of visual detection predict that the slope of psychometric functions for detection will be independent of the spectral power distribution of the test light once the spatial and temporal characteristics of the light are fixed. This article examines reports that the slope of the psychometric function depends on the wavelength of a spectrally-narrowband test light of fixed size and duration. Three sources of difficulty that any experimental measurement of slope must encounter are addressed: (1) the different spatial distributions of the photoreceptor classes across the retina; (2) possible variations in threshold over the course of the experiment; and (3) the large variability of estimates of slope and the need for a method of assessing this variability. Measurements of slope with 2 and 4 deg test flashes against a bright 510 nm field show no significant trend with the wavelength of the test. A novel statistical test bounds the magnitude of possible variations in slope across the visible spectrum.     

A model of the dynamics of retinal activity during natural visual fixation

During visual fixation, small eye movements keep the retinal image continuously in motion. It is known that neurons in the visual system are sensitive to the spatiotemporal modulations of luminance resulting from this motion. In this study, we examined the influence of fixational eye movements on the statistics of neural activity in the macaque's retina during the brief intersaccadic periods of natural visual fixation. The responses of parvocellular (P) and magnocellular (M) ganglion cells in different regions of the visual field were modeled while their receptive fields scanned natural images following recorded traces of eye movements. Immediately after the onset of fixation, wide ensembles of coactive ganglion cells extended over several degrees of visual angle, both in the central and peripheral regions of the visual field. Following this initial pattern of activity, the covariance between the responses of pairs of P and M cells and the correlation between the responses of pairs of M cells dropped drastically during the course of fixation. Cell responses were completely uncorrelated by the end of a typical 300-ms fixation. This dynamic spatial decorrelation of retinal activity is a robust phenomenon independent of the specifics of the model. We show that it originates from the interaction of three factors: the statistics of natural scenes, the small amplitudes of fixational eye movements, and the temporal sensitivities of ganglion cells. These results support the hypothesis that fixational eye movements, by shaping the statistics of retinal activity, are an integral component of early visual representations.     

Contrast sensitivity near borders: significance of test stimulus form, size and duration

The foveal increment threshold is known to be raised at the boundary between dark and light fields, but there are large differences in the magnitude of the effect reported. In the present paper the influence of test stimulus size, form and duration on this magnitude has been investigated. When the test stimulus was a small, 2′ of arc, spot, presented for 10 msec, its threshold was raised by only 0.1 log units or even less. The threshold of a bar, presented parallel to the boundary, was elevated by about 0.3 log units. The raise of the threshold was positively related to the length of the test bar and was a non-monotonic function of the diameter of circular test spot or the width of the bar. Increasing the duration of a small test spot increased slightly the effect but the duration of the bar was of no significance. The results show that the use of different test stimuli might account for the discrepancy in the data reported in literature. The selective lowering of the foveal sensitivity to bars at a linear boundary might be a result of adaptation or masking at the level of receptive fields of prolonged form or at the level of spatial frequency filters of the same orientation as the boundary.     

The role of presaccadic compression of visual space in spatial remapping across saccadic eye movements

When multiple bars are briefly flashed near the saccadic goal on a visual reference just before a saccade, the total width of the multiple bars appears to be compressed toward the saccadic goal. We show that presaccadic compression of visual space is related to the attribution of the displacement of a visual stimulus to the displacement of another stimulus appearing after the saccade. Subjects observed a bar and a ruler. The bar was displaced during a saccade and the ruler disappeared briefly at the same time, and then the ruler reappeared at the same location after the saccade. The subjects had the impression that the bar appeared to remain stationary and the ruler appeared to be displaced after the saccade. This impression occurs strongly when the amount of the compression of visual space reaches the maximum at the saccade onset. Also, it occurs only at the saccadic goal in the same way as presaccadic compression of visual space. Saccadic suppression of displacement was equivalent at the saccadic goal and in the location opposite to the saccadic goal, indicating that the attribution of the bar displacement to the displacement of the ruler appearing after the saccade is not a consequence of saccadic suppression of displacement. Furthermore, performing a direction discrimination task showed that the bar appears stationary at the saccadic goal during compression of visual space even when the bar was actually displaced. We interpret these results as showing that presaccadic compression of visual space establishes the location of the saccadic goal (the bar) as a reference and then the location of the ruler is remapped relative to the reference location after the saccade, resulting in the illusory displacement of the ruler.     

孔源性视网膜脱离年龄相关因素分析

目的 探讨孔源性视网膜脱离不同年龄段的发病相关因素、网脱特点、手术效果比较.方法 对2007年8月到2008年8月收治孔源性视网膜脱离117例,按年龄划分为:≤18岁青少年组29例(34只眼);19～64岁中年组53例(56只眼);≥65岁老年组35例(35只眼).对三组的临床特点、治疗效果进行对比及分析.结果 青少年组引起孔源性网脱相关因素中,高度近视、外伤因素的发病率较老年组均显著增高;青少年组引起孔源性视网膜脱离相关因素高度近视的发病率与中年组比较,差别无显著意义;青少年组引起孔源性视网膜脱离相关因素中外伤的发病率较中年组显著增高.青少年组及中年组孔源性视网膜脱离裂孔其中巨大裂孔、锯齿缘截离发病率较老年组显著增高,老年组中黄斑孔发病率较青年组及中年组显著增高,视网膜脱离范围累及黄斑区的青少年组、中年组及老年组相比较,青少年组低,差别有显著意义.青少年组、中年组、老年组术后视网膜复位率无显著差别.结论 青少年、中年及老年孔源性视网膜脱离比较,临床特点不同,但治疗效果无显著差异.     

34 epibulbar malignant tumours (author's transl)]

Based on a study of 34 cases, the authors make therapeutical and diagnostical references concerning the epibulbar malignant tumours. These were met with a frequency of 10% of the total amount of the malignant tumours of the visual apparatus. The most frequent setting were at the level of the bulbar conjunctiva and of the sclero-corneal limb, especially in front of the opening of the palpebral slit and in the temporal area. The histological examination of the tumours pointed out the following morphological types; epitheliomas (61%), especially spino-cellular, malignant melanomas (32%) and sarcomas (6%). The therapeutical attitude was the surgical one -- the accurate extirpation -- in the limited tumours, followed by radiotherapy when neoplasic lesions were found at the limit of section. In the invaded tumours, the exenteration of the orbit was performed followed by radiotherapy. On the terms of such a therapeutical conduct, the distant prognosis proved to be dependent on two factors: a. The early diagnosis, that is the stage of the therapeutical action. It is insisted upon the importance of the signs of malignization of some benign tumors: papillomas or naevi. b. The nature and origin of the tumour: the secondary tumours are more severe from the beginning.     

Formation of congenital defects during ocular development. I. Embryogenesis of the eye and developmental birth defects

Formation of developmental congenital defects of organ of vision finds its direct basis in the course of eye embryogenesis. Author of this work presents on the base of the literature, the crucial stages of embryonal development of eyeball and its individual structures. At every stage of development, the critical moments are shown, when the disturbance of the developmental process may lead to the origination of different groups of congenital eye defects. The knowledge of the potential period of particular congenital eye defects formation, facilitates the identification of eventual teratogenic factors, that have acted during pregnancy. It enables, to state the activity of gradually discovered genes, that regulate the course of embryonal development of the eye.     

Two waves of virus following anterior chamber inoculation of HSV-1

Following inoculation of herpes simplex virus type 1 (HSV-1) into the anterior chamber of one eye of a Balb/c mouse, a pattern of ocular retinal disease occurs which is characterized by retinal necrosis, disruption of the retinal architecture of the uninoculated contralateral eye, and sparing of the retina of the virus-inoculated eye. Our direct virus culture studies have revealed that, after uniocular anterior chamber inoculation, virus reaches the uninoculated eye in two temporally separate waves. The first wave of virus is detected in the uninoculated eye as early as one day postinoculation (pi), long before virus is found in either of the optic nerves or the brain. The second wave of virus arrives in this eye between 7 and 10 days pi. Sequentially, the path of the second wave of virus appears to move from the injected eye to (1) the ipsilateral optic nerve, (2) the brain, (3) the contralateral optic nerve, and (4) the posterior segment of the uninoculated contralateral eye, suggesting that interocular spread of the second wave of virus after anterior chamber inoculation occurs via neural pathways. Results of histopathologic examinations and virus culture studies suggest that the early wave of virus contributes to the inflammation observed at the angle structures of the contralateral eye 7-8 days pi and that the second wave of virus accounts for the peak virus titer observed on day 10 pi, a peak which coincides with the destruction of the retina of this eye. It is proposed that the first wave is causally related to the development of retinitis, which occurs as the second wave reaches the retina.     

Crossing axons in the third nerve nucleus.

The research presented in this paper studied the pathway taken by the crossed fibers of the third nerve nucleus in an animal whose nucleus has been well mapped and found to correlate well with higher mammals and man. Autoradiography using tritiated amino acid labeled the cell bodies an axons of the left side of the oculomotor nucleus of the cat. Axons so labeled could be seen emerging from the ventral portion of the left nucleus through the median longitudinal fasciculus (mlf) to join the left oculomotor nerve. Labeled axons were also seen to emerge from the medial border of the caudal left nucleus, cross the midline, and pass through the right nucleus and the right mlf to join the right oculomotor nerve. These latter axons must be the crossed axons of the superior rectus and levator palpebrae subnuclei. Since the path of these crossed axons is through the caudal portion of the nucleus of the opposite side, the destruction of one lateral half of the oculomotor nucleus would result in a bilateral palsy of the crossed subnuclei. Bilateral palsy of the superior rectus and bilateral assymetrical palsy of the levator palpebrae muscles would result.     

RPE transplants stabilize retinal vasculature and prevent neovascularization in the RCS rat.

Previous reports indicate that in the Royal College of Surgeons (RCS) rat a decline in the retinal vessel density accompanies the loss of the normal architecture of the deep bed. This begins at about three months with neovascularization that originates in the deep vessel bed and develops in the direction of the retinal pigment epithelial (RPE) cells by four months. A surgical technique has been developed recently for the transplantation of healthy RPE cells into the subretinal space of the RCS rat, resulting in the rescue of photoreceptor cells. This permits evaluation of the possibility that such transplants also protect the retinal vessels. We report for the first time: (1) the stabilization of the normal retinal vasculature by maintenance of the density and architecture of the deep vessel bed; and (2) prevention of neovascularization of the RPE by the surgical transplantation of healthy RPE cells into the subretinal space of the RCS rat. More specifically, we show a maintenance of the deep vessel bed density under the transplant in contrast to a significant reduction in the vessel density that had taken place in corresponding areas in nongrafted and sham injected controls at four months of age. The vessel density in the transplanted group is statistically different from the nongrafted and the sham injected groups. We also report a significant decline in the number of neovascularization profiles around the transplant site of the RPE-grafted RCS retina. We also note that the pathological changes in the vasculature of the RCS rat occur in a predictable central to peripheral gradient.