Monocular Eyelid Closure Research Articles

Histomorphological changes in the cornea of the rat following monocular eyelid closure

Objectives: The aim of this study was to describe structural changes in the cornea of the rat after monocular eyelid closure. Methods: Twenty-six Rattus norvegicus male rats aged three months were used. The rats were randomly assigned into baseline (2), experimental (16) and control (8) groups. Unilateral eyelid closure was performed on the experimental animals by suture tarsorrhaphy. At experiment days 5, 10, 15 and 20, four rats from the experimental group and two rats from the control group were euthanized, their eyeballs harvested, and routine processing was done for paraffin embedding, sectioning and Masson’s trichrome staining. The photomicrographs were taken using a digital photomicroscope. Results: In the closed eyes, there was a time-dependent reduction in the stratification of the corneal epithelium with subsequent disintegration, and an increase in distribution of stromal keratocytes while the corneal endothelial cells showed slight enlargement from squamous shape. The contralateral and control eyes did not exhibit any significant changes through the experimental period. Conclusion: Monocular eyelid closure causes structural changes in the corneal epithelium, stroma and endothelium of the tarsorrhaphy eye. Therefore, tarsorrhaphy should not be prolonged due to risk of corneal diseases and diminution of vision as a result of the structural changes. Keywords: cornea; eyelid closure; rat; tarsorrhaphy

Rapid Structural Remodeling of Thalamocortical Synapses Parallels Experience-Dependent Functional Plasticity in Mouse Primary Visual Cortex

Monocular lid closure (MC) causes a profound shift in the ocular dominance (OD) of neurons in primary visual cortex (V1). Anatomical studies in both cat and mouse V1 suggest that large-scale structural rearrangements of eye-specific thalamocortical (TC) axons in response to MC occur much more slowly than the shift in OD. Consequently, there has been considerable debate as to whether the plasticity of TC synapses, which transmit competing visual information from each eye to V1, contributes to the early functional consequences of MC or is simply a feature of long-term deprivation. Here, we used quantitative immuno-electron microscopy to examine the possibility that alterations of TC synapses occur rapidly enough to impact OD after brief MC. The effect of short-term deprivation on TC synaptic structure was examined in male C57BL/6 mice that underwent 3 and 7 d of MC or monocular retinal inactivation (MI) with tetrodotoxin. The data show that 3 d of MC is sufficient to induce substantial remodeling of TC synapses. In contrast, 3 d of MI, which alters TC activity but does not shift OD, does not significantly affect the structure of TC synapses. Our results support the hypothesis that the rapid plasticity of TC synapses is a key step in the sequence of events that shift OD in visual cortex.

Thalamic activity that drives visual cortical plasticity

Manipulations of activity in one retina can profoundly affect binocular connections in the visual cortex. Retinal activity is relayed to the cortex by the dorsal lateral geniculate nucleus (dLGN). We compared the qualities and amount of activity in the dLGN following monocular eyelid closure and monocular retinal inactivation in awake mice. Our findings substantially alter the interpretation of previous studies and define the afferent activity patterns that trigger cortical plasticity.

Instructive Effect of Visual Experience in Mouse Visual Cortex

We describe a form of experience-dependent response enhancement in the visual cortex of awake mice. Repeated presentations of grating stimuli of a single orientation result in a persistent enhancement of responses evoked by the test stimulus. Response potentiation is specific to the orientation of the test stimulus, develops gradually over the course of several training sessions, and occurs in both juvenile and adult mice. The stimulus-selective response potentiation (SRP) can mask deprivation-induced response depression in adult mice. SRP requires NMDA receptor activation and is prevented by viral delivery of a peptide that interferes with AMPA receptor trafficking. SRP may reveal the mechanisms involved in certain forms of perceptual learning.

Synaptic Density in Geniculocortical Afferents Remains Constant after Monocular Deprivation in the Cat

Monocular eyelid closure in cats during a critical period in development produces both physiological plasticity, as indicated by a loss of responsiveness of primary visual cortical neurons to deprived eye stimulation, and morphological plasticity, as demonstrated by a decrease in the total length of individual geniculocortical arbors representing the deprived eye. Although the physiological plasticity appears maximal after 2 d of monocular deprivation (MD), the shrinkage of deprived-eye geniculocortical arbors is less than half-maximal at 4 d and is not maximal until 7 d of deprivation, at which time the deprived arbors are approximately half their previous size. To study this form of plasticity at the level of individual thalamocortical synapses rather than arbors, we developed a new double-label colocalization technique. First, geniculocortical afferent arbors serving either the deprived or nondeprived eye were labeled by injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin into lamina A of the lateral geniculate nucleus. Then, using antibodies to synaptic vesicle proteins, we identified presynaptic terminals within the labeled arbors in layer IV of the primary visual cortex. Analysis of serial optical sections obtained using confocal microscopy allowed measurement of the numerical density of presynaptic sites and the relative amounts of synaptic vesicle protein in geniculocortical afferents after both 2 and 7 d of MD. We found that the density of synapses in geniculocortical axons was similar for deprived and nondeprived afferents, suggesting that this feature of the afferents is conserved even during periods in which synapse number is reduced by half in deprived-eye arbors. These results are not consistent with the hypothesis that a rapid loss of deprived-eye geniculocortical presynaptic sites is responsible for the prompt physiological effects of MD.

Experiments to study recovery of lateral geniculate nucleus cell size following monocular lid closure and reverse suture in infant monkeys

Following monocular eyelid closure at birth in macaque monkeys, reverse suture at 3 weeks of age cancels the difference in size between deprived and undeprived parvocellular lateral geniculate nucleus (LGN) cells by causing hypertrophy of the initially deprived cells. This means that two weeks after reversal cells in both initially deprived and initially undeprived parvocellular laminae are approximately 15% larger than normal. However, long term survival shows that there is a second phase of change during which all parvocellular cells shrink, but particularly the initially undeprived cells which become considerably smaller than the initially deprived cells. Reopening the secondarily closed eye after a short period of reversal resulted in normal cell sizes in one animal, but two animals developed marked squints and had very small parvocellular cells in all laminae, and one animal developed bilateral amblyopia. Simply reopening the eye of two animals after two months of late closure started at 7 months of age reversed the shrinkage of all parvocellular cells which is caused by late closure.

Effects of enucleation at different ages on the sizes of neurons in the lateral geniculate nucleus of infant and adult monkeys

Cell areas have been measured in the lateral geniculate nucleus following enucleation in 7 infant and 3 adult monkeys. Deafferented cells showed rapid transneuronal degeneration. Following enucleation at birth non-deafferented parvocellular LGN cells underwent hypertrophy which was followed by a delayed return to normal size. Enucleation at 7 months of age caused marked shrinkage of non-deafferented parvocellular cells. These changes in the non-deafferented cells are similar to those following monocular lid closure at the same ages. Sizes of non-deafferented cells were unchanged following enucleation of adult monkeys.

Anatomical consequences of long-term monocular eyelid closure on lateral geniculate nucleus and striate cortex in squirrel monkey.

The effects of long-term monocular deprivation on the geniculostriate system in squirrel monkeys were studied with neuroanatomical methods. Four neonates were visually deprived by monocular eyelid suture during their first 10 days of life and survived from 9 to 40 months. In the lateral geniculate nucleus (LGN), deprivation resulted in severe cell size changes. Neurons in the deprived laminae were smaller compared to those in the undeprived laminae. Deprivation left the reciprocal connections between LGN and striate cortex intact: After horseradish peroxidase (HRP) injections into striate cortex, retrogradely transported enzyme labeled a wedge of neurons in deprived and undeprived LGN laminae; anterogradely transported HRP filled preterminal and terminal axons in this wedge. Following 3H-proline injections into the deprived eye for transneuronal transport, autoradiography showed in the ipsilateral striate cortex a silver grain distribution over most of layer IVc similar to that in normal squirrel monkeys, except for a small strip in the anterior calcarine fissure. Here, a few, irregularly spaced "patches" of higher grain density occurred deep in layer IVc. Layer IVc of contralateral area 17 was also uniformly labeled over most of its extent, except for a very few and inconspicuous accumulations of slightly increased silver grains. After visual stimulation of the deprived eye, the 14C-2-deoxyglucose method showed in the contralateral striate cortex some alternating "patches" of higher uptake superimposed on the heavy labeling in layer IVc. Layer IVc in the ipsilateral cortex was more uniformly labeled. Regularly spaced arrays of labeled "puffs" in layers II/III were present in both hemispheres. Cytochrome oxidase staining showed no change in the distribution pattern of the enzyme in the deprived monkeys from the basic pattern of normal adults. No changes in cell sizes were found in layer IVc in cresyl-violet-acetate-stained sections. These results lead to the conclusion that in area 17 of squirrel monkeys there is no distinct segregation of inputs from the two eyes into anatomically discrete ocular dominance columns and they support the view of a predominantly binocular organization of area 17.

Pattern processing and slow waves in visual cortex of visually deprived cats

The slow-wave response recorded from electrodes within the visual cortex of the awake cat has been shown to reflect the pattern processing activities of the visual cortex. In a cat deprived of pattern vision by monocular lid closure, the response evoked from the non-deprived eye by patterned and diffuse stimuli show differences similar to those found in a normal cat. However, when these same stimuli are presented to the deprived eye, the differences are not present. These slow-wave data confirm previous data from single unit recordings in showing that pattern processing is altered by deprivation.

Plasticity of cat visual cortex

Monocular eyelid closure in kittens mimics certain visual deficits in humans that result in amblyopia ex anopsia. I have now studied the effects of monocular eyelid closure in cat upon the slow-wave response recorded from visul cortex. The pattern of changes in the response closely paralleled the changes in visual function of the amblyopic eye, in particular the suppression during binocular vision.

Visual deficits and recovery following monocular lid closure in a prosimian primate

The behavioral consequences of long-term monocular lid closure in 6 deprived and 2 normal prosimian primates (Galago crassicaudatus) were examined using a visual field perimetry technique and a variety of visuomotor tests. Deprived galagos had their right lid sutured either within 8 (n = 4) or 38 (n = 2) days postpartum and were reared under these conditions for at least 8 months. Normal galagos responded briskly on all visuomotor tests including tracking, response to visual threat, ability to descend from a platform, and obstacle avoidance. Perimetry tests of these animals showed that the visual field with both eyes open was 180 degrees while each eye tested individually had a 135 degrees field. Deprived galagos tested with their experienced eye exhibited normal visuomotor behavior and normal visual fields. Following reverse suture all deprived animals behaved initially as if blind on all tests. Over the next 4-8 weeks these animals showed some limited recovery on the visuomotor tasks. Tests of perimetry indicated that by the end of four weeks of testing, vision was limited to the deprived monocular segment where it remained stable to the end of the testing period. With both eyes open, deprived galagos exhibited a normal 180 degrees visual field. Curiously, however, when stimuli were introduced simultaneously in corresponding points in both monocular segments of the field, the majority of orienting responses of deprived subjects favored the right deprived eye. Under similar testing conditions one normal subject favored its right eye and the other its left eye. The reversibility of monocular deprivation was explored further either by extending the period of forced use of the deprived eye in one animal or by removing the non-deprived eye in another animal. No further improvement in visual field perimetry was seen following a longer period of forced use of the deprived eye. In contrast, a substantial increase in the visual field could be demonstrated one month following removal of the originally experienced eye: the deprived eye field expanded to include the entire 90 degrees ipsilateral hemifield. The contralateral field of the deprived eye, however, continued to remain blind even after one year. These behavioral results suggest that although the original binocular competitive disadvantage suffered by the deprived eye during development is relatively permanent, it can partially be ameliorated by late removal of the non-deprived eye.

Cell size changes in undeprived laminae of monkey lateral geniculate nucleus after monocular closure.

Transneuronal degeneration in the lateral geniculate nucleus (LGN) due to removal of an eye or closure of the eyelids has traditionally been measured by comparing the sizes of cells in the deprived laminae with those of the corresponding undeprived laminae in the nucleus of the opposite side of the same brain. However, the undeprived laminae may themselves change; some hypertrophy of their cells has been found in kittens following early monocular closure1 but such comparisons are difficult in the cat because of the variation between animals2. In a recent study of the effect of binocular closure in the monkey3 it was necessary to measure cell sizes in the LGNs of a series of normal monkeys as controls, because there are no undeprived laminae after binocular closure. From this series of normal animals it was apparent that there is much less variation between individual monkeys than has been found with cats and it was thus possible to make reliable comparison of cell sizes between different animals. In the present study the series of normal monkeys has been extended and the mean cell size and its variability between animals have been established for each geniculate lamina in the normal rhesus monkey. We give here a comparison of the cell sizes of both deprived and undeprived laminae following monocular eyelid closure with normal measurements and report some unexpected findings, particularly in animals where monocular closure has been carried out relatively late.

Neural activity from motor cortex in visuobehavioral deprivation

Visually evoked slow-wave and single-unit activity were recorded from motor cortex in nonanesthetized and chloralose-anesthetized cats with a history of monocular lid closure ranging from 11 to 18 months. In the deeply anesthetized cat no differences were found between the responses evoked from the deprived and nondeprived eyes. During light anesthesia and in the awake cat a robust single-unit and slow-wave response was evoked from the nondeprived eye compared to the weak response evoked from the deprived eye. When the awake cat used only its previously deprived eye, the amount of unit activity in motor cortex was less than the amount of activity present when the cat used only the nondeprived eye. The results indicate deficits in the ability of deprived eye-stimulation to activate the motor cortex. The comparison of responses between deep and light anesthesia and the lower neural activity level observed when subjects use only the deprived eye suggests that these effects may arise in part from changes in the relationship between the deprived eye and arousal mechanisms.

Effects of monocular lid closure on development of receptive-field characteristics of neurons in rabbit superior colliculus.

1. The receptive-field characteristics of superior colliculus neurons were studied in rabbit pups that had one eyelid sutured prior to eye opening. Units recorded from the superior colliculus (SC) receiving input from the unsutured eye provided normal developmental data, and those from the colliculus receiving input from the sutured eye were used to study the effect of visual deprivation. 2. A total of 1,054 cells recorded from 89 animals ranging in age from 7 to 35 days were obtained, 514 cells in the normal colliculus and 540 cells in the deprived colliculus. During normal development, three nonoriented cell types (concentric, uniform, motion) showed a progressive increase in relative frequency of occurrence, starting at about 7 days and reaching the adult level at about 15 days. Directionally selective cells developed slightly later, reaching an adult level at 3 wk. Oriented directional cells were the slowest to mature, requiring about 4 wk to reach the final level. 3. Eyelid suturing significantly affected the oriented directional cell development; these cells developed at a normal rate for about 3 wk, then rather abruptly began to decrease in number; a stable relative frequency of about one-fourth the normal value was reached at about 4 wk. A corresponding increase in the relative number of indefinite cells to above the normal level also occurred. In contrast, the development of nonoriented cells and directionally selective cells was not affected by the deprivation. 4. The development of rabbit superior colliculus receptive fields was found to be, in general, similar to development of kitten SC receptive fields. It also correlates well with developmental changes seen in rabbit ganglion cell receptive fields and with anatomical changes in developing rabbit SC. Indirect support is given for the hypothesis that changes seen in SC with deprivation are secondary to changes in the visual cortex.

Photic evoked activity in the visual cortex of monocularly deprived cats

Monocular eyelid closure during the first 2 months of life has previously been shown to cause a dramatic reduction in the number of neurons that can be activated by photic stimulation of the deprived eye. Those studies recorded single neuron responses to stimulation with discrete forms. In contrast, the author has previously shown that the initial components of the slow-wave response from one region of the cortex evoked by diffuse stimulation of the deprived eye are actually similar to these components when evoked from the nondeprived eye. The disparity in these findings may be due to the fact that the slow-wave data were recorded from only one region of the visual cortex and in response to diffuse photic stimulation. To resolve these differences the slow-wave response from areas 17, 18, and 19 and single-unit activity in response to diffuse photic stimulation were studied. The response evoked from the deprived eye was found to be reduced in area 17 but was sometimes actually larger in areas 18 and 19 than the response evoked from the good eye. Of 45 single units tested, 36 could be driven by stimulation of either eye. Evidently, photic stimulation of the deprived eye is capable of evoking neuronal discharge within the visual cortex. Indeed, the ability of neuronal transmission from the deprived eye to activate neurons within the visual cortex is in keeping with the evidence on behavioral recovery from monocular eyelid closure.

Postnatal development of the synaptic organisation of the lateral geniculate nucleus in the kitten with unilateral eyelid closure.

CLOSURE of the eyelids over one eye during a well defined period of susceptibility in kittens has been shown to result in severe structural changes of the nerve cells in the lateral geniculate nucleus1,2. Neurones in those parts of the lateral geniculate nucleus which are related to the binocular visual field and which receive fibres from the closed eye are smaller than those in the laminae innervated from the open eye. The fact that cellular changes do not occur in the part of the nucleus related to the monocular visual field has led to the hypothesis that there is a process of competition or an imbalance between the neurones in the deprived and stimulated portions of the binocular parts of the laminae, whereas such competition is not present in the monocular segment2,3. We have examined the lateral geniculate nucleus with the electron microscope at different intervals extending through the period of susceptibility after monocular eyelid closure and in a parallel series of normally reared kittens.

Behavioral and morphological evidence for binocular competition in the postnatal development of the dog's visual system.

Two normally-reared dogs and five dogs reared with monocular eyelid closure were studied. The two normal dogs and three of the deprived dogs were tested with a perimetry technique for the extent of behavioral visual field, and all of the dogs were studied histologically to determine the size of cell somata in the lateral geniculate nucleus. Behaviorally, the normal dogs saw with each eye approximately from 120 degrees ipsilateral to 30 degrees contralateral. The deprived dogs had a normal field of view for the non-deprived eye, but with the deprived eye they saw only in the monocular segment (i.e., approximately 120 degrees to 30 degrees ipsilateral). Histologically, geniculate cells in the monocular segment of deprived dogs were of normal size. In the deprived laminae of the binocular segment, however, cells averaged about two-thirds normal size, and in non-deprived laminae of this segment, neurons were hypertrophied. The hypertrophy was greater for cells in lamina A (40%) than for those in lamina A1 (17%). These data indicate that in monocularly deprived dogs, the monocular segment of the visual system develops normally, but the binocular segment does not. Therefore, we conclude that binocular competition operates in the developing dog's visual system much as it does in the cat's.

Development of interocular alignment in cats

Kittens are born with a large divergent strabismus. Fourteen kittens observed daily from birth concurrently developed both visually guided behavior and normal interocular alignment during the second postnatal month. (Interocular alignment is defined as the angle formed by the visual axes.) Once achieved, this normal interocular alignment is resistant to a number if alterations in the cat's visual input. Two cats with adult eyelid closure (one binocular, one monocular), 3 cats with adult unilateral visual cortex ablations, and 3 cats with adult transection of the optic chiasm along the sagittal midline did not develop strabismus. However, all 5 kittens reared with binocular eyelid closure and 9 of 10 reared with monocular eyelid closure developed a stable strabismus. The former all displayed a similar divergent strabismus whereas strabismus in the latter covered a wide range of interocular misalignments. A purely mechanical explanation is discarded since both eyes of cats reared with monocular closure were similarly misaligned. These results indicate that cats require a normal, binocular visual environment during development to achieve the proper interocular alignment. It is hypothesized that during normal development, the binocular patterned visual input stimulates some part or parts of the cat's visual system to align the eyes properly. Binocular closure results in no patterned stimulation, so the eyes remain diverged. Monocular closure allows patterned stimulation through the open eye, and this initiates alignment which results in a wide range of errors due to the lack of data from the closed eye. Apparently the visual system does not require the visual cortex for this alignment since 3 kittens reared with extensive visual cortex ablations made during the 8th postnatal day demonstrated the normal development of both visually guided behavior and interocular alignment.