Effects of enucleation of the fixating eye on strabismic amblyopia in monkeys

The effects of enucleation of the fixating eye on the visual function of the deviating eye were studied in two rhesus monkeys with strabismic amblyopia. An esotropia was surgically induced when the monkeys were approximately 1 mo of age, and the fixating eyes were then enucleated at age 3 yr 11 mo. Four measures of visual function (photopic increment-threshold spectral sensitivity, scotopic spectral sensitivity, spatial modulation sensitivity, and temporal modulation sensitivity) were determined for both eyes prior to enucleation and for the deviating eyes over an 11-month period following the surgical removal of the fixating eye. Both monkeys showed some recovery of contrast sensitivity of their deviating eyes. The extrapolated cut-off values for their spatial modulation sensitivity functions increased from .27 to 2.8 c/deg for one animal and from .28 to 6.3 c/deg for the other. The extrapolated cut-off frequencies for the temporal modulation sensitivity functions of both animals showed an increase of 20-25 Hz compared to the pre-enucleation values. The spectral sensitivity functions of one monkey recovered to near normal values following enucleation, while both the photopic and scotopic functions of the other animal remained at pre-enucleation levels. Overall, the results of the experiments indicate that the removal of the fixating eyes of monkeys with strabismic amblyopia can result in significant improvements in the functional capacity of their deviating eyes.     

Spectral sensitivity of monocularly deprived cats.

The reports of rod-dominated psychophysical spectral sensitivity from the deprived eye of monocularly lid-sutured (MD) monkeys are intriguing but difficult to reconcile with the absence of any reported deprivation effects in retina. As most studies of MD retina have been from cat, we have examined psychophysically the increment threshold spectral sensitivity of MD cats using both reaction time and simultaneous two-choice behavioral procedures. Although the deprived eyes exhibited an absolute increment threshold sensitivity deficit, both rod and cone spectral sensitivity functions were obtained on large white backgrounds. This normal transition from rod to cone vision, as background luminance increased, was also found in threshold vs. intensity functions. Using their deprived eye, some cats exhibited a rod spectral sensitivity function when a smaller, normally photopic, background was used providing some support for a hypothesis that the rod-dominated spectral sensitivity observed in monkey may represent detection of scattered stimulus light. Alternatively monocular deprivation may reveal a rod-dominated mechanism which exists in monkey but not in cat.     

Sensitivity to global form in glass patterns after early visual deprivation in humans

To compare the effects of early monocular versus early binocular deprivation on the perception of global form, we assessed sensitivity to global concentric structure in Glass patterns with varying ratios of paired signal dots to noise dots. Children who had been deprived by dense congenital cataracts in one (n=10) or both (n=8) eyes performed significantly worse than comparably aged children without eye problems. Consistent with previous results on sensitivity to global motion [Vision Research 42 (2002) 169], thresholds in the deprived eyes were significantly better after monocular deprivation than after binocular deprivation of comparable duration, even when there had been little patching of the nondeprived eye after monocular deprivation. Together, the results indicate that the competitive interactions between a deprived and nondeprived eye evident in the primary visual cortex can co-occur with complementary interactions in extrastriate cortex that enable a relative sparing of some visual functions after early monocular deprivation.     

Functional effects of bilateral form deprivation in monkeys

Psychophysical methods were used to study the effects of binocular form deprivation, initiated at 1 month of age, on the visual sensitivities of young monkeys. All the monkeys reared with bilateral form deprivation for 7 weeks or longer had reduced spatial contrast sensitivity for both eyes. Although the contrast sensitivity deficits of the bilaterally form-deprived monkeys generally were larger for one eye than the other, the magnitudes of the deficits were small compared with those produced by similar periods of unilateral form deprivation. For other monocular vision functions investigated, temporal contrast sensitivity and increment-threshold spectral sensitivity, the data for the bilaterally form-deprived animals showed only minor variations from those of the control monkeys. However, none of the bilaterally form-deprived monkeys had binocular vision on either measures of binocular summation or stereodetection, even if the animal had normal monocular vision functions. Therefore, these results show that monocular sensory deficits caused by abnormal early visual experience as a result of bilateral form deprivation are much less severe than those caused by unilateral form deprivation. The differences in the severity of visual deficits may be attributed to the consequences of anomalous binocular competition associated with unilateral form deprivation that was minimized during bilateral form deprivation. Thus, these results illustrate that anomalous binocular competition is more detrimental to the developing visual system of infants than direct deprivation per se.     

Contrast sensitivity in amblyopia due to stimulus deprivation.

Contrast sensitivity functions for sinusoidal gratings of varying spatial frequency and stimulus duration were determined for both eyes of 2 patients with amblyopia due to early occlusion and lid closure. The amblyopic eyes showed reduced contrast sensitivity over a wide range of spatial frequencies and stimulus durations, and the temporal integration time of the amblyopic eye was increased by comparison with the non-amblyopic eye at high spatial frequencies. When the gratings were flickered at 10 Hz the sensitivity for both flicker and pattern detection was reduced in the amblyopic eye over the entire spatial frequency range. Abnormal flicker perception by the amblyopic eyes was also evident in the reduced photopic luminosity functions of these patients.     

Spectral sensitivity of the pupillary system

Background: The spectral sensitivity of the pupillary mechanism is reported to be greater in the blue part of the visible spectrum compared to the sensitivity of the visual system as defined by V(Λ) for a twodegree field. This means that blue light gives rise to smaller pupils compared to those that occur with light of other colours of the same luminance. This has been interpreted as indicating that there may be a rod input to pupil response even at photopic levels of illumination. An alternative explanation is that the smaller pupil size for blue light is an artefact arising from the use of the CIE V(Λ) function which is based on the spectral sensitivity of the eye to a twodegree field. In most investigations of pupil response, the adapting luminous field is much larger than two degrees. Method: The size of the static pupil was measured using an entoptic method when the eye was adapted to a large ‘Ganzfeld’ field of wavelengths of 624, 580, 521, 467 and 429 nm and luminances ranging from mesopic to photopic. Results: The pupil is smaller for light of shorter wavelengths compared to that for light of longer wavelengths of the same luminance. This effect disappears at photopic luminances when luminance is calculated using the V_10° (Λ) function instead of the V_2° (Λ) function but is still evident at mesopic levels. When pupil size is plotted against equivalent luminance for a lodegree field V_10° (Λ, L_eq) pupil size is independent of wavelength. Conclusion: The apparent enhanced sensitivity of the pupil to blue light at photopic levels is an artefact arising from the inappropriate use of the V_2° (Λ) for the measurement of luminance when the adapting field in pupil response measurements is a large field. Pupil size is a simple linear function of the log of equivalent luminance calculated for large fields for adapting luminances from photopic to mesopic and is independent of the wavelength of the adapting field for the whole range of adapting luminances.     

Stimulus alternation and the purkinje shift

Simultaneous spectral sensitivity determinations of the human electroretinogram and the visual evoked potential were made over an adaptation range extending from scotopic to photopic. Determinations were made for the amplitude of the B wave and the late negative potential of the electroretinogram and for the latency of a conspicuous peak of the VEP.³ Alternating stimuli were delivered to a peripheral region of the visual field. Responses at both recording sites matched the scotopic CIE luminosity function at low levels; at high levels they matched Wald's curve for the peripheral cones. The transition from scotopic to photopic function with light adaptation was not abrupt. Mixed activity was the rule over most of the range of adaptation, but the relative prominence of photopic activity increased with light adaptation. The spectral sensitivities of the responses from the two recording sites were similar but not identical with the electroretinogram receiving a stronger contribution from the scotopic system over much of the adaptation range.     

Spatial contrast sensitivity deficits in monkeys produced by optically induced anisometropia

An anisometropia was simulated in infant rhesus monkeys by securing a high-powered minus lens (-10 D) in front of one eye. The anisometropia rearing procedure was initiated at 30 days of age and was continued for durations of 30, 60, or 90 days. Behavioral measurements of spatial contrast sensitivity obtained when the animals were 9 months of age indicated that the monkeys treated for 30 days had equal or nearly equal contrast sensitivities and cut-off spatial frequencies in the two eyes. The 30-day monkeys also demonstrated normal binocular summation for threshold stimuli. In contrast, the monkeys treated for either 60 or 90 days showed a significant reduction in contrast sensitivity in the defocused eyes for spatial frequencies greater than 1.0 cycles/deg and failed to show an improvement in contrast sensitivity under binocular viewing conditions. The cut-off spatial frequencies obtained at moderate luminance levels for the defocused eyes of the 60- and 90-day monkeys were slightly more than 1.0 octave lower than the cut-offs for the nondeprived eyes and, like humans with anisometropic amblyopia, the deficits in the spatial resolving capacity of the defocused eyes were observed over a large range of background luminances. The results indicate that the lens-reared monkey is a promising model for anisometropic amblyopia in humans.     

Effect of a yellow intraocular lens on scotopic vision, glare disability, and blue color perception

PURPOSE: To compare the photopic and scotopic contrast sensitivity with and without glare as well as blue color perception between eyes with an AcrySof SN60AT Natural intraocular lens (IOL) (Alcon Laboratories Inc.) and eyes with a conventional AcrySof SA60AT IOL. SETTING: Ankara University School of Medicine, Department of Ophthalmology, Ankara, Turkey. METHODS: Right eyes of 38 patients with an AcrySof Natural IOL and right eyes of 38 age-matched patients with a conventional AcrySof SA60AT IOL were included in a study. Contrast sensitivity was measured with the Functional Acuity Contrast Test under photopic conditions. Scotopic contrast sensitivity in the presence or absence of glare was measured using the Mesotest II (Oculus GmbH). Blue-green color vision was evaluated with the Moreland equation of the HMC Anomaloskop MR (Oculus GmbH). RESULTS: The mean age of patients was 66.6 years +/- 8.2 (SD) in the Natural IOL group and 66.4 +/- 8.0 years in the conventional IOL group. There was no statistically significant difference in photopic contrast sensitivity, scotopic contrast sensitivity with and without glare, or disability glare between the 2 groups (P>.05). Photopic and scotopic contrast sensitivity with and without glare significantly decreased with age in both groups (P<.01). There was no statistically significant difference in anomaloscope scores between the 2 groups (P>.05). The Moreland middle match point showed a significant shift toward blue with age in both groups (P<.01). CONCLUSIONS: The AcrySof SN60AT Natural IOL provided contrast sensitivity under photopic and scotopic conditions (with and without glare) and blue color perception comparable that obtained with the AcrySof SA60AT IOL. Scotopic vision and blue color discrimination decreased with age with both IOLs.     

Psychophysical studies of monkey vision. I. Macaque luminosity and color vision tests

Macaque monkeys and normal human observers were tested on the same apparatus for the presence of a Purkinje shift, and for spectral sensitivity under scotopic and photopic conditions. The flicker fusion point for different lights was used as a measure of visual sensitivity in these tests and the testing procedure was a four-alternative forced choice. The results show that macaque monkeys and normal human observers show a rod-cone break at the same flicker frequency, and are very similar in both relative and absolute scotopic and photopic sensitivity.Macaque monkeys, normal human observers and color-defective human observers were also tested under identical conditions in several measures of color vision.     

Spectral sensitivity of human infants at absolute visual threshold

The forced-choice preferential looking technique (FPL) was used in conjunction with an infrared TV viewing system to measure the dark-adapted visual sensitivity of human infants at 1 and 3 months of age. Absolute thresholds of 1-month olds were about 50 times higher than adults, and those of 3-month olds were about 10 times higher. Spectral sensitivity at 1 month matched the CIE scotopic luminosity function at wavelengths longer than 430 nm; at wavelengths shorter than 430 nm there was an elevation in sensitivity, but it was not statistically significant. Spectral sensitivity at 3 months matched the CIE scotopic luminosity function at all wavelengths.We conclude that rods in the eyes of human infants contain rhodopsin by 1 month of age; that the scotopic system is functional and more sensitive than any cone system by 1 month of age; and that even in very young infants a single quantal absorption may be sufficient to generate a behaviorally meaningful signal in a rod photoreceptor.     

Monocular deprivation in an identical twin.

PURPOSE: The scientific literature contains minimal human studies of the effect of monocular deprivation on a single eye of twins. This report examines the effects of early visual deprivation on axial length measurements by comparing the refractive findings and axial length measurements of identical twins' 4 eyes, 1 visually deprived and 3 nondeprived. METHODS: A retrospective record review was performed on the medical records of identical 6-year-old Hispanic female twins. Both patients had received eye care at the University Optometric Center, State University of New York, for a period of at least 5 years. Subsequently, an Internal Review Board-approved prospective study was undertaken. Each twin received a comprehensive eye examination and an A-scan. RESULTS: The findings of the 3 nondeprived eyes were compared with those of the 1 deprived eye. The 3 nondeprived eyes had at least 20/20 visual acuity, low hyperopia or astigmatism, low with the rule keratometric findings, 12-mm corneal diameters, and A-scan results ranging from 21.0 to 21.55 mm. The remaining eye, which was visually deprived secondary to congenital cataract from birth to 6 months of age, had an aphakic refractive condition, decreased visual acuity, glaucoma, esotropia, and borderline microcornea. Keratometric findings were comparable with those of the other 3 eyes, whereas the A-scan finding was 25.16 mm. CONCLUSION: This case presented a unique opportunity to directly examine the effect of visual deprivation on axial elongation. Although other factors, such as glaucoma and borderline microcornea, may influence axial elongation, the findings in this patient indicate that a substantial portion, if not all, of the axial elongation can be attributed to early monocular deprivation.     

Electroretinograms and visual evoked potentials in long-term monocularly deprived cats

The effects of long-term monocular lid-suture deprivation on visual-evoked cortical potentials (VEPs) and flash- and pattern-evoked electroretinograms (FERGs and PERGs, respectively) were assessed in the cat. VEPs were virtually eliminated when recorded with the deprived eye, indicating that the lid suture produced a severe amblyopia in that eye. In contrast, FERGs and PERGs were more similar for both deprived and nondeprived eyes and comparable to those recorded in normal animals. The current findings demonstrate that long-term deprivation (3-4 yr) does not produce systematic changes in the electroretinogram.     

Morphological and physiological changes in the monkey visual system after short-term lid suture.

The effects of short-term unilateral form vision deprivation by suturing the lids in macaques were evaluated in terms of cortical neuronal electrophysiology and cell sizes in the lateral geniculate nucleus (LGN). The results show that cell shrinkage in the LGN is proportional to the duration of lid closure, with the parvocellular laminae being somewhat more sensitive than the magnocellular layers. Conversely, the electrophysiologically demonstrable shift of cortical dominance in favor of the nondeprived eye occurs after only 1 week of deprivation, with equal severity in short-term and long-term deprived monkeys, and precedes the appearance of histological abnormalities in the LGN. This increased cortical vs. geniculate sensitivity to visual deprivation is further demonstrated by the findings that (1) lid closure near the end of the second month of life does not affect the LGN cell size but causes the cortical electrophysiological deprivation effect and (2) the cortical eye dominance but not the size of the geniculate cells can be reversed in favor of the previously deprived eye by surgical closure of the formerly dominant eye.     

Scotopic versus photopic pattern onset-offset electroretinograms

We investigated the contribution of rods and cones to the human pattern electroretinogram to onset and offset checkerboards of different spatial frequency and wavelength in a 39° × 39° field. Under strictly scotopic conditions, there was a negative potential at onset and a positive potential at offset, whereas under photopic conditions, there was a positive potential at onset and a negative/positive potential at offset. Thus, the waveform to pattern onset (offset) was that of the luminance electroretinogram to decreasing (increasing) luminances. For pattern onset, the sensitivity difference 486–601 nm under scotopic and photopic conditions closely followed the luminosity function of rods and cones. The amplitude of the scotopic onset response increased with check size up to 3°30 and that of the photopic onset response, up to 30. With larger checks, the scotopic and photopic onset response markedly decreased. This indicates antagonistic center-surround organization of the receptive fields under both scotopic and photopic conditions. By contrast, the offset response monotonically increased with check size under scotopic and photopic conditions, which suggests a luminance component in the pattern electroretinogram. Consequently, the pattern electroretinogram to reversing checker-boards has to be regarded as a mixture of both pattern- (contrast) and luminance-specific components.     

Rod-cone dependence of saccadic eye-movement latency in a foveating task

This study examines the relations between some well known oculomotor functions (saccades) and well known retinal physiology (dark adaptation): it deals with the overall latency versus target luminance functions, with the underlying rod and cone latency-luminance functions, and with the synergistic interaction between these latency functions for mesopic targets. Saccadic latency was measured to small lit targets presented at 10 deg retinal eccentricity in complete darkness. Target luminance and wavelength were varied. Additional measurements were made during dark adaptation or on backgrounds, or at different retinal eccentricities. Luminance matched stimuli and Palmer's (1968) equivalent luminance transformation were also used. Latency is determined by an achromatic luminance mechanism that receives substantial rod inputs above the cone threshold. Latencies for pure rod or pure cone inputs increase rapidly as target luminance decreases. For the rods this latency increase appears to represent the waiting time for the 140 or so photons (lambda = 507 nm) that are required for a saccade. Errors in direction occur at scotopic luminances, or at low photopic luminances when only cones are functioning.     

Observations on the effects of form deprivation on the refractive status of the monkey

The consistency of the refractive error alterations produced by monocular form deprivation in developing monkeys and the influence of the duration and the age at the onset of deprivation on the magnitude of these alterations was investigated. Refractive error and axial length measurements are presented for a group of monkeys which had one eye sutured closed for a period exceeding 18 months beginning at various ages ranging from 26 days to 25 months. In addition, we pooled and reanalyzed refractive error and axial length data for monocularly form-deprived monkeys from previous studies. When the alterations in the deprived eye's refractive status are specified with respect to the fellow nondeprived eye, the results are, with a few noteworthy exceptions, consistent between laboratories and individual animals. In most cases, early monocular form deprivation causes the treated eye to develop a longer axial length and to manifest a more myopic/less hyperopic refractive error than the fellow nontreated eye. The magnitude of this deprivation-induced alteration is generally dependent on the duration and the age at the onset of form deprivation. The earlier the deprivation is initiated and the longer it is maintained, the greater the degree of the relative myopia produced in the deprived eye.     

Better perception of global motion after monocular than after binocular deprivation.

We used random-dot kinematograms to compare the effects of early monocular versus early binocular deprivation on the development of the perception of the direction of global motion. Patients had been visually deprived by a cataract in one or both eyes from birth or later after a history of normal visual experience. The discrimination of direction of global motion was significantly impaired after early visual deprivation. Surprisingly, impairments were significantly worse after early binocular deprivation than after early monocular deprivation, and the sensitive period was very short. The unexpectedly good results after monocular deprivation suggest that the higher centers involved in the integration of global motion profit from input to the nondeprived eye. These findings suggest that beyond the primary visual cortex, competitive interactions between the eyes can give way to collaborative interactions that enable a relative sparing of some visual functions after monocular deprivation.     

Evidence that increased scleral growth underlies visual deprivation myopia in chicks

The authors evaluated three measures of scleral growth in chicks that were visually deprived with the use of translucent occluders. The authors sought to determine whether the ocular elongation and myopia that results from this deprivation is associated with increased growth of the sclera. The authors found that the dry weight of the sclera of deprived eyes increased 65% faster than that of nondeprived eyes. Furthermore, the uptake of labeled methionine and thymidine was significantly increased by visual deprivation, whether expressed as incorporation per sclera, per milligram of sclera, per milligram of protein, or per milligram of DNA. In addition, the amount of DNA and soluble protein was significantly greater in the scleras of deprived eyes than in those of nondeprived eyes. Finally, the degree of hydration of the scleras from deprived eyes was greater relative to their weight than that of the scleras from nondeprived eyes. These results suggest that visual deprivation causes increased cellular proliferation and increased protein synthesis in the sclera of chicks.