Orientation anisotropy of visual stimuli in rhesus monkey: a behavior study.

The contrast sensitivity of the rhesus monkey was tested, according to a modified reaction-time paradigm, for sine-wave grating targets at different orientations. The monkey possesses an oblique effect slightly larger than that of humans. A reaction time analysis showed the oblique effect to be a suprathreshold as well as a threshold phenomenon. The presence of this effect further strengthens the use of the monkey as a model for the human visual system.

Effect of corticosteroids on rabbits corneal keratocytes after photorefractive keratectomy.

BACKGROUND: To determine the corticosteroid effect on the activity and repopulation of keratocytes after photorefractive keratectomy (PRK). METHODS: A 193-nm excimer laser (VISX Twenty/Twenty) created a central ablation depth of 22 microns (diameter:5 nm) on 22 corneas of 16 albino rabbits. Two ablated eyes were examined 6 hours following PRK. Twelve eyes received no postoperative corticosteroids and eight were treated with topical fluoromethalone for 3 months. Corneas were examined 1, 3, 6, and 12 months after PRK by immunofluorescence and transmission electron microscopy. RESULTS: Corticosteroids reduced haze (p=0.02), but all corneas (treated or untreated) cleared 6 months after PRK. Keratocytes were absent from the anterior 100 microns of the stroma 6 hours after PRK. However, the number and activity of keratocytes were significantly greater in this area in untreated corneas at 1 month and then gradually decreased. By 6 and 12 months, the number of keratocytes approached controls. Treated corneas had fewer keratocytes than either controls or untreated eyes (p<0.01) and by 3 months, a subepithelial acellular zone of 30 to 50 microns thickness appeared and persisted until at last 12 months after PRK. CONCLUSIONS: Corticosteroids have a transient effect in reducing haze and seem to inhibit keratocyte movement, leading to an acellular subepithelial region beneath the ablated area.

Behavioral studies on the effect of abnormal early visual experience in monkeys: spatial modulation sensitivity.

Spatial modulation sensitivity functions have been investigated by behavioral methods in two monkeys reared with normal visual experience and 12 monkeys reared with abnormal, early visual experience. Experimental treatments were initiated when the animals were approximately one month of age. Two monkeys were each treated with one of the following procedures: (1) long-term monocular lid suture, (2) short-term monocular lid suture, (3) surgically induced esotropia, (4) surgically induced exotropia, (5) optical dissociation of binocular vision with ophthalmic prisms, or (6) chronic monocular cycloplegia. The results of the studies showed a severe loss of contrast sensitivity of the treated eyes compared to the control eyes for monkeys reared with monocular lid suture or surgically induced esotropia. Surgically induced exotropia resulted in a moderate reduction in sensitivity of the deviated eye while optical dissociation resulted in a mild reduction in sensitivity of one eye compared to the other. One of the two monkeys reared for seven months with chronic monocular cycloplegia had a relative reduction in contrast sensitivity of the treated eye, but the other monkey had equal sensitivities in the two eyes. However, binocular summation experiments showed that even though the relative difference between the monocular sensitivities was small or absent for the monkeys reared with optical dissociation or chronic monocular cycloplegia, none of them demonstrated binocular vision in these experiments.

Behavioral studies on the effect of abnormal early visual experience in monkeys: temporal modulation sensitivity.

Temporal modulation sensitivity functions were investigated by behavioral methods in two monkeys reared with normal visual experience and 12 monkeys reared with abnormal early visual experience. Experimental treatments were initiated when the animals were approximately one month of age. Two monkeys were each treated with one of the following procedures: (1) long-term monocular lid suture, (2) short-term monocular lid suture, (3) surgically induced esotropia, (4) surgically induced exotropia, (5) optical dissociation of binocular vision with ophthalmic prisms, or (6) chronic monocular cycloplegia. The temporal modulation sensitivity functions for uniform field flicker for both eyes of the control subjects and for the untreated eyes of the experimental subjects were similar to functions for humans measured on the same apparatus using the same behavioral procedures. All eight of the monkeys preconditioned by lid suture or surgically induced strabismus showed reduced sensitivity at all temporal frequencies with the difference between the experimental and control eyes being larger for the high than low temporal frequencies. The monkeys reared with optical dissociation of binocular vision or chronic monocular cycloplegia showed equal temporal modulation sensitivities of the two eyes, but failed to show binocular summation. It was concluded from these studies that abnormal early visual experience in monkeys results in deficiencies in the processing of both spatial and temporal information, but the differences between the treated and untreated eyes were usually greater in the spatial domain than the temporal domain.

Contrast increment thresholds of rhesus monkeys.

Increment contrast thresholds ware determined behaviourally as a function of background contrast for sinusoidal gratings with spatial frequencies between 0.5 and 8.0 c/deg. As has been previously reported for humans, the contrast discrimination functions ere characterized by a facilitation effect for low background contrasts and a masking effect for high background contrasts. The shapes of the functions varied with spatial frequency, but for all spatial frequencies, portion of the functions were adequately described by a power law. The shapes of the reaction time distributions obtained for near threshold stimuli suggested that at least two mechanisms with different spatio-temporal sensitivities were involved in detecting the stimulus and that the exact shape of the contrast discrimination function was dependent on which mechanism dominated detection.

Effect of humidity on the deswelling function of the human cornea.

The present study determined the effect of the full range of humidities on the deswelling function of the human cornea. The closed-eye deswelling function and the open-eye deswelling responses for five different levels of humidity (0%, 25%, 60%, 85%, and 100%) were assessed for 8 normal, young-adult subjects. Open-eye corneal deswelling for the 8 subjects was unaffected by relative humidities from 0 to 100%. Therefore, osmotically-driven corneal thinning effect of tear evaporation does not significantly contribute to the deswelling function of the human cornea. We conclude, contrary to recent reports, that the endothelial pump is the primary mechanism that maintains normal corneal thickness and provides recovery from stromal edema.

The expanding scope of optometry and the impact on the curriculum of schools and colleges.

The expansion of the profession of optometry has placed pressures on the schools and colleges of optometry to reduce traditional curricular elements, add new ones, obtain faculty trained for the new scope of the profession, and provide additional patient experiences for hands-on learning. While the schools have adapted to these pressures and are leading the profession into new areas, the traditional 4-year optometry curriculum is critically filled. Further expansion will require difficult decisions.

Fusional vergence ranges of the monkey: a behavioral study.

Fusional vergence ranges were behaviorally determined for two rhesus monkeys and two human subjects using random dot stereograms as stimuli. The subject performed a two-alternative forced choice form discrimination of discretely presented stimuli and the fusional vergence stimulus was changed during the intertrial intervals. The fusional vergence measurements for the monkey were similar to those for the human subjects.

A study of specific optical characteristics of biomicroscopes.

This study evaluated specific optical characteristics of eight biomicroscopes along with mechanical capabilities and accessories. Although the instruments were not ranked, the information in table form should be helpful in the selection of a biomicroscope for the practitioners requirements.

Computerized fixation disparity measurement.

Fixation disparity (FD) curves have many clinical implications. A new method for evaluating binocular function using FD curves is described. This method is shown to compare very well with traditional methods of measurement. During clinical applications, it has proven to be very useful in diagnosis of problems and monitoring of training. The operation of the program as well as clinical data are presented.

Meridional amblyopia in monkeys.

Contrast sensitivity as a function of the orientation of a grating stimulus was determined by behavioral methods for four rhesus monkeys. Two of the monkeys had been reared with normal binocular experience, had spherical refractive errors, and showed a normal oblique effect. The other two monkeys which had been reared with one eyelid sutured (which was still sutured during these experiments), showed astigmatic refractive errors in the non-deprived eye and grating contrast sensitivity as a function of orientation that was correlated with the principal meridians of their astigmatism. Control experiments showed that the meridional amblyopia was not due to an uncorrected refractive error. Reaction time measures of contrast sensitivity for suprathreshold grating patterns showed that meridional amblyopia was not present for high contrast gratings.

Rigid lens base curve stability upon hydrogen peroxide disinfection.

Because of the possibility of transmitting communicable diseases, in particular the HIV virus, it has been recommended that all diagnostic contact lenses, including rigid lenses, be disinfected after each use. Hydrogen peroxide is a recommended disinfection agent, but its effect on rigid lens polymers is relatively unknown. We soaked 50 lenses of 5 different polymers in 3% hydrogen peroxide for 10 min and measured the base curves to determine if any changes occurred. Our results showed no statistically significant change in base curve for the lenses measured, but 22% of the lenses did exhibit a small amount of warpage (mean 0.0382 mm) of the base curve. We were not able to determine what caused these lenses, and not all the lenses, to warp. Under the conditions of this study, hydrogen peroxide does not appear to cause clinically significant parameter changes, but lenses should be checked for warpage before use.

Deswelling of the cornea after hypoxia.

Central corneal thickness changes were monitored in nine subjects who wore low water content hydrogel lenses of varying thickness for 3 h in the open and closed eye conditions. At the end of 3 h of lens wear, corneal deswelling to prelens thickness for the different levels of hypoxia was determined by pachometry performed every 5 min for the first 30 min, every 10 min for the next 40 min, and every 15 min thereafter. Examination of the individual deswelling profiles indicates (1) a biphasic response in those with corneal edema greater than 14%, and (2) a large portion of the deswelling occurred during the first 30 min, followed by a more gradual rate until baseline was reached. Small amounts of corneal edema (less than 5%) required up to 25 min to return to baseline, whereas 6 to 15% required 30 to 70 min, and 20% required 95 min. The time taken for the cornea to deswell to baseline was linearly related to the amount of corneal swelling (r = 0.88; p less than 0.0001).