Spatial load factor in prediction of reading performance.

Abstract This study investigated whether there is a relationship between reading age and clinical optometric tests that have varying degrees of spatial loading in their design. Spatial loading in this context is the demand on the visual system to process information about the relative position and orientation of stimuli. A total of 112 children aged 8-11 years were assessed using saccadic eye movement and rapid naming tasks with varying spatial loads. All were subtests of Garzia's Developmental Eye Movement test and Liubinas' SeeRite Reading Diagnostic Programme. Variability in load was achieved by comparing rapid naming of numerals vs the spatially loaded letters p, d, b, q; and by comparing the speed of reading numerals presented in increasingly complex arrays. Reading Age was assessed independently and results were analysed by multiple logistic regression. Spatially loaded naming tasks performed at speed exposed a Spatial Loading Factor which clearly differentiates children at risk with reading.

The effect of age on the edema response of the central and mid-peripheral cornea.

Corneal edema was induced by a 2-h closed-eye wear of a thick hydrogel contact lens. Corneal edema production and edema recovery responses were measured in the central and mid-peripheral (3 mm inferior) cornea of 10 younger (20.4 +/- 2.0 years) and 10 older (69.0 +/- 3.4 years) subjects using optical pachometry. The amount of edema produced was not significantly different between the two corneal locations nor between the two age groups. The edema recovery rates of the central and mid-peripheral cornea were not significantly different in either the younger or older group. The edema recovery rates at both corneal positions were significantly faster in the younger group than in the older group. The results of this study suggest that the mechanisms that control edema production and recovery in the inferior mid-peripheral cornea are similar to those previously described in central and temporal mid-peripheral locations. Ageing appears to influence the edema production and recovery mechanisms of both the central and mid-peripheral cornea in similar ways.

Factors influencing normal perimetric thresholds obtained using the Humphrey Field Analyzer.

The relationships between dioptric blur, pupil size, retinal eccentricity, and retinal sensitivity were investigated in the central 5 degrees of the visual field in 10 normal subjects using the Humphrey Field Analyzer. Pupil size did not influence the foveal sensitivity or retinal profile in the unblurred condition. The slope of the retinal profile was significantly steeper in the 3 mm pupil size condition (-0.62 dB/degree) than when compared to the 8 mm pupil size condition (-0.34 dB/degree), when averaged over all dioptric blur conditions. The depth of focus for the 3 mm pupil size condition (3.86 diopters) was significantly greater than that found for the 8 mm pupil size condition (1.82 D). The retinal threshold doubling eccentricity (E2) was calculated to be similar to that of grating acuity and contrast sensitivity (3.71). The data suggest that while large depth of focus effects in small pupil sizes appear to reduce the need for accurate refractive error corrections in determining perimetric retinal sensitivities, variations in the slope of the retinal profile under conditions of uncontrolled dioptric blur and pupil size may result in the artifactual sensitivity decreases. Therefore, it is recommended that measurement of pupil size and accurate correction of near refractive errors be performed to minimize the possibility of incorrect detection of central visual field defects.

Altitudinal hemianopsia: a case report and mini-review.

Altitudinal hemianopsia is a relatively rare visual field defect. A case report is presented describing a patient suffering from sudden-onset traumatic altitudinal hemianopsia. Other conditions which may produce sudden or gradual onset altitudinal hemianopsias are discussed. The importance of the optometrist being aware of the possibility of an altitudinal visual field defect in conditions of trauma to the face after motor vehicle accidents is stressed.

Problem of between-eye correlation for statistical hypothesis testing: rabbit corneal thickness.

The two eyes of a subject often yield correlated data. Statistical analysis which treats correlated data as if it were independent is most likely to be biased toward statistical significance; that is, the probability of a type I error is likely to be inflated. To illustrate the importance of lack of independence to the inferential process, data from an experimental design commonly used in optometric research are used to demonstrate (1) the potential magnitude of between-eye correlation, (2) the statistical bias toward a significant outcome when the between-eye correlation is ignored via inappropriate analysis, and (3) simple ways by which the bias can be avoided. The researcher must be aware of the between-eye correlation which exists for the particular effect under study, and the statistical bias that ensues from the correlation when the data are not handled correctly.

Corneal hydration control in normal and alloxan-induced diabetic rabbits.

Reports of increased corneal thickness and altered endothelial morphology suggest that there is abnormal corneal hydration control in diabetic patients. To study the possible influence of hyperglycemia on corneal hydration control, experiments were done on normal and alloxan-induced diabetic rabbits to assess: (1) stromal dry weight, hydration, and swelling pressure; (2) corneal thickness and contact lens-induced edema recovery responses; and (3) endothelial homogenate sodium/potassium adenosinetriphosphatase (Na+/K+ ATPase) activity. The data show that 10 weeks of uncontrolled hyperglycemia in the rabbit results in abnormal corneal hydration control indicated by increased corneal thickness, increased stromal hydration, and a decreased ability to recover from contact lens-induced corneal edema. The stroma appears to be minimally involved in these changes; swelling pressures and dry weights of the normal and diabetic stroma were not significantly different. The measured decrease in diabetic rabbit endothelial homogenate Na+/K+ ATPase activity strongly suggests that endothelial fluid pump dysfunction is a major component in the abnormal corneal hydration control found in the uncontrolled diabetic rabbit.

Corneal edema recovery dynamics in the rabbit. A useful model?

Open-eye and closed-eye recovery from contact lens-induced corneal edema was measured on the right eyes of 10 New Zealand White rabbits using ultrasound pachometry. Edema was produced by either 2 hours of eye closure (Patch Test) or 2 hours wearing of a thick hydrogel contact lens over the closed eye (Lens Test). The measured corneal edema recovery data were analyzed by both linear-regression analysis and nonlinear-regression analysis based on a previously published exponential open-eye edema recovery model. Linear-regression analysis revealed that the initial rabbit open-eye edema recovery rate of 36.3 +/- 11.9 (standard deviation) micron/hr was not significantly different (P greater than 0.25) from the initial young human open-eye edema recovery rate of 35.6 +/- 3.4 micron/hr. Similarly, the initial rabbit closed-eye edema recovery rate of 14.3 +/- 5.4 micron/hr was found to be not significantly different (P greater than 0.25) from the initial young human closed-eye edema recovery rate of 15.0 +/- 2.2 microns/hr. Corneal edema recovery indices derived using nonlinear-regression analysis also had a strong similarity between rabbit and human corneal edema recovery rates. For example, the calculated percent recovery per hour (PRPH) for rabbit open-eye corneal edema recovery of 41.4 +/- 11.3%/hr agreed well with the reported human open-eye PRPH of 34.2-58.9%/hr. The rabbit closed-eye PRPH of 25.6 +/- 10.7%/hr was also found to lie between the calculated human closed-eye PRPH values of 19.9-30.2%/hr.(ABSTRACT TRUNCATED AT 250 WORDS)

Physical and psychophysical properties of a liquid crystal diffuser.

A number of optical and psychophysical properties of a new liquid crystal diffuser were measured. The data suggest that the liquid crystal window offers an inexpensive and readily adjustable source of diffusive blur, as well as being a useful shutter, luminance filter, and variable contrast attenuator. The wavelength dependency of the scattering produced by the liquid crystal varies with applied voltage. The diffusive blur, overall luminance and contrast reduction, and short wavelength light attenuation produced by the liquid crystal window may provide a useful model for the intraocular scattering associated with cataract.

Recovery from contact lens-induced edema is prolonged in the diabetic rabbit cornea.

The recovery of the cornea from contact lens-induced edema has been proposed as a clinical test for the assessment of corneal hydration control, with prolonged times being seen in aged subjects and in conditions associated with endothelial dysfunction. Because diabetes is known to be associated with altered endothelial morphology, a study was performed to determine the influence of diabetes on recovery times from contact lens-induced edema. Corneal edema was produced in 10 normal and 10 diabetic rabbits by the monocular wear of a thick hydrogel contact lens on a closed eye for a period of 2 h. Corneal thicknesses measured by ultrasound pachometry at the end of the wearing period showed significantly (p less than 0.0001) more edema in the normal (75 +/- 11 microns) than in the diabetic group (56 +/- 11 microns). Open eye and closed eye recovery times from the induced edema were calculated for both groups using linear and nonlinear regression analysis. The calculated edema recovery rates for the diabetic cornea were significantly slower than those of the normal in both the open eye and closed eye deswelling conditions. The results of this study indicate that the cornea of a diabetic rabbit has a decreased ability to recover from contact lens-induced edema, and suggests that the practitioner should remain aware of the blood glucose levels of his poorly controlled diabetic contact lens patients.

Diurnal and long-term variations in corneal thickness in the normal and alloxan-induced diabetic rabbit.

The variation of central corneal thickness with age was measured weekly at 10am by ultrasound pachometry in 10 normal (5 to 19 weeks of age) and 10 alloxan-induced diabetic (9 to 19 weeks of age) rabbits. The diurnal variation of corneal thickness was also measured at 2 hour intervals for a period of 24 hours on both groups of rabbits at 19 weeks of age. A nonlinear increase in corneal thickness with age was noted in both groups, with the diabetic cornea becoming significantly thicker (p less than 0.05) than the normal control after 7 weeks of hyperglycemia. At 19 weeks of age, the diabetic rabbit cornea (379 +/- 18um) was 5.2% thicker than the normal control (360 +/- 10um). Both normal (26um) and diabetic (26um) rabbits demonstrated a similar sinusoidal diurnal variation of corneal thickness, with the cornea being thinnest in both groups near midnight. A negative exponential function derived using nonlinear regression analysis was found to adequately describe the age data, and could be used to provide a number of possibly useful clinical indices of corneal thickness variation with age. The data from this study demonstrate that 1) the variation of rabbit corneal thickness with age can be modelled using a negative exponential function, 2) alloxan-induced hyperglycemia is associated with an increase in rabbit corneal thickness, 3) alloxan-induced hyperglycemia does not appear to influence the diurnal variation of rabbit corneal thickness and 4) care should be taken in the interpretation of in vivo corneal thickness studies using normal rabbits of under 12 weeks of age (2kg body weight).

Contrast sensitivity for letter and grating targets under various stimulus conditions.

Measurement of visual acuity for letters of different contrasts has been suggested as a clinical way to evaluate contrast sensitivity in patients with vision abnormalities. If variable-contrast letter acuity provides information similar to the contrast sensitivity function (CSF), then comparable effects should be seen in stimulus manipulations which simulate decreased vision. Using both our own and published data, we compared the effects of diffusive blur, dioptric blur, and eccentric viewing on contrast sensitivity for letter and grating targets. A diffuser placed close to the eye reduces contrast sensitivity fairly evenly across all spatial frequencies, with similar results for letters and gratings. However, dioptric blur reduces sensitivity substantially more to letters than to comparably fine gratings. Eccentric viewing also produces a larger sensitivity loss for letters than for gratings. Because some stimulus manipulations produce dissimilar changes in contrast sensitivity for letters and gratings, it is questionable whether the results of one measure can be used to draw inferences about the other. It is proposed that local or relative phase discrimination has an important role in explaining the different responses to letter and grating targets.

A review of manifestations of diabetes mellitus in the anterior eye and cornea.

Diabetes mellitus can lead to blindness through its effects on the retina and lens, yet diabetes is a systemic disease influencing the entire eye. The current interest in extended wear contact lenses emphasizes the need for the practitioners to have a thorough knowledge of the pathophysiology of the anterior eye and cornea of the diabetic. The current literature is reviewed, and its relevance to contact lens wear is discussed.