Reading rates with artificial central scotomata with and without spatial remapping of print.

People with central field defects resulting from age-related macular degeneration (ARMD) read very slowly. In this study, oral reading rates were determined for unrelated sequences of words in samples of normal young and old subjects with simulated central scotomata of 2 degrees, 4 degrees, and 8 degrees. Scotomata were stabilized at the fovea of the right eye by electronic feedback of eye position, monitored using a SRI dual-Purkinje Eyetracker. Reading rates were determined by jumping print after each stationary presentation through an increasing number of character spaces on different trials. This procedure mimicked the sequence of retinal images produced during the saccades and fixations of normal reading, but without requiring subjects to make accurate eye movements. In Experiment 1, the letter size that yielded the optimal reading rate was found to increase systematically with scotoma size. However, the optimal reading rate decreased more or less linearly as the scotoma size increased. Experiment 2 showed that the optimal reading rate was obtained for essentially the same duration of text presentation, regardless of scotoma size. Experiment 3 investigated the effect of spatial remapping, in which print obscured by the scotoma was stretched electronically to reappear at the scotoma margin. Compared to a nonremapped control condition, spatial remapping produced small but significant increases in reading rate for both 4 degrees and 8 degrees scotomata. Across experiments, average reading rates were faster for the young than the old subjects. Overall, the results define how reading rate is expected to decrease for central scotomata of different sizes and suggest that spatial remapping of print may improve reading rates in patients with ARMD.

Interrelations between measures of visual acuity and parameters of eye movement in congenital nystagmus.

The authors assessed relationships between visual acuity and the amplitude, frequency, intensity, and duration of foveation periods in a retrospective study of 32 patients. Twenty-four patients had congenital idiopathic nystagmus, and eight patients had nystagmus and albinism. Visual acuity was determined for Landolt ring optotypes and, as the extrapolated high-frequency cutoff of the contrast sensitivity function, for horizontal and vertical gratings. No significant correlation existed between acuity and any measured eye movement parameter; however, optotype acuity was related to the magnitude of astigmatic refractive correction, both in patients with idiopathic nystagmus and in albinos. In a subgroup of patients with idiopathic nystagmus whose astigmatic refractive error was -1.50 D or less, nystagmus intensity (amplitude x frequency) correlated significantly with acuity for optotypes (r = 0.71), but not for gratings. Although resolution for vertical gratings was correlated with astigmatic refractive correction, the ratio of resolution for gratings parallel and orthogonal to the meridian of nystagmus was not. Thus, the belief that poorer acuity in patients with substantial astigmatism is attributable to an optically induced meridional amblyopia is supported only partly by these results. The authors concluded that among patients with congenital nystagmus, the influence of eye motion on visual acuity is not readily predicted either from the parameters of nystagmus that they evaluated or from the comparison of resolution for horizontal and vertical gratings.

Image evaluation of ophthalmic devices.

Methods used in the ophthalmic industry to evaluate the optical image quality of correcting lenses have remained the same over many years. This is especially true for single-vision ophthalmic lenses. With the development of new optical designs and materials for correcting refractive errors and presbyopia, new procedures for evaluating lens performance are being explored. One such method is the measurement of the modulation transfer function. Over the last 2 years, we have been measuring modulation transfer functions on a variety of single-vision and bifocal contact lenses, progressive addition spectacle lenses, holographic-diffractive lenses, and intraocular lenses. In this article, the modulation transfer function procedure and its application and results from one study on safety goggles will be presented and discussed.

Grating acuity overestimates Snellen acuity in patients with age-related maculopathy.

In foveally fixating patients resolution acuity is approximately the same when measured with a Snellen chart (Snellen acuity) or when estimated from the high frequency cutoff of the contrast sensitivity function (CSF) (grating acuity). In contrast, with peripheral viewing, normal patients have better grating than Snellen acuity. This suggests that grating acuity deteriorates less rapidly with eccentricity than does Snellen acuity. If patients with bilateral age-related maculopathy (ARM) fixate with a single, healthy peripheral-retinal locus, grating and Snellen acuities should be similar to those in the normal periphery. As expected from the normal data, grating acuity was better than Snellen acuity in all 19 patients with ARM. Although some showed acuities which were similar to those of the normal periphery, others had Snellen acuities that were even worse than predicted from the normal peripheral acuity. Possible explanations for the superiority of grating over Snellen acuity are discussed.

The programmable remapper: clinical applications for patients with field defects.

NASA, Johnson Space Center is developing an electronic image remapper which will warp an image from one coordinate system onto another with great flexibility and speed. The Programmable Remapper will transform images at conventional video frame rate. The Remapper was designed to be used in conjunction with an optical correlator to enhance object recognition through "real time" Fourier analysis. We are investigating an additional potential application for the Remapper as a low-vision aid. In diseases which result in obvious field defects such as age-related maculopathy (ARM) or retinitis pigmentosa (RP), the Remapper can be used to redistribute onto the still-functioning retina the image information that would otherwise be lost due to the associated field defect. Compared with eccentric viewing, this process makes use of the acuity of a larger area of the retina. We envision the future aid to consist of a portable spectacle-mounted display with miniaturized camera input and the Remapper. The patient will view the remapped world on this display. Patients may require training with feedback as to eye and scotoma position in order to use the Remapper most effectively. The Remapper might be reduced in cost, weight, and size to the point of being a feasible low-vision prosthesis as a result of development required by military, space, and industrial utilization. In order to demonstrate how such an aid may work, we have generated static images on an image processor which have undergone radial-only remapping; i.e., image points are slid along radii, with their azimuths unchanged. The remapping process and the application to low vision along with static images are presented in this paper.

Contour interaction function in the preschool child.

Contour interaction was investigated in 12 preschool children 3 to 4 years of age, and compared to the results obtained from 5 normal adults tested under an identical paradigm. Observers viewed the display from a distance at which they could identify the location of a gap (up or down) in an isolated C correctly on 90 to 95% of the trials. The isolated C and C's with bars tangentially located at various positions above and below the test optotype were intermixed randomly. Percent correct was plotted as a function of the angular subtense of the gap width of the test optotype. A significant decrease in performance was found when the bars were positioned at 0.71 to 1.42 times the angular subtense of the gap for both the preschool children and the adults. The results suggest that preschool children demonstrate contour interaction that is quantitatively similar to adults. Because the spacing of letters on standard acuity charts is typically larger than the range over which contour interaction occurs, the poorer acuity often measured with charts compared to isolated letter presentation in preschool children suggests that factors other than contour interaction (perhaps attentional factors) are involved.

Contrast sensitivity. The visual rehabilitation of the patient with macular degeneration.

Contrast sensitivity functions (CSFs) were determined on a large group of patients with macular degeneration to better assess qualitative aspects of their residual vision. Contrast sensitivity was also determined through the telescopic low-vision aids of selected patients. In general, the patients have a substantial loss of contrast sensitivity for all spatial frequencies. The preferred eye of these patients appeared to be related to the peak of the CSF more so than to the Snellen visual or resolution acuity. Through the low-vision aid, not only was there the predictable increase in resolution acuity but also a notable increase in peak contrast sensitivity. These findings indicate that contrast sensitivity testing provides useful information that should be considered in the visual rehabilitation of the patient with macular degeneration.

Suprathreshold contrast perception in functional amblyopia.

Three experiments were conducted to investigate suprathreshold contrast perception of a group of strabismic and anisometropic amblyopes. In the first experiment, simple reaction-time was measured as a function of the contrast of sinusoidal gratings. At all contrast levels, the amblyopic eyes showed prolonged reaction-times compared to the non-amblyopic eyes. In the second experiment, the perceived contrast of suprathreshold sinusoidal gratings was measured using an interocular successive matching paradigm. The third experiment compared perceived contrast of the amblyopic and non-amblyopic eye by contrast magnitude estimation. The results of the matching and magnitude estimation experiments showed that while amblyopic observers show large losses of contrast sensitivity at threshold, at suprathreshold contrast levels perceived contrast is essentially normal or near normal in the amblyopic eye. Taken together, these experiments suggest that: 1) the amblyopic eye has a higher contrast gain than the non-amblyopic eye, and 2) the amblyopic eye shows prolonged response latencies. Similar results can be obtained in normal observers under conditions of peripheral viewing and noise masking.

Contrast sensitivity in albinotic patients.

Vertical and horizontal grating contrast sensitivity functions (CSF) were measured on eight albinotic patients. Several of these patients also had CSF measurements performed through their telescopic aids. For all the patients, the horizontal grating CSF was more sensitive than the vertical grafting CSF. Although there are many pathological problems that could result in this sensitivity difference (high astigmatism, neural modification, etc.), we propose that in this case, the nystagmoid eye movements are the main causative factor. Through the telescopic aid, the CSF resolution acuity and peak sensitivity increased. The increase in resolution was somewhat less than would be predicted based on magnification. The increase in sensitivity is believed to be the result of stimulation of more sensitive peripheral retina. This increase in sensitivity is an added benefit to these patients corrected with low vision aids.

Contrast sensitivity as a function of exposure duration in the amblyopic visual system.

The spatiotemporal differences between the normal and amblyopic visual systems were investigated by measuring contrast sensitivity as a function of exposure duration for specific spatial frequencies of a sine-wave grating. At threshold, normal subjects had contrast-time reciprocity curves with slopes of approximately 0.7, indicating less than perfect temporal summation. The integration time (critical duration) varied as a function of spatial frequency with longer integration times for higher spatial frequencies (in agreement with previous studies). Amblyopic subjects had integration times and reciprocity slopes that were similar to the normals for low spatial frequencies; however, at high spatial frequencies there were significant differences. These results show that an amblyopic eye may either lack sustained neurons or alternately have sustained neurons with abnormal response characteristics.

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.

Space shuttle suit alert system visual acuity problem.

The National Aeronautics and Space Administration (NASA) requested solution of a visual acuity problem that presbyopic astronauts had when viewing an alert system visual display attached to their Space Shuttle suits. Solutions wee analyzed in the context of constraints which prohibited major modifications of the suits. The solution to the problem was to mount Fresnel lens strips on the helmet so that presbyopic astronauts could view the visual display clearly.

Ray tracing through a toric refracting surface.

A procedure has been developed for tracing a ray through a toric refracting surface. The surface is defined in terms of a rectangular coordinate system. The ray is specified by the rectangular coordinates of a point through which it passes and its direction-cosines.