Angela C. Kothe

Crowding and Contrast in Amblyopia

The crowding effect, defined as the ratio of visual acuities for letters presented in Snellen (i.e., line) format and isolated-letter format, was measured in the amblyopic eyes of 15 children and 15 adults with unilateral amblyopia. Normal limits were based on data from 20 children and 20 adults with no history of amblyopia. The crowding effect was compared for letters of high (96%) contrast and for letters of low (11%) contrast. We report that amblyopia can differentially affect line and isolated-letter acuity for both high- and low-contrast letters. For the patients that show an abnormal crowding effect, this effect can be: (1) significantly stronger, (2) significantly weaker, or (3) not significantly different for high- than for low-contrast letters. These findings are consistent with the hypothesis that the crowding effect is contrast-dependent in some amblyopic eyes of both child and adult unilateral amblyopes

Method for identifying amblyopes whose reduced line acuity is caused by defective selection and/or control of gaze

Three visual tests were administered to a group of 15 amblyopic children. 15 adult amblyopes and two age‐matched control groups, each of 20 subjects. Test results comprised visual acuity for recognizing high contrast letters presented in line (i.e. Snellen) Formal, isolated‐letter format and repeal‐letter format. The classical Snellen format confounds the effects of gaze control defects with the effects of adjacent contours on a patients ability to recognize a foveated letter. We designed a repeat‐letter format intended to unconfound these effects. The repeal letter formal is much less sensitive to gaze control defects, and somewhat more sensitive to adjacent contour interactions than is the Snellen format. We report that amblyopic eyes can he subdivided empirically into three repeat‐letter categories: repeat‐letter acuity significantly better than Snellen acuity; repeat letter acuity not significantly, different from Snellen acuity: and repeal letter acuity significantly worse than Snellen acuity. We report that this subdivision cuts across the clinical subclassification of amblyopic and also across the crowding no crowding subclassification. We suggest that, rather than abnormal lateral interactions, defective selection and or control of gaze is an important factor in depressed visual acuity in amblyopic eyes of the first repeal‐letter category but not for the third type, in which abnormal lateral interactions may be important. To test the hypothesis that the response to patching and refractive therapy may be less satisfactory in our first category of amblyopic eyes, we are carrying out a prospective study. If the hypothesis is confirmed, it might then he worthwhile to identify these patients at an early stage and investigate whether therapies designed to improve the control of gaze might be more effective in this group than is patching or refractive therapy.

Physical characteristics and perceptual effects of blue-blocking lenses

The transmission-optical properties of a “blue-blocking” lens and its influence on several aspects of human visual performance were assessed. Results showed that the lens was effective in absorbing ultraviolet and blue wavelengths and that its effects on contrast sensitivity and visual evoked potentials (VEPs) were similar to those produced by an equivalent neutral density (ND) filter. Although the lens did not alter stereopsis, it did produce severe color discrimination losses for normal and dichromatic subjects.

Crowding depends on contrast

Previous studies of visual “crowding” in children have been restricted to the use of high contrast optotypes. In this study, visual acuity was measured in a group of 30 normally sighted children using high (96%), medium (11%), and low (4%) contrast Snellen charts and isolated letter cards. The crowding effect was found to be less for low contrast letters than for high contrast letters.

Comparison of noninvasive methods to derive the mean central retinal artery pressure in man.

Purpose. The pressure within the ophthalmic artery can be estimated by several noninvasive procedures based on measurements of the pressure either within the central retinal artery (CRA) or the brachial artery. In this study we compared 5 methods of deriving the mean pressure within the ophthalmic artery in 10 healthy volunteers 21 to 31 years of age. Methods. The pressure within the ophthalmic artery was calculated from estimates of the systolic and diastolic pressures within the CRA derived by suction ophthalmodynamometry (s- ODM), compression ophthalmodynamometry (c-ODM), interpolation from scleral compression/intraocular pressure (IOP) conversion tables, and measurements of the brachial blood pressure (BP). Results. Group average CRA pressure values varied significantly across techniques, with the largest difference among methods being about 15 mm Hg. CRA pressures derived by s-ODM or c-ODM and direct measurements of the IOP yielded statistically identical values. These latter values were significantly lower than CRA pressures estimated by either scleral compression/lOP conversion tables, or those predicted from brachial BP measurements with the arm held up alongside the head, both of which produced equivalent values. The highest estimates of CRA pressures were obtained when brachial BP values were derived with the arm in its normal anatomical position. Conclusion. The patency of the vascular network to the eye and subsequent perfusion of intraocular neural tissue essential to normal visual function can be evaluated by simple clinical procedures. Although all techniques to estimate the pressure in the ophthalmic artery are relatively simple to use, they do not all yield the same absolute values and consequently should be interpreted in this light when used for either clinical or research purposes.

The component of gaze selection/control in the development of visual acuity in children.

Visual acuity was tested for 180 eyes of 90 children in four age groups using three types of test charts. Subjects read the same 10 high-contrast letters in Snellen (line) format, as isolated-letter flash cards, and as repeat-letter flash cards. Group mean line and group mean isolated-letter acuity showed similar progressive improvements with age. A subgroup of 24 of 50 eyes of 4- to 5-year-olds (15 of 25 subjects) and 3 of 50 eyes of 6- to 7-year-old (2 of 25 subjects) had low Snellen acuity. Of this low-acuity subgroup of 27 eyes, 10 scored above average for their age group on the repeat-letter chart. We concluded that abnormal lateral interactions were not the explanation for the immaturity of Snellen acuity in these 10 eyes. We suggest that an important factor in the low acuities of these 10 eyes is delayed development of the selection and/or control of gaze direction. Some eyes with excellent Snellen acuity showed high crowding. For example, there were five such eyes in the oldest group. We suggest that the excellent acuities of at least two of these eyes are limited by minor inaccuracies in gaze selection and/or control rather than by lateral interaction.

Variability in clinically measured photopic oscillatory potentials

Oscillatory potentials found on the ascending phase of the electroretinogram b-wave probably originate in some element(s) of the inner plexiform layer. As oscillatory potentials are particularly sensitive to changes in retinal, and possibly choroidal, blood flow, they have been used extensively to provide clinical measures of the degree of retinal ischemia during the progression of diabetic retinopathy. Recent studies in our laboratories have disclosed previously unreported significant variability in the photopic oscillatory potentials on repeated measures even in tightly controlled conditions. The amplitude of five recordable light-adapted wavelets exhibited considerable intra- and inter-subject variability. Until further investigation can determine factors affecting standardization of testing, it appears that changes in oscillatory potential implicit times rather than in amplitudes are a better measurement in clinical neurophysiology.

Improving the diagnostic power of electroretinography by transient alteration of the ocular perfusion pressure.

In this report we present the results of a series of studies focusing on the effects of transient changes in the ocular perfusion pressure (OPP) on retinal function in normals as assessed by the flash electroretinogram (ERG). A transient increase or decrease in the OPP affected by body inversion and compression/suction ophthalmodynamometry (ODM), respectively, is shown to affect differentially the b-wave of scotopic and photopic ERGs. However, under dark-adapted conditions, the cone component of the red flash ERG b-wave exhibited a vulnerability to decreased OPP which approached that seen for the b-wave of the scotopic blue flash ERG b-wave. Similar test procedures used to investigate the functional response of the inner plexiform layer during altered OPP revealed component-specific changes in white flash scotopic oscillatory potentials (OPs). The results of these provocative tests of retinal function offer new insights into basic retinal physiology and encouraging prospects for practical clinical diagnostic procedures with enhanced sensitivity and specificity for subclinical retinal disorders.

VARIATION OF DARK FOCUS OF ACCOMMODATION WITH LASER SPECKLE EXPOSURE DURATION

Abstract Dark focus of accommodation (DFA) was measured in 10 subjects using a computer‐aided He‐Ne Badal laser optometer having high temporal and amplitude resolution. Accommodative responses were tracked during 5 min sessions for 10 speckle exposure durations ranging from 100 to 1000 ms. DFA‐speckle exposure functions fell into four distinct groups. This identifies the laser speckle exposure time as a possible contaminant in measures of DFA made by laser optometers if individual variability across exposure times is ignored. Group averaging of accommodative responses for laser exposure times obfuscates the reported individual variability.

Influence of retinal vascular perfusion pressure on macular photostress recovery time

The photostress recovery time (PSRT) was used as an index of macular function under conditions of altered retinal vascular perfusion pressure (RVPP) in 10 healthy human observers. RVPP was decreased by scleral indentation and increased by body inversion. All 10 subjects showed prolonged PSRTs with decreased RVPP. The change in PSRT, associated with increased RVPP, varied among subjects allowing a sub‐classification into two groups. Group 1 (n = 4) showed significant increases in PSRT whereas Group 2 (n = 6) showed no significant change in PSRT. This group difference in PSRT could not be related to test procedure‐induced changes in vascular dynamics, such as IOP, or diastolic and systolic brachial and ophthalmic artery pressures. Three possible interpretations are presented to account for the significant increase in PSRT demonstrated by some individuals. Results from this preliminary study suggest that the assessment of a subjects PSRT, subsequent to short‐term alterations in RVPP, may provide a non‐invasive provocative test for the detection of systemic vascular disease.