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Biomedical subjects

J M Enoch

Publications and source records attributed to J M Enoch.

At least 37 records · Page 2Linked to original sources
PubMed1995

Vernier acuity and aging.

PURPOSE/BACKGROUND: Many visual functions change with age. Hyperacuity vernier thresholds, however, seem to remain practically invariant with age. METHODS: We critically review reports on age-related changes in hyperacuity thresholds and present new data using two and three-point vernier hyperacuity thresholds. RESULTS: It appears as though the vernier (two dot or two line) alignment hyperacuity thresholds are not sensitive to age. SIGNIFICANCE: Based on these studies we propose that hyperacuity (vernier) thresholds be used as a benchmark, or a gold standard for aging studies. This measurement can serve as a valuable control and reference for other clinical studies of aging.

Adult↗
PubMed1995

Stability of the Stiles-Crawford function in a unilateral amblyopic subject over a 38-year period: a case study.

The relative luminous efficiency curves for light entering different parts of the entrance pupil [Stiles-Crawford functions (SCFs)] of subject TG, whose right eye is amblyopic, were determined earlier for both the normal and the amblyopic eye. The left eye was found to have a normal SCF, varying from the center to the edge of the entrance pupil by an entire log unit. However, the SCF for the right eye was found to be abnormal, showing peak sensitivity far toward the temporal edge of the pupil and varying from center to edge by only one-third of a log unit. From these data it was inferred that the photoreceptors were in poor alignment. In October, 1993 the subject experienced a retinal detachment in the left (nonamblyopic) eye, requiring cryoretinopexy, drainage of subretinal fluid, and a scleral buckle. Postoperatively, visual acuity recovered to 6/9 to 6/7.5 (20/30 to 20/25), but vision was poor when illumination or contrast was low. In order to determine whether the decrement in visual performance was the result of misalignment of the photoreceptors, the subject's SCFs were determined 9 months postoperatively for both eyes, using a procedure similar to that used earlier. The relative luminous efficiency curves for both eyes were found to be surprisingly similar to the corresponding curves found 38 years earlier. It was concluded that any decrease in visual function resulting from the retinal detachment and repair was not caused by misalignment of the photoreceptors. We infer that the mechanisms resulting in the exceptionally high degree of photoreceptor alignment in the left eye and the disturbed alignment in the right eye are still present.

Aged↗
PubMed1995

Hyperacuity test to evaluate vision through dense cataracts; research preliminary to a clinical study. I. Studies conducted at the University of California at Berkeley before travel to India.

BACKGROUND. Patients with dense ocular media disorders retain the ability to project or point to an intense source of light. Using this response capability and high luminance points of light as stimuli, Vernier judgments (a hyperacuity test) can be made by these patients, even without the presence of a "window" through a leucoma, cataract, or bleed. Without coaching, these individuals are able to locate the centers of the individual degraded point images if the individual light sources are adequately separated (i.e., if sufficient "gaps" exist between the individual stimuli), and they can spatially align the degraded images. Advanced cataracts are the main cause of blindness in the developing world, and this is a treatable condition. In these nations, only a modest proportion of affected patients receive surgery, and only 5% or less of these individuals obtain treatment in two eyes. There are incredibly large and rapidly growing backlogs of advanced cataract patients requiring care (many millions). Because of the 20 to 30% failure rates that occur after treatment (all causes) in many developing world settings, a test performed before surgery, which offers a meaningful estimate of postsurgical visual outcome, can be valuable. Using the principle defined above, we seek to determine before surgery those individuals who will derive most benefit from cataract removal, and which of two cataractous eyes has the better postsurgical visual prognosis. EXPERIMENTAL. In Berkeley, we performed a series of preliminary studies on a Vernier acuity test before initiating a clinical study in a developing world setting. These studies were conducted upon young adult normal subjects wearing their usual vision corrections, with and without induced refractive errors, and/or with or without simulated dense nuclear cataracts. We sought (1) to determine the number of repeat trials necessary for reliable outcomes; (2) to compare a two-point and a three-point Vernier acuity display; (3) to determine the shape of the measured response function at large gap separations between test points; (4) to define optimal test distance and stimulus size; (5) to assess the effect(s) of a broad range of uncorrected refractive errors upon outcomes; and (6) to consider means to minimize refraction-based errors by using a pinhole, a refractive correction, and/or selective spatial filtering. We compared responses obtained using the current CRT/VDT-based, computer-driven (Berkeley) instrument with a new precision optical/mechanical computer-driven (India) instrument. The India instrument is needed to determine design parameters for a next stage simpler, cheaper, more rugged field instrument(s).(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1995

Hyperacuity test to evaluate vision through dense cataracts; research preliminary to a clinical study. II. Initial trials of the India instrument and HASP protocol at Aravind Eye Hospital, Madurai, India.

BACKGROUND. Our purpose is to develop a clinical test capable of assessing vision through dense cataracts (and other advanced media opacities) before surgery. Because such anomalies are endemic in the developing world, it was desirable to conduct a clinical study of a prototype India instrument and HyperAcuity Study Protocol (HASP) in such a setting. This investigation, preliminary to a planned clinical study, was conducted at the Aravind (Free) Eye Hospital, Madurai, India. EXPERIMENTAL. The preliminary HASP protocol, developed in Berkeley, was adapted to existing clinical practices in Madurai. Included was a new test of visual acuity, the Gap "VA" test, developed as part of the revised protocol. The rational for this test, methods used, and sample data are presented. A Vernier alignment test (one of the hyperacuities) is used to assess vision through dense ocular media disorders. The task of the patient is to align vertically three high luminance, discrete visual stimuli. The revised HASP protocol and adapted India instrument were tested upon patients with advanced cataracts. The cataract grading system used in these studies (provided by Drs. N. V. Projna and G. Rohini) is defined. Patients were tested before and shortly after surgery. This investigation allowed us to refine the test protocol and instrument design preparatory for a clinical study of HASP. A much simpler/cheaper version of the India instrument will be used in the planned clinical study. Additional experiments are scheduled in order to address issues which require resolution before initiation of next-stage testing. In part, in these papers, we seek to help others understand some problems encountered when conducting research in a developing world environment, even a very good one!

Aged↗
PubMed1995

Performance on three-point vernier acuity targets as a function of age.

With certain (but not all) vernier acuity (VeA) stimulus displays observers exhibit no or slight performance decrements with aging. Because three-point VeA test displays are much more easily applied experimentally and clinically, we examined visual performance on this task as a function of age. Ages of groups tested ranged from 20 to 70+ years (some individuals were 90+ years old). Eyes were examined monocularly. We tested three- and two-point sharply defined VeA arrays and a three-point ground-glass-degraded display (simulating a 20/100 nuclear cataract). Gap or feature separations were varied. Results obtained with the three-point display showed little alteration with age. Image degradation resulted in reduction in performance when small gap separations were used between the test points, but only slight decrements for large gap separations were recorded. The finding that performance with certain VeA targets is largely unaffected by age is confirmed and is expanded.

Adult↗
PubMed1995

Aniseikonia, metamorphopsia and perceived entoptic pattern: some effects of a macular epiretinal membrane, and the subsequent spontaneous separation of the membrane.

Following cryo surgery for retinal hole repair, visual effects in the senior author's eyes were caused by an epiretinal membrane. Subjectively, metamorphopsia was noted, and entoptically perceived radial striae were observed centred near fixation. After subsequent cataract surgery, metamorphopsia was not detectable. Several months later, the centre of the striate entoptic pattern was observed to be centered several degrees from fixation. Concurrently, loss of contrast in the central visual field was noted. In time, both the entoptic pattern and the contrast effects became less visible (contrast improved much faster). The epiretinal membrane had spontaneously separated from the fovea. Repeated measurements of aniseikonia were obtained before cataract surgery, and after cataract surgery both prior to, and after the separation of the epiretinal membrane. Inferences are drawn.

Aniseikonia↗
PubMed1994

Photoreceptor orientation in iris coloboma.

PURPOSE: Investigate photoreceptor orientation and alignment in an observer with a simple coloboma of the iris and no retinal involvement. METHOD: Using a psychophysical procedure, extensive measurements of the Stiles-Crawford functions of the first kind (SCE I) were obtained from this observer. RESULTS: In this observer, the usual center-of-the-exit-pupil-of-the-eye alignment tendency of photoreceptors (inferred from the peaks of the SCE I functions) is absent. The inferred orientation of photoreceptors at numerous retinal loci tested lies outside the pupil, opposite the direction of the downwardly displaced pupil center. CONCLUSIONS: These results add support to previous suggestions that nonphototropic forces such as retinal tractional forces should be considered as an integral part of the mechanism(s) subserving human photoreceptor alignment in both normal and anomalous observers.

Coloboma↗
PubMed1993

Residual aniseikonia among patients fitted with one or two intraocular lenses (pseudophakic corrections)

In the presence of one or two intraocular lenses (IOL's; pseudophakic corrections) or in the presence of naturally occurring or developmental anisometropia (due to a tumescent cataract) some induced or residual refractive aniseikonia is usually encountered. We wish to call this problem to the attention of the optometric practitioner. We provide a discussion of the resultant aniseikonia and simple rules of thumb for management of such patients with refractive aniseikonia. Aniseikonia in these patients may be less important than once had been suggested, but certainly it is more significant than it is currently being considered. Use of personal computers to calculate this aniseikonia makes appropriate correction relatively easy and readily applicable.

Aniseikonia↗
PubMed1993

Cataract: a critical problem in the developing world.

Cataract blindness is a major public health problem worldwide. Unlike other leading causes of blindness, cataract can be treated effectively with surgery. Demographic, social, and economic factors are important determinants of surgical treatment of this disorder in the developing world, resulting in the accumulation of very large numbers of unoperated cases. Because of many factors, the problem is really out of hand. In this paper, we try to raise awareness among optometrists of this major challenge. At the same time, because many individuals do not achieve good vision after surgery and because in most developing countries only one eye is operated upon, we emphasize the use of improved prognostic techniques to make the surgical outcome more predictable, avoiding nonproductive surgery and resultant disappointment on the part of all concerned. We feel optometry can play important roles in diagnosis, visual correction, and management of these patients, hopefully in cooperation with ophthalmic surgeons.

Blindness↗
PubMed1993

Optics of phakic, pseudophakic, and aphakic eyes: effects on the Stiles-Crawford (SCE I) function.

In many experimental situations one has to work with suitable optical corrections for ocular elements which have been eliminated (lens in aphakia, for example). Analysis of the optics of the total system (eye plus correction and other optical elements) shows substantial changes in the positions of the cardinal points. These changes are meaningful and must be taken into account in subsequent analysis. We discuss the pertinent optical considerations and show as an example the results of measurements of the Stiles-Crawford effect of the first kind in an aphakic subject with spectacle and contact lens correction.

Aphakia↗
PubMed1992

Variation of the hyperacuity gap function with age.

Responses to hyperacuity stimuli are processed in quite a different manner by the central nervous system than many other stimuli such as resolution targets or increment threshold stimuli. With increased attention being paid to visual response in aging individuals, it is important to study how these response paradigms are affected by age. In this analysis, otherwise normal observers 20 to 85 years of age were studied. The hyperacuity gap test was used. As defined, this is a vernier test using a two-point test array. This particular format was chosen because the resultant data are less distorted by image blur, scatter, and astigmatism than other designs we have studied. The gap or feature separation between the two small targets (which were aligned vertically) was increased logarithmically using octave steps and the hyperacuity threshold (setting variance) was measured for each step. No systematic variation of vernier hyperacuity threshold was found with increasing age within the range of ages or gaps tested. This is important because it can serve as a valuable control and reference for other clinical studies of aging.

Adult↗
PubMed1992

Everett Kinsey Lecture. Hyperacuity, a technique for assessment of results of refractive surgery.

In recent years there have been rapid advancements in corneal refractive surgery. The results of keratorefractive procedures are generally not predictable and wound healing effects interfere with the stability. The problems with predictability and stability make the usual visual performance indices such as visual acuity and contrast sensitivity testing insufficiently refined for functional assessment of visual performance after keratorefractive surgery. We argue that there is a need for precise visual functional analysis that parallels and complements the advanced technologies employed in ophthalmic surgery. Results from hyperacuity test paradigms are put forth as one set of precise, reliable bench marks for characterizing the results of refractive surgery. Hyperacuity testing is well suited for analysis of visual performance after keratorefractive surgery because the procedure is resistant to optical image degradation and requires a higher-order analysis by the visual system. The uses of hyperacuity techniques are discussed.

Humans↗
PubMed1991

Anomalous kinetic visual fields found in family members of patients with a confirmed diagnosis of Gilles de la Tourette syndrome.

Three visual manifestations of Gilles de la Tourette syndrome (TS), a neuropsychiatric disorder, have been previously reported. In this paper we report anomalies in the measured kinetic paracentral visual field of TS patients and their family members. We found that 96% of TS patients exhibited paracentral kinetic visual field defects (nasal and temporal steps, enlargement, "ringing," and/or baring of the blind spot). Ninety-three percent of fathers of TS children and, surprisingly, about 80% of mothers exhibit these characteristic visual field anomalies. Sample data and examples of family studies are reported.

Adolescent↗
PubMed1991

Quantification of metamorphopsia using hyperacuity techniques.

We have developed a new technique for quantification of metamorphopsia using a simplified and modified Amsler grid. Furthermore, the technique used is a bisection task, a form of hyperacuity determination. This allows for very fine assessments of metamorphopsia even in the presence of cataracts.

Aged↗
PubMed1990

Effect of bleaching on the width and index of refraction of goldfish rod and cone outer segment fragments.

Data on goldfish rod and cone fragments were obtained before and after a strong bleach by using a Zeiss Jamin-Lebedeff infrared interference microscope and computer image processing techniques. The receptor fragments were fractured in the inner segment and were immersed in goldfish aqueous humor medium. On bleaching we found that: (1) there is a small increase in rod diameter. This effect is of the same magnitude reported earlier in Rana pipiens outer segments (Vision Res 1973; 13:171). (2) There is an inferred decrease in rod refractive index. (3) There is a decrease in cone width. (4) There is a slight increase in inferred cone refractive index. These data are presented.

Animals↗
PubMed1990

Dopamine and myopia.

Dopamine may play a role in form-deprivation myopia in chickens. Anomalies of the dopaminergic system are believed to be responsible for manifestations of symptoms in Tourette Syndrome (TS), a neuropsychiatric disorder. Visual anomalies have been identified in TS. In this paper, refractive error in people suffering from TS is compared with available population studies. Refractive errors in TS are found to be similar to those in the normal population.

Adult↗