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Biomedical subjects

F Thorn

Publications and source records attributed to F Thorn.

At least 19 recordsLinked to original sources
PubMed1995

A dynamic relationship between myopia and blur-driven accommodation in school-aged children.

Previously we reported that recently myopic children accommodated insufficiently to blur induced by negative lenses. The purpose of the present study was to relate changes in blur-driven accommodation to myopia development in children. Refractive errors and the accommodation response function (ARF) were measured in 23 myopic and 40 emmetropic children on two occasions separated by periods ranging from 6 to 12 months. Repeated measures of accommodation were made with a Canon R-1 autorefractor while negative lenses of increasing power were placed in front of the child's right eye viewing 20/100 letters at 4 m. Concomitant changes in refractive error and in accommodative function over periods of 6-12 months were found to be highly correlated in myopes (r = 0.77) but not in emmetropes (r = 0.09).

Accommodation, Ocular↗
PubMed1995

Shifts in tonic accommodation after near work are related to refractive errors in children.

A link between changes in tonic accommodation (TA) produced by sustained near work and the development of adult-onset myopia has been suggested in studies of young adults. Measures of TA before and after near work have been lacking in children of school age, which is the most susceptible period for the development and progression of juvenile-onset myopia. In the present study accommodation was measured in 87 children, aged 7 to 16 years, before and after 15 minutes of video game playing. All children were refracted before testing and wore optical correction during measures of accommodation with a Canon R1 autorefractor. Most children showed initial values of TA (far focus minus dark focus) between 0.0 and 1.0 D, with a mean of 0.68 D. Grouped by refractive status, the myopic children initially showed 0.30 D of TA, while the emmetropic children showed 0.75 D and the hyperopic children showed 0.94 D. After playing the video game, TA of the myopes increased by 1.15 D, compared to smaller increases for the emmetropes (0.68 D) and hyperopes (0.24 D). Comparable values have been obtained from young adults. These results indicate that the smallest initial values of TA and the largest inward shifts in TA are found during the period of acquisition and progression of myopia, regardless of age.

Accommodation, Ocular↗
PubMed1994

Research and scholarship at the New England College of Optometry.

BACKGROUND: The development of the research and scholarly environment of the New England College of Optometry is recounted. We note three distinct phases in this development during the 1970s, 1980s, and 1990s. Unique interests of the faculty are discussed as well as affiliations with other institutions. An emphasis is placed on the current building of a strong research and scholarship program.

Fellowships and Scholarships↗
PubMed1994

The development of eye alignment, convergence, and sensory binocularity in young infants.

PURPOSE: To measure and compare the development of ocular alignment, sensory binocularity, and convergence in infants during the first 5 months of life. METHODS: Healthy infants were tested between 2 and 21 weeks of age. Ocular alignment was measured by the Hirschberg test; convergence was determined by visual examination as an illuminated toy approached an infant's face; and sensory binocularity was measured by preferential looking for fusible versus rivalrous gratings. In experiment 1 we compared the proportion of infants at different ages demonstrating orthotropic ocular alignment with those showing convergence. In experiment 2, we compared the age of onset of convergence to that of sensory binocularity. RESULTS: Experiment 1: Most infants were orthotropic during the first month, and almost all of the others showed small amounts of exotropia. None of the infants showed accurate convergence until 6 weeks of age. By 4 months of age virtually all were orthotropic and had good convergence. Experiment 2: The onset of sensory binocular fusion occurred at 12.8 +/- 3.3 weeks. Full convergence did not occur until 13.7 +/- 3.2 weeks, although the first signs of convergence occurred slightly earlier. For individual infants there was a high correlation between the age of onset of sensory binocularity and convergence, and both onsets occurred earlier in girls than in boys. CONCLUSIONS: Ocular alignment did not require the development of binocularity mechanisms, and the development of binocularity mechanisms did not await the onset of good ocular alignment. The relatively sudden onset of binocularity, both sensory (preference for fusion and stereopsis) and motor (convergence) at about 3 months of age and the high correlation between these measures indicate a common causal mechanism that probably involves refinements in striate cortex circuitry.

Child Development↗
PubMed1993

Myopic children show insufficient accommodative response to blur.

PURPOSE: The study was performed to establish the relationship between the slope of the accommodative response function and refractive error in children. METHODS: Using an autorefractor, accommodative responses were measured in children under the following conditions. The subjects wore their best subjective refraction to view targets (a 3 x 3 array of 20/100 letters) displayed at seven distances (4.0 to 0.25 m). They viewed letters placed at 4.0 m through a series of negative lenses and letters placed at 0.25 m through a series of positive lenses. RESULTS: Myopic children accommodate significantly less than emmetropic children for real targets at near distances. Compared with emmetropic subjects, myopic children use blur poorly to increase accommodation, as shown by shallow slopes of the accommodative response functions for negative lenses. However, with positive lenses, requiring relaxation of accommodation, there is no significant difference in slope between myopic and emmetropic children. CONCLUSIONS: Blur is not an effective stimulus for accommodation in myopic children.

Accommodation, Ocular↗
PubMed1990

Effects of dioptric blur on Snellen and grating acuity.

We compared the effects of dioptric blur on Snellen acuity and grating acuity. Dioptric blur had a strong negative effect on Snellen acuity, consistent with previous studies, but had little effect on grating acuity. Between 1 and 12 D both types of acuity were reduced as a linear function of blur. However, 12 D of blur reduced grating acuity to only 6/24 (20/80), whereas letter acuity was worse than 6/300 (20/1000). We suggest that these differences are due to the presence of "spurious resolution" in which phase-reversed gratings are readily detectable. But the phase reversals so distort the relative positions of linear segments within the letters that the letters become unrecognizable. These results indicate that Snellen letters are more sensitive than gratings to a patient's refractive errors, emphasize the differences between Snellen and grating acuity, and indicate that the minimum angle of resolution (MAR) concept is not applicable to letters.

Adult↗
PubMed1990

Effects of dioptric blur on the Vistech contrast sensitivity test.

We studied the effect of dioptric blur on contrast sensitivity using the Vistech Vision Contrast System Chart. One diopter of blur affected the Vistech gratings by an amount consistent with expectations; sensitivity loss was greatest with higher spatial frequencies. However, a second diopter of blur induced an equal sensitivity loss for all spatial frequencies. A third diopter induced the greatest loss at the lowest frequency and an actual increase in sensitivity at the highest frequencies. This surprising finding is due to an interaction between "spuriously resolved" gratings and the border of the grating disc which resulted in the finest grating discs appearing to contain low frequency borders aligned with the fine gratings. Actually the fine gratings in the disc were invisible. When performing scanning photometry on the Vistech chart, we found that the contrasts of the unblurred gratings with higher contrast levels are 2.7 to 5.4 times higher than those stated by the manufacturer. The presence of this aliasing artifact indicates that the Vistech Vision Contrast System may produce erroneous results when used under conditions of high dioptric blur.

Adult↗
PubMed1989

Television and vision: reading captions when vision is blurred.

Closed captions for television have become an important means of communication for hearing-impaired people. Yet, previous studies show that hearing-impaired people are very likely to have visual problems that may interfere with caption reading. In this study, the effect of reduced vision on caption reading is examined. Six hearing-impaired and 14 hearing subjects viewed segments of captioned movies on a 19-inch television screen at a distance of three meters. All of the subjects had normal vision. Their vision was blurred with plus lenses of five different strengths. The data show that caption reading is affected by relatively small amounts of blur. If a person has even a slight visual problem, the caption letters under normal television viewing conditions are too small for maximum reading performance.

Adult↗
PubMed1989

Speechreading with reduced vision: a problem of aging.

Presbycusis, a progressive loss of hearing in the elderly, causes people to become dependent on vision for understanding spoken language at the same time that visual disorders of aging compromise their vision. This combination of sensory losses disrupts the lives of millions of elderly Americans but remains virtually unstudied. We tested the ability of hearing and hearing-impaired adults to understand sentences spoken by an actress without sound. She was seen through various amounts of dioptric blur. Both young and elderly adults were relatively unaffected by as much as 4 D of blur. They quickly adjusted to new levels of blur, showing improved performance within a few minutes. Ocular pathologies altered the effect of blur on speechreading.

Adult↗
PubMed1987

Compensatory eye movements during active head rotation for near targets: effects of imagination, rapid head oscillation and vergence.

Because the center of natural head rotation lies some distance behind the centers of eye rotation, the VOR has to operate with a gain substantially above 1 for there to be stable fixation of targets lying near the head. In humans, VOR gain was increased inversely proportional to fixation distance and changed with the angle of the head for very near targets. These effects were also evident when the subject imagined the target. However, this "high-gain" VOR was found to deteriorate substantially at frequencies beyond ca 2.5 Hz. In conditions without visual feedback, the VOR gain enhancement due to near fixation was disrupted by monocular viewing. When the subjects wore lenses to relax or increase accommodation, the lenses were found to have no effect on VOR gain. On the other hand, prisms of equivalent power to the lenses had a large effect whereby gain was adjusted according to the vergence state of the eyes. This suggests that VOR gain modulation is under the direct control of convergence.

Accommodation, Ocular↗
PubMed1987

Effect of luminance level on contrast sensitivity in myopia.

High myopia results in stretching of the retina. We expected this stretching to impair visual function. Accordingly, we tested static and dynamic spatial contrast sensitivity and temporal modulation sensitivity at photopic, mesopic, and scotopic luminance levels. None of these functions differed significantly for high myopes (6 to 10 D) relative to age-matched normals. We conclude that retinal stretching need not induce visual deficits.

Adult↗
PubMed1987

Orthogonal astigmatic axes in Chinese and Caucasian infants.

Caucasian infants are known to have a high incidence of astigmatism. The axis of greatest power is usually in the orientation orthogonal to the most common type found in Caucasian adults, with-the-rule astigmatism. We now find that Chinese infants also have a high incidence of astigmatism relative to adults, but its orientation is orthogonal to that of Caucasian infants. The source of this racial difference is not clear. It is unlikely to be due to the most obvious difference, the structure of the eyelids.

Adolescent↗
PubMed1986

High myopia does not affect contrast sensitivity.

The axial elongation of high myopia is known to induce tangential stretching forces on the retina. Such forces might be expected to compromise retinal circuitry and thus affect the temporal and spatial contrast sensitivity function. In fact, we find that simple high myopes have normal contrast sensitivity for stationary gratings, moving gratings, and uniform field flicker. This suggests that, unlike many other retinal disease processes, the high myope's retina retains its normal integrity until the outer retina is compromised.

Adult↗
PubMed1986

Congenital myopic esotropia: a case study.

A congenital myope developed a 20 delta left esotropia at 8 months of age. His eyes aligned immediately with a -4.00 D correction and would become esotropic when the glasses were removed. Before eyeglass wear best corrected acuity was reduced for both eyes with the left eye acuity significantly lower than the right. From age 8 to 12 months with part-time eyeglass wear, acuity improved and became equal for the two eyes. At 12 months of age the eyeglasses were lost and within 1 month the strabismus no longer responded to minus lenses and amblyopia reappeared in the left eye. This indicates that a myopic infant may stop bifixating blurred images beyond his far point and then cease to be able to relax convergence thereafter. It has the broader implication that tonic vergence posture and AC/A ratio may develop according to the visual experience of each infant.

Esotropia↗
PubMed1984

Consensual accommodation.

Consensual accommodation, like other consensual responses, may be a useful diagnostic indicator. The consensual interocular lag (direct accommodative response minus consensual accommodative response) was measured under a variety of accommodative conditions in four normal subjects, one of whom later underwent cryosurgery for a retinal tear. There was no consensual interocular lag when subjects were orthophoric , but when slightly exophoric at near distances a small lag does occur. A subject with large exophorias at near showed a more complex accommodative pattern. The consensual interocular lag proved to be the most sensitive measure of accommodative loss and subsequent recovery after retinal cryosurgery.

Accommodation, Ocular↗
PubMed1984

Progressive addition lenses for deaf lip readers and signers.

Deaf patients depend on intermediate vision for interpersonal communication. Deaf presbyopes are at a particular disadvantage since they are usually corrected for near and far vision with bifocals without consideration of their intermediate vision. Progressive addition lenses were used in this study to provide an intermediate correction for a group of deaf presbyopes. The patients' communication ability was tested through Varilux II lenses and habitual bifocals on a series of standardized lip reading and sign language tests. The patients showed better visual communication skills with the progressive adds, a preference for the progressive adds, and the impression of better vision through the progressive addition lens as opposed to their habitual bifocals.

Aged↗
PubMed1983

Use of various measures of visual acuity and contrast sensitivity in the evaluation of monocular occlusion and active vision training of three adult amblyopes.

Three adult amblyopic patients who underwent vision training involving monocular occlusion of the nonamblyopic eye and stimulation of the amblyopic eye using simple fixation exercises and a spinning grating procedure were evaluated using five different types of acuity testing and two methods of contrast sensitivity. It was found that contrast sensitivity and isolated Landolt C acuity improved sooner and to a greater extent than the more complex visual acuity tasks. One patient who improved only on the simpler tasks complained that the enhanced contrast due to training made vision in that eye less acceptable because it was more confusing. The results show that multiple measures of visual acuity and contrast sensitivity are necessary to adequately monitor the effects of occlusion therapy.

Adult↗