PubMed Health⌕ Search

Biomedical subjects

F Thorn

Publications and source records attributed to F Thorn.

16 recordsLinked to original sources
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↗
PubMed1977

Does refractive error change twice as fast as corneal power in orthokeratology?

We analyzed data from 4 orthokeratology studies of refractive error change (deltaRE) and keratometer-reading change (deltaK). Instead of the reported deltaRE/deltaK ratio of 2:1, we calculated from linear-regression analysis a ratio (slope) of 2:3. The ordinate intercept of the straight line fitted to the data (N = 181 eyes) indicates that when deltaK = 0, deltaRE = 0.72. The correlation between deltaK and deltaRE is poor (+0.63). Causes and interpretations of these results are discussed.

Contact Lenses↗