Biomedical subjects

S Diether

Publications and source records attributed to S Diether.

5 recordsLinked to original sources

PubMed2001

Changes in contrast sensitivity induced by defocus and their possible relations to emmetropization in the chicken.

PURPOSE: To test whether the level of contrast adaptation (CA) relates to refractive development in the chicken. (CA refers to a spatial frequency-selective increase of suprathreshold contrast sensitivity after exposure to low-contrast patterns). METHODS: CA was determined in individual chicks by comparing their optomotor gain in response to drifting low-contrast stripe patterns before and after treatment with spectacle lenses. The amount of CA was compared with the loss of contrast predicted from defocus at the tested spatial frequency. The reversion of CA and recovery from deprivation myopia were studied while the retinal image features were controlled by forcing the animals to watch spatially filtered digital video clips. RESULTS: CA was induced by wearing positive and negative lenses for 1.5 hours, both without and with cycloplegia, but was less pronounced in the case of positive lenses when accommodation was intact. The amount of CA at a tested spatial frequency was predicted from the loss of contrast calculated from the modulation transfer function for a defocused optical system. Watching low-pass-filtered video clips induced deprivation myopia and inhibited recovery from it. It also prevented the reversal of CA that was previously induced by deprivation. Both recovery from deprivation myopia and recovery from CA occurred with sharp video clips, although less so than with normal visual exposure. CONCLUSIONS: CA changes with retinal image sharpness and occurs even when accommodation is intact. Because CA correlates with myopia induced by frosted occluders, negative lenses, and low-pass-filtered video clips, and its reversal correlates with recovery from myopia, it is possible that shifts in CA may represent a signal related to refractive error development.

Accommodation, Ocular↗

PubMed1999

Interactions of spatial and luminance information in the retina of chickens during myopia development.

Degrading the retinal image by frosted eye occluders produces elongated eyes and 'deprivation myopia' in a variety of animal models. The postulated retinal 'deprivation detector' is quite sensitive to even small changes in image contrast or spatial frequency composition. Because psychophysical experiments have shown that a decline in luminance shifts the contrast sensitivity function to lower spatial frequencies, it is likely that only a reduced spatial frequency range is available for image analysis to control eye growth. It is even possible that the compression might be sufficient to promote deprivation myopia. We have tested this hypothesis, using the animal model of the chicken. (1) At an ambient illumination of 550 lux (about 76 cd m-2), neutral density (ND) filters placed in front of the eye with 0.0, 0.5 or 1.0 log unit attenuation did not change refractive development. However, monocularly or binocularly attached filters with 2 log units attenuation produced 5-7 D of myopia relative to normal eyes. Black occluders were not more effective. Frosted eye occluders with little effect on image brightness (about 0.5 log units attenuation) produced much more myopia (about 16 D compared with the controls). (2) The effects of the ND filters on refractive development could not be reproduced if the ambient illumination was reduced by 2 log units. Probably, minor effects on image quality were introduced by optical imperfections of the ND filters which were more critical at low retinal image brightness. (3) In an optomotor experiment (spatial frequency 0.2 cyc deg-1, stripe speed 57 deg sec-1), it was shown that the chickens' contrast sensitivity was severely reduced when the eyes were covered by 2.0 ND filters. (4) Since there is evidence that changes in dopamine release from the retina may be one of the factors affecting the development of myopia, we have tested how selective these changes were for spatial information. It was found that dopamine release was controlled by both spatial and luminance information and that the inputs of both could be scarcely separated. (5) Because the experiments show that the eye becomes more sensitive to image degradation at low light, the human eye may also be more prone to develop myopia if the light levels are low during extended periods of near work.

Animals↗

PubMed1999

The growing eye: an autofocus system that works on very poor images.

It is unknown which retinal image features are analyzed to control axial eye growth and refractive development. On the other hand, identification of these features is fundamental for the understanding of visually acquired refractive errors. Cyclopleged chicks were individually kept in the center of a drum with only one viewing distance possible. Defocusing spectacle lenses were used to stimulate the retina with defined defocus of similar magnitude but different sign. If spatial frequency content and contrast were the only cues analyzed by the retina, all chicks should have become myopic. However, compensatory eye growth was still always in the right direction. The most likely cues for emmetropization, spatial frequency content and image contrast, do therefore not correlate with the elongation of the eye. Rather, the sign of defocus was extracted even from very poor images.

Accommodation, Ocular↗

PubMed1999

Long-term changes in retinal contrast sensitivity in chicks from frosted occluders and drugs: relations to myopia?

Experiments in animal models have shown that the retinal analyzes the image to identify the position of the plane of focus and fine-tunes the growth of the underlying sclera. It is fundamental to the understanding of the development of refractive errors to know which image features are processed. Since the position of the image plane fluctuates continuously with accommodative status and viewing distance, a meaningful control of refractive development can only occur by an averaging procedure with a long time constant. As a candidate for a retinal signal for enhanced eye growth and myopia we propose the level of contrast adaptation which varies with the average amount of defocus. Using a behavioural paradigm, we have found in chickens (1) that contrast adaptation (CA, here referred to as an increase in contrast sensitivity) occurs at low spatial frequencies (0.2 cyc/deg) already after 1.5 h of wearing frosted goggles which cause deprivation myopia, (2) that CA also occurs with negative lenses (-7.4D) and positive lenses (+6.9D) after 1.5 h, at least if accommodation is paralyzed and, (3) that CA occurs at a retinal level or has, at least, a retinal component. Furthermore, we have studied the effects of atropine and reserpine, which both suppress myopia development, on CA. Quisqualate, which causes retinal degeneration but leaves emmetropization functional, was also tested. We found that both atropine and reserpine increase contrast sensitivity to a level where no further CA could be induced by frosted goggles. Quisqualate increased only the variability of refractive development and of contrast sensitivity. Taken together, CA occurring during extended periods of defocus is a possible candidate for a retinal error signal for myopia development. However, the situation is complicated by the fact that there must be a second image processing mode generating a powerful inhibitory growth signal if the image is in front of the retina, even with poor images (Diether, S., & Schaeffel, F. (1999).

Adaptation, Ocular↗

PubMed1997

Local changes in eye growth induced by imposed local refractive error despite active accommodation.

We have tested whether defocus imposed on local retinal areas can produce local changes in eye growth, even if accommodation is available to clear part of the imposed defocus. Hemi-field lenses were attached to little leather hoods that were worn by young chickens from day 11-15 post-hatching. The lens segments defocused either the nasal or the temporal visual field, or covered the full field. We found that negative lenses (-7.5 D) were incompletely compensated in all three cases but caused significant myopia in the defocused parts of the visual field (differences to fellow eyes with normal vision: nasal visual field -3.13 +/- 1.56 D, P < 0.001; temporal visual field -4.02 +/- 1.38 D, P < 0.001; full field -3.82 +/- 2.48 D, P = 0.01). Myopia was not enhanced if the lenses covered the entire visual field. Positive lenses (+6.9 D) caused larger changes in refraction than negative lenses and, again, there was no significant difference in the amount of induced hyperopia in the nasal or temporal retina, or in the amount of hyperopia with full-field lenses (difference to fellow eyes with normal vision: nasal visual field +6.2 +/- 2.69 D, P < 0.001; temporal visual field +5.95 +/- 2.22 D, full field +7.22 +/- 2.44 D, P < 0.001). To compare the shapes of the excised eyes after lens treatment, we wrote a fully automated image processing program that traced their outlines in digitized video images. We found that the shapes of the eyes treated with positive lenses did scarcely differ from their fellow eyes with normal vision, indicating that hyperopia over this 4 day period was caused mostly by choroidal thickening. Full field negative lenses produced significant axial eye elongation; the effects of locally imposed defocus on eye shape were less conspicuous and were significant only in some areas. That local compensation of defocus was possible for both negative and positive lenses, suggests that the retina can recognize the sign of defocus without accommodation cues. Even more striking is that the presence of accommodation is apparently ignored since the drift in the plane of focus during accommodation does not disturb the compensation process. We re-analyze previous experimental results that argue for different mechanisms for deprivation myopia and lens-induced refractive errors. We propose that lens-induced refractive errors are compensated by similar retinal mechanisms as the ones proposed by Bartmann and Schaeffel [(1994). Vision Research, 34, pp. 873-876] to explain deprivation myopia. The proposed mechanisms can integrate with long time constants over the spatial frequency content in the retinal image while the viewing distances change, and control both choroidal thickening and scleral growth. However, it turns out that the compensation of imposed myopia cannot be explained if only one constant viewing is available. Apparently, there is more than a retinal blur detector to guide refractive development.

Accommodation, Ocular↗