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F Schaeffel

Publications and source records attributed to F Schaeffel.

At least 19 recordsLinked to original sources
PubMed1995

A negatively powered lens in the chameleon.

Chameleons are arboral lizards that spot their prey visually and catch it by highly precise shots with their long sticky tongue. They scan their environment by large-amplitude independent saccadic eye movements; once an insect is detected, the head axis is aligned towards the target ('head tracking', both eyes come forward to fixate the insect and, in a phase called 'initial protrusion', the sticky tongue is loaded with tension by a special hyoid apparatus and subsequently shot out of the mouth with great precision. Lenses placed in front of the eyes produce predictable errors in distance estimation, suggesting that chameleons rely on accommodation cues when measuring the distance to their prey, but focusing has never been measured directly. Using a new technique to measure accommodation, we now show that accommodation is precise enough to serve as the major distance cue. Because accurate focusing requires large retinal images, we have tested image magnification and find that it is higher than in any other vertebrate eye scaled to the same size. This is a result of a unique optical design: unlike other vertebrate eyes, the crystalline lens of the chameleon has negative refractive power. Although there is a trend among vertebrates to increase corneal power and to decrease lens power with higher visual acuity, only in the chameleon eye has this tendency led to a reversal of the sign of the power of the lens.

Accommodation, Ocular↗
PubMed1995

Studies on the role of the retinal dopamine/melatonin system in experimental refractive errors in chickens.

We have found that development of both deprivation-induced and lens-induced refractive errors in chickens implicates changes of the diurnal growth rhythms in the eye (Fig. 1). Because the major diurnal oscillator in the eye is expressed by the retinal dopamine/melatonin system, effects of drugs were studied that change retinal dopamine and/or serotonin levels. Vehicle-injected and drug-injected eyes treated with either translucent occluders or lenses were compared to focus on visual growth mechanisms. Retinal biogenic amine levels were measured at the end of each experiment by HPLC with electrochemical detection. For reserpine (which was most extensively studied) electroretinograms were recorded to test retinal function [Fig. 3 (C)] and catecholaminergic and serotonergic retinal neurons were observed by immunohistochemical labelling [Fig. 3(D)]. Deprivation myopia was readily altered by a single intravitreal injection of drugs that affected retinal dopamine or serotonin levels; reserpine which depleted both serotonin and dopamine stores blocked deprivation myopia very efficiently [Fig. 3(A)], whereas 5,7-dihydroxy-tryptamine (5,7-DHT), sulpiride, melatonin and Sch23390 could enhance deprivation myopia (Table 1, Fig. 5). In contrast to other procedures that were previously employed to block deprivation myopia (6-OHDA injections or continuous light) and which had no significant effect on lens-induced refractive errors, reserpine also affected lens-induced changes in eye growth. At lower doses, the effect was selective for negative lenses (Fig. 4). We found that the individual retinal dopamine levels were very variable among individuals but were correlated in both eyes of an animal; a similar variability was previously found with regard to deprivation myopia. To test a hypothesis raised by Li, Schaeffel, Kohler and Zrenner [(1992) Visual Neuroscience, 9, 483-492] that individual dopamine levels might determine the susceptibility to deprivation myopia, refractive errors were correlated with dopamine levels in occluded and untreated eyes of monocularly deprived chickens (Fig. 6). The hypothesis was rejected. Although it has been previously found that the static retinal tissue levels of dopamine are not altered by lens treatment, subtle changes in the ratio of DOPAC to dopamine were detected in the present study. The result indicates that retinal dopamine might be implicated also in lens-induced growth changes. Surprisingly, the changes were in the opposite direction for deprivation and negative lenses although both produce myopia. Currently, there is evidence that deprivation-induced and lens-induced refractive errors in chicks are produced by different mechanisms. However, findings (1), (3) and (5) suggest that there may also be common features. Although it has not yet been resolved how both mechanisms merge to produce the appropriate axial eye growth rates, we propose a scheme (Fig. 7).

5,7-Dihydroxytryptamine↗
PubMed1995

Myopia.

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Accommodation, Ocular↗
PubMed1994

A simple mechanism for emmetropization without cues from accommodation or colour.

We propose and test the simple hypothesis that a chicken eye can emmetropize without cues derived from accommodation or colour just by maximizing retinal image contrast. Using different translucent occluders with known modulation transfer functions we found that deprivation myopia is correlated with the amount of image degradation. Equipped with a long-term integrator, a mechanism minimizing image degradation by changing the axial eye growth rate would therefore be sufficient to place the plane of focus of the eye at the average viewing distance.

Accommodation, Ocular↗
PubMed1994

6-Hydroxy dopamine does not affect lens-induced refractive errors but suppresses deprivation myopia.

Degradation of the retinal image by translucent occluders during postnatal development induces axial myopia in chickens, tree shrews and monkeys. Local visual deprivation produces myopia even in local regions of the eye and neither accommodation nor intact connection between the eye and the brain are necessary. Therefore, it is an important question whether a similar local-retinal pathway translating visual information into growth or stretch signals to the underlying sclera is acting to emmetropize the growing eye. It is not known until now whether occluder deprivation triggers similar eye growth (or scleral stretch) mechanisms that are also responsible for visual guidance of normal refractive development. We here report that, in chickens, 6-hydroxy dopamine suppresses deprivation-induced myopia but has no effect on the magnitude of changes in axial eye elongation that are induced by spectacle lenses. The result suggests that, in chickens with normal accommodation, two pharmacologically different feedback loops may be responsible for deprivation myopia and lens-induced refractive errors.

Animals↗
PubMed1994

Lower-field myopia and astigmatism in amphibians and chickens.

In some afoveate vertebrates refractive state appears to vary over the eye to match the average viewing distances of different areas of the visual field. However, precise measurements are difficult to obtain even in anesthetized animals, because standard methods of refraction are not designed for off-axis measurements and because the presence of astigmatism may fog the results. Therefore we developed a new automated objective technique, automated infrared photoretinoscopy, and measured off-axis refractions in alert chickens and amphibians. We found, in agreement with previous studies, that chickens (Gallus domesticus) are myopic and also have some astigmatism in the lower visual field. Lower-field myopia was, however, variable. It did not match the distance to the ground precisely, but it declined with age (as increased head height would predict). With-the-rule astigmatism was noticed in early posthatching development; it was striking even along the optic axis. The astigmatism lessened with age, as it does in human infants. Frogs (Rana pipiens and Rana temporaria) displayed pronounced myopic astigmatism that was confined to the lower visual field. Salamanders (Salamandra salamandra) and toads (Bufo bufo) showed less variation in refractive state across the visual field, although toads also were myopic in the lower visual field.

Animals↗
PubMed1994

Chick eyes under cycloplegia compensate for spectacle lenses despite six-hydroxy dopamine treatment.

PURPOSE: To test whether eye growth changes produced by spectacle lens wear are mediated by changes in ciliary muscle tonus in chicks. METHODS: Because there is evidence that deprivation myopia is based on a local-retinal mechanism in the eye that probably remains functional after cycloplegia as well as after ciliary ganglion or Edinger-Westphal lesions, none of these treatments provides insight into whether accommodation tonus is also important in the control of axial eye growth. Because 6-hydroxy dopamine (6-OHDA) suppresses deprivation myopia, to isolate growth changes mediated by accommodation the authors injected 6-OHDA and paralyzed accommodation in addition (by corneal application of vecuroniumbromide). To quantify the state of cycloplegia, the abnormal pecking responses of cyclopleged chickens were studied. RESULTS: The authors found that cycloplegia could be maintained for 3 hours daily by corneal application of vecuroniumbromide. To ensure that visual exposure was restricted to the time period of cycloplegia, chickens were transferred to a 3-hour light/21-hour dark cycle. Control experiments showed that emmetropization was still functional under the changed light cycle. Strikingly, even with suppressed local-retinal growth control mechanisms (as indicated by the lack of deprivation myopia in a 6-OHDA injected group of chickens with occluders) and paralysis of accommodation, the eyes compensated for the defocus imposed by spectacles by changing their axial growth rates to be similar to those of eyes with functional accommodation. CONCLUSIONS: The findings show that the ciliary muscle and the activity of the iris sphincter muscle are not involved in emmetropization in chicks. If accommodation mediates the growth effects with lenses, it must happen via another pathway. Based on previous results, the authors propose that either the choroidal nerves from the ciliary ganglion to the choroid are important or that another yet unknown pathway from the Edinger Westphal nucleus to the eye transmits the necessary information.

Accommodation, Ocular↗
PubMed1993

Diurnal growth rhythms in the chicken eye: relation to myopia development and retinal dopamine levels.

1. If the eyes of young chickens are deprived of clear vision by translucent occluders, they develop considerable amounts of axial myopia within days. At the same time, the day time retinal dopamine levels drop by about 30%. Because the retinal dopamine levels of normally sighted chicks also differ diurnally and are low at night, we expected that the rate of axial eye growth might also differ during this time. 2. Unexpectedly, eyes grew in length only during the day (about 0.13 mm/day) and even shrank during the night (about -0.04 mm/night, average net growth +0.09 mm in 24 h). 3. If the eyes were occluded, they grew both during the day and also at night (average net growth: +0.16 mm in 24 h). Therefore, development of deprivation myopia was a result of the lack of growth inhibition at night rather than of excessive growth during the day when the actual deprivation occurred. 4. Suppression of dopaminergic retinal pathways by intravitreal injections of the neurotoxin 6-hydroxy-dopamine (6-OHDA) also suppressed development of deprivation myopia and it restored the growth inhibition at night. With normal visual experience, the drug had no effect on axial eye growth and refractive state. 5. Diurnal growth rhythms of the eyes disappeared under continuous light.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

[Age dependence of pupillary near reflex].

BACKGROUND: This study was performed to gain age correlated normal values for the pupillary near reflex. METHODS: Accommodation and pupillary near reaction were measured by means of simultaneous infrared video retinoscopy and pupillography in 64 healthy volunteers aged between 5 and 55 years. Measurements were done at a reduced, near mesopic, light condition with accommodation to 10, 14, 20 and 33 cm. RESULTS: The pupillary near response varied highly with age: persons younger than 20 years of age showed a significantly smaller pupillary near response as compared to those older than 20 years. In most of the children younger than 10 years the pupil near response was very small (less than 10% constriction) at accommodation distances longer than 10 cm. There was a significant difference between the age groups younger and older than 20 but no statistically significant differences within these age groups. CONCLUSION: A change of the pupillary near reaction takes place around the age of 20. We conclude that this change does not only reflect the aging of the cristalline lens but is due to an age related change of the supranuclear control.

Accommodation, Ocular↗
PubMed1993

Inter-individual variability in the dynamics of natural accommodation in humans: relation to age and refractive errors.

1. To study the relationship between accommodation under natural viewing conditions, age and refractive errors, we have measured time courses of accommodation in thirty-nine human subjects aged 5-49 years using a newly developed technique. The technique is based on infrared photoretinoscopy and involves fully automated on-line image processing of digitized video images of the eyes with a sampling rate of 5.3 Hz. 2. The distance between the subject and the video camera was about 1.3 m. Head movements of the subject required little restriction because the eyes were automatically tracked in the video image by the computer program. All subjects were tested under binocular viewing conditions. 3. Both refraction of the right eye and pupil diameter were measured with a precision of 0.2-0.4 dioptres (D) and 0.1 mm, respectively, and were plotted on-line. The data were subsequently automatically analysed. 4. Automated infrared photoretinoscopy proved to be very convenient and easy to handle in both children and adults. 5. The maximal speed of accommodation for a target at a distance of 5 D declined in the subjects with age (from up to 21.7 D s-1 for accommodation and 32.7 D s-1 for subsequent accommodation to a distant target ('near to far accommodation') in children down to 2-18 D s-1 in adults). There was a striking inter-individual variability in the maximum possible speed of accommodation and near to far accommodation. 6. Speed of accommodation and of near to far accommodation was correlated for each subject. However, in most of the subjects, the process of near to far accommodation was faster than accommodation (P < 0.005, if averaged over all subjects). This correlation was independent of age. 7. The accommodation-induced pupillary constriction (pupillary near response) was absent in children for a 4 D target; even at 10 D, there was no reliable pupillary response. The pupillary near response increased to about 1.6 mm D-1 of accommodation at the age of 47. Since a pupillary near response could still be elicited in presbyopic subjects unable to accommodate, the ratio of pupillary constriction per dioptre of accommodation approached infinity. 8. The magnitude of the pupillary near response was highly variable even among subjects of the same age but was typical for each subject. There was a correlation (P < 0.01) to refractive error: corrected myopes had weaker pupillary near responses than emmetropes or hyperopes.(ABSTRACT TRUNCATED AT 400 WORDS)

Accommodation, Ocular↗
PubMed1992

Barn owls have symmetrical accommodation in both eyes, but independent pupillary responses to light.

We have studied accommodation behaviour in the barn owl (Tyto alba). By defocussing one eye with various spectacle lenses and recording the refractive state in both eyes continuously during pecking, we found that the owls' accommodation was symmetrical in both eyes, with no regard to the power of the lens used. Even with no visual input to one eye, the amount of accommodation was always identical in both eyes. On the other hand, pupillary responses to light were independent. This finding differs from an earlier observation in the chicken, where both accommodation and pupillary responses were found to be independent. The result is discussed with regard to current knowledge on the central pathways for control of accommodation and pupillary responses in birds.

Accommodation, Ocular↗
PubMed1992

Dose-dependent effects of 6-hydroxy dopamine on deprivation myopia, electroretinograms, and dopaminergic amacrine cells in chickens.

We found that a single intravitreal injection of 6-hydroxy dopamine (6-OHDA) is highly efficient in blocking the development of deprivation-induced myopia in young chickens. To investigate the effects of 6-OHDA on retinal function, we studied electroretinograms (ERGs) in chickens aged 15-25 days, 4 days subsequent to the injection. Both spectral sensitivity and oscillatory potentials were tested. In addition, a histological examination was performed of dopaminergic amacrine cells labeled by a monoclonal antibody against tyrosine hydroxylase. We found that, at doses of 6-OHDA sufficient to suppress deprivation myopia entirely, no effect could be detected on either the ERGs or on the density and appearance of dopaminergic amacrine cells. For higher doses, spectral sensitivity and the number of dopaminergic amacrine cells declined gradually. In contrast, as doses increased, oscillatory potentials 1 and 2 grew in amplitude only to decline at the highest doses. The results indicate that (1) development of deprivation myopia requires normal retinal function and that (2) slight changes in the gains of dopaminergic pathways are sufficient to block the development of deprivation myopia.

Animals↗
PubMed1992

Longitudinal chromatic aberration and emmetropization: results from the chicken eye.

1. Due to the chromatic dispersion of the ocular media, the focal length of the optics of the eye is about 3 diopters longer for red light than for blue light. Because emmetropization in the chicken (Gallus domesticus) does not require colour cues and operates properly in monochromatic light, one can, therefore, expect that chickens raised in red light become more myopic (with longer eyes) than chicks raised in short wavelength light. Prior to conducting this experiment, we matched the brightness of both light conditions by means of flicker electroretinograms such that equiluminance was obtained for the chickens. 2. Unexpectedly, refractive development was not different from controls in white light for either red or near-ultraviolet light. 3. We tested whether the visual mechanisms guiding refractive development were still sensitive to defocus under both illuminations by treating the chicks with spectacle lenses. 4. Similar to a previous experiment in white light, the growth of the eye in red light also changed such that it compensated for the imposed defocus. It failed to do so, however, in near-ultraviolet light. 5. A histological analysis of the sampling intervals for the ultraviolet receptor system revealed that its spatial resolving power was too low to detect the defocus imposed by the lenses, whereas the long wavelength receptors provided sufficiently good visual acuity. 6. The results show that, during emmetropization, the chicken eye elegantly bypasses the problem of multiple chromatic focal planes by having a low sensitivity to defocus in the blue end of the spectrum. Because the chromatic dispersion function is steep in the blue range but flat at the red end of the spectrum, the remaining chromatic defocus in the spectral range of high visual acuity is low and may match the depth of field of the eye.

Animals↗
PubMed1991

Properties of the feedback loops controlling eye growth and refractive state in the chicken.

Recent experiments in chickens provide evidence that axial eye growth and refractive state are guided by mechanisms sensitive to refractive error. To determine whether or not the sign of refractive error is derived from longitudinal chromatic aberration we raised chicks with spectacle lenses in monochromatic light. The eyes showed an appropriate growth response to correct for the defocus imposed by the lenses no different than in previous experiments in white light. Thus, in normally accommodating chicks chromatic cues are not necessary for emmetropization to occur. We examined the linearity of feedback loops controlling axial eye growth: positive spectacle lenses were found to inhibit axial growth very efficiently making the eyes shorter than normal whereas negative lenses had little effect on axial elongation: feedback loops for regulation of axial growth are highly nonlinear and act most efficiently on the myopic side. We found that, subsequent to a period of binocular deprivation of form vision, the refractive errors acquired are highly correlated in both eyes. Since both eyes grew without visual feedback we conclude that the gains in the feedback loops that control axial growth must be similar in both eyes. We suggest that the gains are genetically determined and are typical for each individual. Chicks made near-sighted in both eyes by "deprivation of form vision" were corrected by appropriate negative lenses. Three out of five chicks recovered from myopia despite the correction. Also two chicks that were made near-sighted in one eye recovered with no regard to the correcting lens. Three chicks remained more myopic than the correcting lens required and finally started to recover while the lens was still in place. Two out of three chicks that were made far-sighted showed recovery despite appropriate correction by positive lenses. We conclude that there must be a nonvisual mechanism highly sensitive to abnormal eye shape. During expt (4) we found unexpectedly that the development of form deprivation myopia is inhibited if no part of the retina in an animal is exposed to normal visual experience. The result indicates that some communication between both eyes exists, although form deprivation myopia itself has been shown to develop independently in both eyes.

Animals↗
PubMed1991

Diurnal control of rod function in the chicken.

We studied rod function in the chicken by recording corneal electroretinograms (ERGs). The following experiments were performed to demonstrate rod function during daytime: (1) determining the dark-adaptation function; (2) measuring the spectral sensitivity by a a-b-wave amplitude criterion in response to monochromatic flickering light of different frequencies ranging from 6.5-40.8 Hz (duty cycle 1:1); (3) analyzing the response vs. log stimulus intensity (V-log I) function in order to reveal a possible two phase process; and (4) determining the spectral sensitivity function either in a non-dark adapted state or after dark adaptation of the animals for 1 and 24 h. None of these experiments demonstrated clear evidence of rod function during daytime. On the other hand, we found rods histologically by light- and electron microscopy. Therefore, we repeated our ERG recordings during the night (between midnight and 3:00 A.M.). Without previous dark adaptation, rod function could be seen immediately in the same experiments described above. The result shows that, in the chicken, rods are turned on endogenously during the night but are scarcely functional during the day.

Animals↗
PubMed1990

Refractive state, ocular anatomy, and accommodative range of the sea otter (Enhydra lutris).

Sea otters are carnivorous, amphibious mammals that are active both above and under water. Accordingly, it might be expected that their eyes are adapted for both aerial and aqueous vision. We examined the anatomy and physiological optics of the sea otter eye with a view towards describing and explaining its amphibious visual characteristics. We employed photokeratoscopy to measure the refractive power of the sea otter cornea, which we found to be 59 D. Using video dynamic photorefraction, we found that sea otters can focus targets clearly both in air and water, relying on accommodation to compensate for the refractive loss of their corneas upon immersion in water. Our anatomical investigations revealed that the anterior epithelium of the cornea is extensively developed, as is the iris musculature, meridional ciliary muscle, and the corneoscleral venous plexus. The first feature is most likely an adaptation to the salinity of the marine environment. We believe the latter features are part of a novel, well-developed lenticular accommodative mechanism.

Accommodation, Ocular↗
PubMed1990

Developing eyes that lack accommodation grow to compensate for imposed defocus.

The eyes of growing chicks adjust to correct for myopia (eye relatively long for the focal length of its optics) or hyperopia (eye relatively short for the focal length of its optics). Eyes made functionally hyperopic with negative spectacle lenses become myopic and long, whereas eyes made functionally myopic with positive spectacle lenses become hyperopic and short. We report here that these compensatory growth adjustments occur not only in normal eyes but also in eyes unable to accommodate (focus) because of lesions to the Edinger-Westphal nuclei. Thus, at least in chicks, accommodation is not necessary for growth that reduces refractive errors during development, and may not be necessary for the normal control of eye growth.

Accommodation, Ocular↗