[Comparative analysis of light sensitivity, depth and motion perception in animals and humans]

BACKGROUND

This study examined how humans perform regarding light sensitivity, depth perception and motion vision in comparison to various animals.

OBJECTIVE

The parameters that limit the performance of the visual system for these different functions were examined.

METHODS

This study was based on literature studies (search in PubMed) and own results.

RESULTS

Light sensitivity is limited by the brightness of the retinal image, which in turn is determined by the f‑number of the eye. Furthermore, it is limited by photon noise, thermal decay of rhodopsin, noise in the phototransduction cascade and neuronal processing. In invertebrates, impressive optical tricks have been developed to increase the number of photons reaching the photoreceptors. Furthermore, the spontaneous decay of the photopigment is lower in invertebrates at the cost of higher energy consumption. For depth perception at close range, stereopsis is the most precise but is available only to a few vertebrates. In contrast, motion parallax is used by many species including vertebrates as well as invertebrates. In a few cases accommodation is used for depth measurements or chromatic aberration. In motion vision the temporal resolution of the eye is most important. The ficker fusion frequency correlates in vertebrates with metabolic turnover and body temperature but also has very high values in insects. Apart from that the flicker fusion frequency generally declines with increasing body weight.

CONCLUSION

Compared to animals the performance of the visual system in humans is among the best regarding light sensitivity, is the best regarding depth resolution and in the middle range regarding motion resolution.

[Current recommendations for deceleration of myopia progression]

BACKGROUND

Epidemiologic data demonstrate a rise in myopia prevalence. Therefore interventions to reduce the risk of myopia and its progression are needed and increasingly often asked for.

METHODS

Systematic literature search via PubMed in MEDLINE.

RESULTS

Myopia progression can be reduced by the following means which are listed according to their efficacy: (1) Atropine eye drops low dosed to avoid clinically relevant side effects, (2) optical means aiming at the correction of peripheral hyperopic defocus, e. g., multifocal contact lenses, and (3) increased daylight exposure.

CONCLUSION

Daylight exposure reduces the risk of incident myopia. Children should be advised to spend sufficient time outdoors, especially before and in primary school. Myopia progression can be effectively attenuated by low-dose topical atropine and multifocal contact lenses.

[Myopia update 2011]

This review summarises some recent aspects of myopia research. The following conclusions have been drawn. As long as myopia progression is visually controlled, at least three different interventions are possible: (i) spectacles/contact lenses which correct only the centre of the visual field and leave the periphery somewhat myopic, (ii) outdoor activity or equivalent temporary increase in illuminance, (iii) pharmacological intervention of retinal growth signals that are transmitted to the underlying sclera. Options (i) and (ii) can be used without risks although there is still room for improvement of the variables. Option (iii) has re-entered a new phase of orientation with new searches for candidate targets after previous testing with muskarinic antagonists (pirenzepine) in children did not enter phase 3 level. If myopia is outside the range over which it is visually controlled by emmetropisation (in the case of high and pathological myopias), in principle the possibility exists to improve the mechanical stability of the sclera pharmacologically. However, there is still a need for more research. Up to now, the mechanical weakness of the sclera in highly mopyic eyes is surgically stabilised by "scleral buckling". However, these procedures have found limited acceptance since the effects were not very reliable. In 40 - 50 % of the cases of high myopia, degenerative processes are found in the retina which can be seen as consequence of the mechanical tension in the fundus, but may also be indepedent of this factor (no significant correlation with axial length!). In part they can be slowed down by intravitreal anti-VEGF therapy. A long-term study from Denmark has shown that most patients with myopia of between 6-9 dpt during puberty reach retirement age without disabling visual loss.

Accommodation behaviour during prey capture in the vietnamese leaf turtle ( Geoemyda spengleri )

Vietnamese leaf turtles ( Geoemyda spengleri) were tested for their ability to focus on prey objects at various distances. Accommodation was continuously measured by infrared photoretinoscopy. All animals investigated during this study showed a surprisingly high precision of accommodation over a range of over 30 D. Measured accommodation matched the target distance accurately for distances between 3 and 17 cm. The turtles switched between independent and coupled accommodation in the two eyes. Independent accommodation was observed when the turtles inspected their environment visually without a defined object of interest. Coupled accommodation was only observed during binocular prey fixation. When a turtle aimed at a target, the symmetrical focus of both eyes persisted even if vision was totally blocked in one eye or altered by ophthalmic lenses. This suggests that the eyes were linked by internal neuronal mechanisms. The pupil of the eye responded clearly to changes in ambient light intensity. A strong decrease in pupil size was also observed when the eye was focused on a close target. In this case, the constriction of the pupil probably aids in the deformation of the eye lens during near-accommodation.

Variations in the off-axis refractive state in the eye of the Vietnamese leaf turtle ( Geoemyda spengleri )

Lower-field myopia has been described for various vertebrates as an adaptation that permits the animal to keep the ground in focus during foraging, and, at the same time, to look out for distant objects, such as predators, in the upper visual field. Off-axis measurements with infrared photoretinoscopy in the eye of Geoemyda spengleri revealed a constant refractive state in the horizontal plane of the visual field but variable refraction in the vertical plane. In the three turtles investigated, the refractions increased continuously from the ventral to the dorsal visual field over a range of 35, 40 and 56 D, respectively. While this finding confirms the presence of an adaptive change of the refractive state equivalent to lower field myopia, subsequent measurements with a rotated retinoscope showed that at least part of the variation in the ventral field was attributed to astigmatism. The reason for this astigmatism is unknown. Anatomical investigation of the retina revealed that the constant refractive values in the horizontal plane corresponded to a stripe of increased ganglion cell density. A maximum density of 4,200 ganglion cells mm(-2) was counted in the centre of this visual streak.

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.

Pineal control of the dopamine D2-receptor gene and dopamine release in the retina of the chicken and their possible relation to growth rhythms of the eye

Retinal dopamine (DA) and the DA D2-receptor have been implicated in the development of "deprivation myopia", induced by frosted eye occluders. We have studied the changes in D2-mediated dopaminergic transmission in the retina, their possible relations to eye growth rhythms and myopia, and their control by the pineal gland. (1) We found that the sensitivity of eye growth to retinal image degradation varied over the day. Intermittent periods of normal vision inhibited deprivation myopia more if they occurred in the evening than in the morning. (2) Diurnal growth rhythms in both eyes interacted even though it was previously shown that both deprivation myopia and the accompanying changes in retinal DA release can be monocularly induced. (3) The D2-receptor mRNA concentration in the retina showed no systemic diurnal changes and was not affected by deprivation myopia, but was increased after 2 days in darkness. Since DA release varies over the day, the gain of dopaminergic transmission may also vary, which could explain the observation described in (1) above. (4) Depletion of retinal DA by intravitreal application of reserpine, which lowers DA content severely, had little effect on D2-receptor mRNA concentration. (5) Selective illumination of the pineal gland reduced the D2-receptor mRNA content in the retina to a similar level to full illumination, indicating that the pineal gland controls the D2-receptor mRNA content in the retina. The pineal also controlled DA release in the retina. These results show that the pineal has a surprisingly large influence on both the retinal DA receptor gene transcription and DA release. It can probably control the gain of dopaminergic transmission in the retina and deprivation myopia and mediate the interactions of the growth rhythms in both eyes.

Developmental compensation of imposed astigmatism is not initiated by astigmatic accommodation in chickens

PURPOSE

It is not clear whether emmetropization is confined to spherical refractive errors, or whether astiqmatic errors are also corrected via visual feedback. Experimental results from the animal model of the chicken are equivocal since compensation of imposed astimatic defocus was found in some but not all studies. Astigmatism could only be compensated by changes in the geometry of the cornea or lens. One has tested whether astigmatic spectacle lenses induce astigmatic accommodation as a possible first step of long-lasting compensation.

METHODS

Thirty-five chickens were treated with cylinder lenses (+3/0D or -3/0D) for 5 h. Refractions were determined at 1.38 m distance without cycloplegia in hand-held chicks before attaching the lenses, with the lenses on (0 h), and after 3 and 5 h, and after removal of the lenses. Spheres (S), cylinders (C) and axes (A) were determined using infrared photoretinocopy in three axes (the 'PowerRefractor', equipped with a 135 mm lens).

RESULTS

(1) The performance of the 'PowerRefractor' was tested in the chickens with trial lenses and gave correct refractions. (2) Astigmatic trial lenses induced refractive errors as expected from their powers in the case of +3/0D lenses: (S) +3.26 +/- 0.93D, (C) -3.45 +/- 0.87D). In the case of -3/0D lenses, slightly more hyperopic spheres were induced (refractions (S) +4.5 +/- 0.48D) but the cylinders were still as expected (-3.25 +/- 0.49D). The axes of astigmatism were correctly reproduced, since rotating the lenses changed the axes of the induced cylinders as expected. (3) Neither after 3 nor after 5 h of lens wear were there significant changes in the axes or the magnitude of astigmatism. Directly after removal of the lens, the refractions did not differ from their start-up values (with +3/0D lenses: (S) +3.31 +/- 1.05D vs. +3.22 +/- 0.76D, (C) -1.19 +/- 1.77D vs. -0.65 +/- 0.94D, (A) 96 +/- 49 vs. 113 +/- 45 deg; with -3/0D lenses: (S) 2.63 +/- 1.12D vs. 2.97 +/- 0.94D, (C) -1.11 +/- 1.15D vs. -0.53 +/- 0.56D, (A) 78 +/- 24 vs. 131 +/- 35 deg).

CONCLUSIONS

The most intuitive mechanism for compensation of astigmatic refractive errors, astigmatic accommodation, could not be demonstrated in chickens. In light of this finding, it seems unlikely that a visually controlled mechanism is operating during development to reduced astigmatism by changing corneal or lenticular growth.

Laboratory, clinical, and kindergarten test of a new eccentric infrared photorefractor (PowerRefractor)

PURPOSE

Photorefraction is a convenient way to determine refractive state from a distance. It is, therefore, useful for measuring infants and noncooperative subjects. However, its reliability (or precision) and accuracy (or validity) has been questioned. In a study in subjects without cycloplegia, we have tested whether, after complete automatization, eccentric photorefraction at a 1-m distance can be as reliable as a common autorefractor.

METHODS

In a laboratory study of 15 student subjects without the use of cycloplegia (30 eyes, refractive errors ranging from -6 D to +6 D), age 25 to 31 years, the photorefractive measurements were compared with spectacle prescriptions. In a clinical study, photorefraction, autorefraction, and subjective refraction were performed in 40 patients without cycloplegia (refractive errors ranging from -4 D to +4 D), most of them with various ocular pathologies. Subjective refractions were obtained by an experienced clinical ophthalmologist but were not accessible to the examiner who used the two refractors. Visual acuity was 20/20 or better except for five subjects. Ages ranged from 6 to 75 years. In the kindergarten screening study, 108 children aged 3 to 6 years were screened for refractive errors.

RESULTS

In the laboratory study, it was found that the mean difference between spectacle prescription and PowerRefractor measurements was < 0.6 D for spheres and below 0.4 D for cylinders. In the clinical study, data were obtained by all three procedures in 78 eyes. The photorefractor and the autorefractor performed similarly for spheres (mean absolute dioptric difference between refractor and subjective measure: 0.593 D and 0.696 D) and cylinders (mean absolute dioptric differences: 0.399 D and 0.389 D). However, the photorefractor was superior with regard to the measurement of the magnitude and axis of astigmatism (mean weighted difference between objective and subjective axis 0.644 D and 0.769 D, respectively). In the kindergarten study, it was found that the PowerRefractor was very convenient to handle. The autorefractor measured more myopic refractions than the PowerRefractor (mean of the left eyes 0.11 +/- 1.1 D vs. 0.62 +/- 0.53 D, p < 0.001). There was no indication that the PowerRefractor failed to detect hyperopia, because all but one child with more than 2 D of hyperopia measured with autorefractor (n = 7) was also hyperopic with the PowerRefractor. Furthermore, presenting an interesting fixation target at a 3-m distance did not cause more hyperopic refractions, indicating that the camera of the PowerRefractor at a 1-m distance was not a significant stimulus to accommodation.

CONCLUSIONS

The PowerRefractor was shown to have comparable or slightly better reliability and accuracy than a modern autorefractor; however, it has major advantages over current autorefractors in that it is faster, measures both eyes at once, and gives interpupillary distance, pupil size, and information on the alignment of the eyes at the same time.

Changes in retinal and choroidal gene expression during development of refractive errors in chicks

PURPOSE

During growth, the retina analyzes the projected image to achieve a close match between eye length and focal length. Because the messengers released by retina and choroid are largely unknown, genes that are differently expressed in response to changes in the retinal image were identified. In addition, because glucagon may be important in the visual control of eye growth, the transcript levels of proglucagon were studied.

METHODS

Reverse transcription-polymerase chain reaction differential display was used to identify genes that were differentially expressed in chick eyes that were deprived of sharp vision or treated with positive or negative lenses. Differences were analyzed through sequencing and database searches and confirmed by Northern blot analyses.

RESULTS

Combining 40 and 33 arbitrary primers with 3 oligo-dT-primers, approximately 48% and 40% of the retinal and choroidal mRNAs were screened, respectively. Twelve differences were detected in retinal tissue and five in choroidal tissue after 6 to 24 hours of exposure to defocus. Only one of 10 sequenced products could be identified as cytochrome-c oxidase, subunit I. Northern blot analysis confirmed its twofold upregulation after positive lens wear and also changes in four other unknown genes. Finally, it was shown that retinal glucagon mRNA content increased after treatment with positive lenses.

CONCLUSIONS

Visual conditions that induce refractive errors produce changes in gene expression in retina and choroid within 1 day. In line with previous immunohistochemical data, it was found that the amount of glucagon mRNA was upregulated during wearing of positive lenses.

Effects of atropine on refractive development, dopamine release, and slow retinal potentials in the chick

Atropine has previously been found to suppress visually induced myopia both in animals and humans. The mechanism of its action is unclear. We have studied its retinal effects in an in vitro preparation, using the retina-pigment epithelium-choroid complex of the chick eye. In vivo, deprivation myopia was induced by translucent goggles. Atropine solution was injected into the vitreous at two-day intervals. Dopamine release from the retina following atropine injection in vivo and from the in vitro retina preparation was quantified by HPLC-EC. In vitro preparations of the isolated chick retina-pigment epithelium-choroid were superfused with atropine. Light-induced potentials (local ERG), slow standing potentials from the retinal pigment epithelium/neural retina, and extracellular potassium concentrations were recorded. In line with previous findings, intravitreal injections of atropine (25 microg, 250 microg) reduced deprivation myopia in a dose-dependent manner. Atropine increased the release of the neurotransmitter dopamine into the superfusate in vitro at 100-500 microM and into the vitreous in vivo at 250 microg. Before an increase was measured in the vitreous, the retinal dopamine content was elevated. In concentrations equivalent to the intravitreal concentration to suppress myopia in vivo (200-800 microM), atropine induced spreading depression (SD) in the in vitro preparation. In contrast, muscarinic agonists, acetylcholine and pilocarpine, did not induce SD. Atropine reduced the ERG b- and d-wave, led to damped oscillations of RPE potentials, and reversed the ERG c-wave. Atropine suppressed myopia only at doses at which severe nonspecific side effects were observed in the retina. Atropine seems to intrude massively into the vital functions of the retina as indicated by the occurrence of SD. We conclude that atropine, by inducing SD, boosts neurotransmitter release from cellular stores, which may cancel out a presumed retinal signal that controls eye growth and through this, myopia.

Visually induced changes in components of the retinoic acid system in fundal layers of the chick

Eye growth is visually regulated via messengers that are released from the retina. The retina involves a yet unknown algorithm to analyse the projected image so that the appropriate growth rates for the back of the eye are ensured. One biochemical candidate that could act as a growth controller, is retinoic acid (RA). Previous work (Seko, Shimokawa and Tokoro, 1996; Mertz et al., 1999) has shown that retinal and choroidal RA levels are indeed predictably changed by visual conditions that cause myopia or hyperopia, respectively. We have studied in which fundal tissues aldehyde dehydrogenase-2 (AHD2) and retinaldehyde dehydrogenase-2 (RALDH2), enzymes involved in RA synthesis, are expressed and at which levels the effects of vision on RA levels may be controlled. Using Northern blot analysis, we have found that the retinal mRNA level of the AHD2 is up-regulated after 3 days of treatment with negative lenses (negative lenses place the image behind the retina). The abundance of the retinal mRNA of a RA receptor, RAR-beta, was up-regulated already after 6 hr of treatment with positive lenses (positive lenses place the image in front of the retina). The up-regulation persisted for at least 1 week. Finally, we have studied the effects of an inhibitor of RA synthesis, disulfiram, on the visual control of eye growth. We found inhibition of myopia as induced by frosted goggles ('deprivation myopia') but no significant inhibitory effects on refractive errors induced by +7D or -7D lenses. Our results are in line with the hypothesis that RA may play a role in the visual control of eye growth. The RA system differs from a number of other candidates (dopamine, cholinergic agents, opiates) in that it distinguishes between positive and negative defocus, similar to the immediate early gene ZENK (Stell et al., 1999). The exact time kinetics of the changes have still to be worked out since it is possible that the changes in RA relate to already occurring changes in growth rather than to initial steps of the signaling cascade.

Accommodation in the cuttlefish (Sepia officinalis)

We have studied natural accommodation in the eye of six specimens of cuttlefish (Sepia officinalis) as they were fed with fish and shrimp. Using infrared photoretinoscopy, we observed (1) that the resting refractive state of the cuttlefish was emmetropic or slightly hyperopic, (2) that accommodation took place only a fraction of a second before a strike and (3) that accommodation focused selectively only in the frontal visual field while no change in refraction could be measured in the lateral field of view. Accommodation was bilateral and amounted to approximately 5 diopters (the reciprocal of the focal length expressed in meters). Simultaneously, the eyes converged. It appears that, as in most teleost fishes, accommodation in the cuttlefish involves a movement of the crystalline lens perpendicular to the axis of the eye. In histological sections, we observed the position and arrangement of the ciliary muscles, confirming earlier anatomical descriptions, and developed a model of how accommodation could be achieved.

Light- and focus-dependent expression of the transcription factor ZENK in the chick retina

Ocular growth and refraction are regulated by visual processing in the retina. We identified candidate regulatory neurons by immunocytochemistry for immediate-early gene products, ZENK (zif268, Egr-1) and Fos, after appropriate visual stimulation. ZENK synthesis was enhanced by conditions that suppress ocular elongation (plus defocus, termination of form deprivation) and suppressed by conditions that enhance ocular elongation (minus defocus, form deprivation), particularly in glucagon-containing amacrine cells. Fos synthesis was enhanced by termination of visual deprivation, but not by defocus and not in glucagon-containing amacrine cells. We conclude that glucagon-containing amacrine cells respond differentially to the sign of defocus and may mediate lens-induced changes in ocular growth and refraction.

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).

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.

How good is the match between the plane of the text and the plane of focus during reading?

Because results from animal models demonstrate that retinal image defocus is a crucial factor in the visual control of eye growth, we have measured the precision of accommodation during reading at 1 m and at 30 cm distance. A newly developed photorefractor was used to sample both the refraction in the vertical meridian and direction of gaze at 25 Hz. Using these two parameters, a three-dimensional "refraction map" of the visual field was plotted. It showed the optic disc as an area with more myopic refractions and the course of refractions across a visual field of about +/- 25 deg. A special calibration scheme was employed to ensure that the precision of the refractions was 0.2 dpt or better (as estimated from the standard deviations of repeated measurements and the noise in the calibration curve). Twelve young adults (students from the lab) served as subjects. We found considerable inter-individual variability in the off-axis refractions but little variability among repeated measurements in the same subjects. Inter-individual variability reached a minimum in the foveal region. Both myopes wearing their spectacle corrections (n = 6) and emmetropes (n = 6) under-accommodated by about 0.3 D during reading at 30 cm distance but, at 1 m distance, only the emmetropes under-accommodated. Since both refraction groups under-accommodated similarly during reading at close distance, it remains unclear whether the small amount of defocus is critical for their future myopia development. Either accommodation errors differ at earlier times when myopia first appears (as suggested by the literature), or the subjects' eye growth was differently sensitive to defocus, or our simple protocol did not pick up existent differences in accommodation among the two groups.

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.

Mathematical procedures in data recording and processing of pupillary fatigue waves

Spontaneous pupillary behaviour in darkness provides information about a subject's level of vigilance. To establish infrared video pupillography (IVP) as a reliable and objective test in the detection and quantification of daytime sleepiness, the definition of numerical parameters is an important precondition characterising spontaneous pupil behaviour adequately for further statistical procedures. The correct measurement of the pupil size, even if the lid or eyelashes are occluding the pupil, is of particular concern when testing vigilance. In this case many edge points of the pupil are detected and a fitting procedure is described that fits these edge points to a circle and excludes outliers. The first step of data preparation consists of a mathematical artefact management consisting of blink detection and elimination, followed by interpolation. Second, a fast Fourier transformation is carried out for frequencies from 0.0 to 0.8 Hz for each time segment of 82 s. Results are given in absolute and relative power of each frequency band per time segment and mean values over the entire record of 11 min. Third, the changes of the mean pupillary diameter per data window against time are shown graphically. An additional parameter referring to the pupil's tendency to instability, the pupillary unrest index (PUI), is defined by cumulative changes in pupil size based on mean values of consecutive data sequences. These mathematical procedures provide a high level of quality in both data collection and evaluation of IVP as an objective test of vigilance. In a pilot study, the pupillary behaviour of two groups were measured. One group rated themselves as alert (ten men), the other group as sleepy (12 men). The power and PUI were compared using the Mann-Whitney U-test. Both parameters show significant differences between the two groups.

Flicker parameters are different for suppression of myopia and hyperopia

Axial eye growth rates in the chicken are controlled by local retinal image-processing circuits. These circuits quantify the loss of contrast for different spatial frequencies and promote axial eye growth rates in correlation with the amount of retinal image degradation ("deprivation myopia"). They also distinguish whether the plane of focus lies in front of or behind the retina. How the sign of defocus is detected still remains unclear. Cues from chromatic aberration are not important. In an attempt to isolate retinal circuits controlling the development of myopia or hyperopia, young chickens were raised in flickering light of different frequencies (12 and 6 Hz) and duty cycles (4-75%) produced by rotating chopper disks. The effects of flickering light on refractive errors and change in axial growth rates induced by translucent occluders or defocusing lenses were measured by infrared retinoscopy and A-scan ultrasound, respectively. Retinal electrical activity was evaluated by flicker ERG after matching flicker parameters and stimulation brightness at retinal surface. Changes in retinal and vitreal dopamine content caused by flicker in occluded and normal eyes were determined by HPLC-ECD. Strikingly, suppression of myopia occurred for similar flicker parameters, whether induced by translucent occluders ("deprivation") or negative lenses ("defocus"). The degree to which myopia was suppressed was correlated with the duration of flicker dark phase and with the ERG amplitude. In contrast, suppression of hyperopia did not correlate with these parameters. We conclude that two different retinal circuits with different temporal characteristics are involved in the processing of hyperopic defocus/deprivation and of myopic defocus, the first one dependent on flicker ERG amplitude. However, we did not find any correlation between the rate of dopamine release and the degree of inhibition of deprivation myopia in flickering light.

Measurement of astigmatism by automated infrared photoretinoscopy

BACKGROUND

There are basically two possibilities to measure cylindrical refractive errors by eccentric photorefraction. The first is to determine the size and the tilt of the light crescent in the subject's pupil. Sphere, cylinder, and axis can be obtained from two pictures with the knife edge at two different orientations by using equations derived by Wesemann et al. In natural eyes, the procedure has limitations because undetermined factors (not considered in the theory) affect size, shape, and intensity of the light crescent. A second possibility is to perform eccentric photorefraction separately in at least three different meridians.

METHODS

We have tested the power of the second possibility. The three critical parameters (sphere, cylinder, and axis) were calculated from Euler's law, which describes curvatures (or refractions) at any given angle. The procedure relied only on empirical calibration and not on a theoretical treatment of the optics. Therefore, it was not necessary to identify all factors that determine the path of light.

RESULTS

The procedure compared favorably with subjective refractive (first population: students, age 26-30 years, N = 7 (14 eyes); correlations: sphere, r = 0.983; cylinder, r = 0.867; axis, r = 0.935) and with a Canon R-1 Autorefractor (second population: children, age 4-14 years, N = 48 (96 eyes); correlations: sphere, r = 0.955; cylinder, r = 0.600; axis, r = 0.846).

CONCLUSIONS

Because it is fast, the technique may be suitable for screening in children. The refractions in the different meridians are performed in real time (25 to 30 Hz) and a single reading (the average from 4-6 refractions in each of the 6 meridians) is obtained in 1-2 s. It constitutes a major improvement to commercially available videorefractors which use measurements only in two meridians in conjunction with the formula by Wesemann et al., although it is still not precise enough to permit spectacle prescription.

Concentrations of biogenic amines in fundal layers in chickens with normal visual experience, deprivation, and after reserpine application

Previous experiments in chickens have shown that dopamine released from the retina may be one of the messengers controlling the growth of the underlying sclera. It is also possible, however, that the apparent relationship between dopamine and myopia is secondary and artifactual. We have done experiments to assess this hypothesis. Using High Pressure Liquid Chromatography with electrochemical detection (HPLC-ED), we have asked whether changes in dopamine metabolism are restricted to the local retinal regions in which myopia was locally induced. Furthermore, we have measured the concentrations of biogenic amines separately in different fundal layers (vitreous, retina, choroid, and sclera) to find out how changes induced by "deprivation" (= removal of high spatial frequencies from the retinal image by translucent eye occluders which produce "deprivation myopia") are transmitted through these layers. Finally, we have repeated the deprivation experiments after intravitreal application of the irreversible dopamine re-uptake blocker reserpine to see how suppression of dopaminergic transmission affects these changes. We found that (1) Alterations in retinal dopamine metabolism were indeed restricted to the retinal areas in which myopia was induced. (2) The retina was the major source of dopamine release with a steep gradient both to the vitreal and choroidal side. Vitreal content was about one-tenth, choroidal content about one-third, and scleral content about one-twentieth of that of the retina. (3) There was a drop by about 40% in vitreal dopamine, DOPAC (3,4-dihydroxyphenylacetic acid) and HVA (homovanilic acid) concentrations following deprivation which occurred already at a time where little changes could yet be seen in their total retinal contents. (4) Choroidal and scleral dopamine levels were not affected by deprivation, indicating that other messengers must relay the information to the sclera. (5) A single intravitreal injection of reserpine lowered dopamine and HVA levels in retina and vitreous for at least 10 days in a dose-dependent fashion and diminished or suppressed further effects of deprivation on these compounds. DOPAC levels continued to change upon deprivation even after reserpine injection (Fig. 3). Our results suggest that the release rates of dopamine from retinal amacrine cells can be estimated from vitreal dopamine concentrations; furthermore, they are in line with the hypothesis that there is an inverse relationship between dopamine release and axial eye growth rates. Although our experiments do not ultimately prove that dopamine has a functional role in the visual control of eye growth, they are in line with this notion.

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.

A dose related response of 6-OHDA on chicken spectral sensitivity and oscillatory potentials of recording electroretinograms

OBJECTIVE

To further study the contribution of dopamine system to the local growth controlling mechanisms, a dose related response of 6-hydroxydopamine (6-OHDA) was studied by recording electroretinograms (ERGs).

METHODS

The spectral sensitivity of the b-waves and spectral efficiency function of oscillatory potentials (OPs) including OP1, OP2 and OP3 in 4 different doses group were measured. The effect of ascorbate that must be contained in solution of 6-OHDA was first tested with the spectral sensitivity of the b-waves and a correlation between response of the OPs and age, as well as a difference in both own eyes was analyzed for determining an intra-subject and inter-subject variance.

RESULTS

An enhanced response was found in OP1, OP2 with doses of 175 micrograms and OP3 with dose of 150 micrograms, and the effect of OPs was mainly in wavelength from 620 nm to 480 nm. No significant increase was found in the spectral sensitivity of the b-waves. The dose 200 micrograms seemed to be toxic to the retina estimated by both spectral sensitivity of the b-waves and spectral efficiency function of the OPs.

CONCLUSIONS

The dose 175 micrograms and 150 micrograms of 6-OHDA yielded an effect on the chicken retina.

Melatonin and deprivation myopia in chickens

Chicken eyes elongate and become myopic if they are covered with translucent diffusors which degrade the retinal image ('deprivation myopia'). Since it has been shown that dopamine D2/D4 receptors (which mediate inhibition of melatonin synthesis) are also implicated in deprivation myopia, we have studied the role of melatonin in the visual control of eye growth. We have found that (1) diurnal melatonin rhythms and melatonin content in the retina are unchanged during deprivation myopia development despite the breakdown of both diurnal growth rhythms of the eye and diurnal rhythms in retinal dopamine metabolism, (2) diurnal melatonin rhythms and melatonin content in the retina remain unchanged after application of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) and presumably also after 6-hydroxydopamine (6-OHDA) application which both have a suppressive effect on deprivation myopia and (3) deprivation myopia was slightly reduced in both eyes after unilateral intravitreal injection of melatonin, despite that deprivation myopia is based on a mechanism intrinsic to the eye. We conclude that melatonin is not involved in the retinal signaling pathway translating visual experience to deprivation myopia.

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).

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.

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.

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.

Constant light affects retinal dopamine levels and blocks deprivation myopia but not lens-induced refractive errors in chickens

Chickens were raised with either translucent occluders or lenses, both under normal light cycles (12-h light/12-h dark) and in constant light (CL). Under normal light cycles, eyes with occluders became very myopic, and eyes with lenses became either relatively hyperopic (positive lenses) or myopic (negative lenses). After the treatment, retinal dopamine (DA), DOPAC, and serotonin levels were measured by high-pressure liquid chromatography (HPLC-EC). A significant drop in daytime retinal DOPAC (-20%) was observed after 1 week of deprivation, and in both DOPAC (-40%) and DA (-30%) after 2 weeks of deprivation. No changes in retinal serotonin levels were found. Retinal DA or DOPAC content remained unchanged after 2 or 4 days of lens wearing even though the lenses had already exerted their maximal effect on axial eye growth. When the chickens were raised in CL, development of deprivation myopia was reduced (8 days CL) or entirely blocked (13 days CL). Lens-induced changes in eye growth were not different after either 6 or 11 days in CL, compared to animals raised in a normal light cycle. Thirteen days of CL resulted in a dramatic reduction of DA and DOPAC levels, but serotonin levels were also lowered. The results suggest that lens-induced changes in refraction may not be dependent on dopaminergic pathways whereas deprivation myopia requires normal diurnal DA rhythms to develop.

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.

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.

[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.

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)

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)