Intraocular pressure fluctuations of growing chick eyes are suppressed in constant light conditions.

We report measurements of intraocular pressure (IOP) in growing domestic chicks at 12 h intervals, with three different lighting conditions. One group of chicks was raised in 12 h light and 12 h darkness (N), another in constant light (CL), and the third group was initially exposed to CL for three weeks then returned to N for either one week or four weeks (CLN). Pressures were measured in the middle of the light and dark periods (noon and midnight) for N and CLN birds, and at corresponding 12 h intervals for CL birds (also noon and midnight). The IOP of N chicks fluctuated from a light period average value of 25 mm Hg ( ±1.3 SD), to a dark period average value of 17.5 mm Hg ( ±1.1 SD mm Hg; P < 0.0001). These pressures were established by 4 days of age. At 7 weeks, (N) IOP continued to fluctuate: light values were 21.7 mm Hg (±1.2 SD), and dark values were 18.3 mm Hg ( ±0.7 SD). The IOP of CL birds did not fluctuate, remaining steady at 17 mm Hg ( ±1.4 SD). Chicks exposed to CL for 3 weeks required more than one week in N to re-establish (N) IOP values. We conclude that IOP fluctuates in hatchling chicks under N light conditions, that fluctuation is suppressed in CL light conditions, and that IOP recovery from 3 weeks suppression in CL requires more than one week in N light conditions.

Lack of oblique astigmatism in the chicken eye.

Primate eyes display considerable oblique off-axis astigmatism which could provide information on the sign of defocus that is needed for emmetropization. The pattern of peripheral astigmatism is not known in the chicken eye, a common model of myopia. Peripheral astigmatism was mapped out over the horizontal visual field in three chickens, 43 days old, and in three near emmetropic human subjects, average age 34.7years, using infrared photoretinoscopy. There were no differences in astigmatism between humans and chickens in the central visual field (chicks -0.35D, humans -0.65D, n.s.) but large differences in the periphery (i.e. astigmatism at 40° in the temporal visual field: humans -4.21D, chicks -0.63D, p<0.001, unpaired t-test). The lack of peripheral astigmatism in chicks was not due to differences in corneal shape. Perhaps related to their superior peripheral optics, we found that chickens had excellent visual performance also in the far periphery. Using an automated optokinetic nystagmus paradigm, no difference was observed in spatial visual performance with vision restricted to either the central 67° of the visual field or to the periphery beyond 67°. Accommodation was elicited by stimuli presented far out in the visual field. Transscleral images of single infrared LEDs showed no sign of peripheral astigmatism. The chick may be the first terrestrial vertebrate described to lack oblique astigmatism. Since corneal shape cannot account for the difference in astigmatism in humans and chicks, it must trace back to the design of the crystalline lens. The lack of peripheral astigmatism in chicks also excludes a role in emmetropization.

Visual accommodation and active pursuit of prey underwater in a plunge-diving bird: the Australasian gannet.

Australasian gannets (Morus serrator), like many other seabird species, locate pelagic prey from the air and perform rapid plunge dives for their capture. Prey are captured underwater either in the momentum (M) phase of the dive while descending through the water column, or the wing flapping (WF) phase while moving, using the wings for propulsion. Detection of prey from the air is clearly visually guided, but it remains unknown whether plunge diving birds also use vision in the underwater phase of the dive. Here we address the question of whether gannets are capable of visually accommodating in the transition from aerial to aquatic vision, and analyse underwater video footage for evidence that gannets use vision in the aquatic phases of hunting. Photokeratometry and infrared video photorefraction revealed that, immediately upon submergence of the head, gannet eyes accommodate and overcome the loss of greater than 45 D (dioptres) of corneal refractive power which occurs in the transition between air and water. Analyses of underwater video showed the highest prey capture rates during WF phase when gannets actively pursue individual fish, a behaviour that very likely involves visual guidance, following the transition after the plunge dive's M phase. This is to our knowledge the first demonstration of the capacity for visual accommodation underwater in a plunge diving bird while capturing submerged prey detected from the air.

Photorefraction of eyes: history and future prospects.

A brief history of photorefraction, i.e., the refraction of eyes by photography or computer image capture, is given. The method of photorefraction originated from an optical scheme for secret communication across the Berlin wall. This scheme used a lens whose focus about infinity was modulated by a movable reflecting surface. From this device, it was recognized that the vertebrate eye was such a reflector and that its double-pass pointspread could be used to compute its degree of defocus. Subsequently, a second, totally independent invention, more accurately termed "photoretinoscopy," used an eccentric light source and obtained retinoscopic-like images of the reflex in the pupil of the subject's eyes. Photoretinoscopy has become the preferred method of photorefraction and has been instantiated in a wide variety of devices used in vision screening and research. This has been greatly helped by the parallel development of computer and digital camera technology. It seems likely that photorefractive methods will continue to be refined and may eventually become ubiquitous in clinical practice.

Role of the pineal gland in ocular development of the chick in normal and constant light conditions.

PURPOSE: To evaluate the role of the pineal gland in development of the chick eye in normal and constant light (CL) conditions. METHODS: Chicks (Gallus gallus) were raised in either a 12-hour light-dark cycle (12L/12D) or in CL, with or without opaque, removable hoods that covered the top of the head for 12 hours each day. An additional group was raised with opaque eye occluders over the right eye for 12 hours daily. Half of the chicks in each group had their pineal glands surgically removed at 3 to 6 days after hatching. Corneal curvature was measured with keratometry, anterior chamber depth with ultrasound, and refraction with infrared photoretinoscopy. RESULTS: Pinealectomy does not affect the development of the chick eyes either in 12L/12D or CL. Covering the right eyes provided the same amount protection against CL's effects on the corneal curvature of both eyes, with or without pinealectomy. Pinealectomized chicks were not protected from CL's effects by 12L/12D head covers. A similar pattern of responses was obtained for refraction and anterior chamber depth. CONCLUSIONS: Although 12L/12D covering of the pineal gland can protect chick eyes from CL's effects (corneal flattening, shallowing of the anterior chamber, and hyperopia), the pineal gland does not appear to be necessary for normal growth in 12L/12D conditions, and its absence does not affect eye growth in CL conditions, with or without hoods or occluders. Pinealectomy does not influence the protection of an eye exposed to CL that is afforded by covering the other eye in a 12L/12D cycle.

Changes in the refractive state during prey capture under low light in the nocturnal cardinalfish Apogon annularis.

Many nocturnal and crepuscular fish use vision to feed and function under low light levels. However, little is known about their ability to accommodate or their visual acuity under these light levels. We used Infrared Photoretinoscopy to track the refractive state of the eye during prey capture under low light in Apogon annularis, a nocturnal reef fish. Anatomical measurements of the eyes allowed calculations of visual acuity. Changes in the refractive state were observed in approximately 75% of the prey capturing strikes, preceding the strikes by 30 ms. These changes were rare between strikes or when prey was absent. Anatomical measurements indicated that the number of photo-detection units in a retinal image greatly exceeded the minimal number needed to detect prey. We conclude that nocturnal vision in A. annularis is sufficiently sensitive to allow accommodation during prey capture.

Growth of the cornea from infancy to adolescence.

The intraocular distance and iris diameter of children and young adolescents were measured, with the aid of a measuring microscope, from photographs of their faces. True intraocular distance was measured with an intraocular caliper at the same time that the photographs were taken. These data were then compiled and horizontal visible iris diameters (HVIDs) were calculated. An equation was derived from the optics of the Gullstrand model eye to calculate horizontal corneal diameter (HCD) from HVID. Comparisons of HVIDs revealed no significant correlation with age in either a regression plot of cross-sectional data for subjects aged 1 month to 1 year, or all subjects whose ages ranged from 1 month to adolescence. Additional longitudinal data for 13 individuals, who had been photographed as both an infant (mean = 3.4 months) and as an older child or adolescent (mean = 8.6 years), were then compiled and HVIDs for these subjects at two different ages were compared. A Wilcoxon signed rank test revealed a small but significant amount of growth, 0.318 mm (p-value = 0.013), in the HVIDs over a mean age difference of 8.3 years for individuals measured twice during their lifetimes. The regression equation for this growth was: HVID = 10.52 (+/-0.095 S.E.) + 0.0305 (+/-0.014) x Age (years). From a comparison of data from earlier literature and our own measurements, we conclude that, after birth, the fastest growth of the cornea must occur during the first few months of life.

Allometry and scaling of wave aberration of eyes.

The scaling of root mean square (RMS) wave aberration in an isometrically growing eye is investigated, along with changes due to measurements made at different relative pupil sizes. It is found that, relative to an initial state, if an eye expands in all directions by the factor k, and the wave aberration is then measured at a relative pupil size which has changed over the pupil size used for the original measurement by a factor b, the new wave aberration will be increased or decreased by a factor kb(n), where n is the exponent relating RMS wave aberration, to pupil radius, r, in the equation: RMS=qr(n) in the initial eye. This implies that, if wave aberration is measured in a growing eye with a constant measurement pupil size, the measured RMS will decrease by the factor 1/k(n-1).

Accommodative state of young adults using reading spectacles.

We examined the accommodative state of young adults wearing +2D and +3D reading spectacles under normal conditions and with the elimination of accommodative cues. Subjects' refractions were measured with an infrared PowerRefractor. Power of the vertical meridian was recorded for subjects viewing far and near targets in free space and through a Badal lens apparatus with and without reading spectacles. Additionally, refractive measurements were taken after subjects wore +2D reading spectacles for 30 min (post-adaptation). In free viewing and viewing through the Badal lens, subjects uniformly over-accommodated relative to the target while wearing reading spectacles (i.e., with the spectacles, they focused at a plane in front of the target). Subjects in the first post-adaptation test showed no significant difference in accommodation between viewing a near target with and without +2D spectacles after having read with them for 30 min, though they had without post-adaptation. Subjects in the second post-adaptation test were not significantly differently accommodated before and after reading when binocularly viewing a near target with +2D reading spectacles. The results imply that no adaptation of the subjects' accommodative postures while viewing visual targets occurred as a result of a 1/2 h near work task with the spectacles. The over-accommodation of subjects using reading spectacles while they are performing visual tasks shows the necessity of measurement if their true accommodative posture is to be determined.

Compensation of corneal horizontal/vertical astigmatism, lateral coma, and spherical aberration by internal optics of the eye.

Both the anterior surface of the cornea and the internal optics (the posterior cornea, crystalline lens) contribute to the aberration of a wavefront passing through the eye. Artal, Guirao, Berrio, and Williams (2001) reported that the wavefront aberrations produced by the internal optics offset, or compensate for, the aberrations produced by the cornea to reduce ocular wavefront aberrations. We have investigated the wavefront aberrations of the cornea, internal optics, and complete eye on both the population and individual level to determine which aberrations are compensated and probable paths leading to that compensation. The corneal and ocular aberrations of 30 young subjects at relaxed accommodation were measured with the Topcon Wavefront Analyzer, which simultaneously measures refraction, corneal topography (videokeratoscope), and wavefront aberrations (Hartmann-Shack sensor). We found strong evidence for compensation of horizontal/vertical (H/V) astigmatism (Zernike term Z5) lateral coma (Z8) and spherical aberration (Z12). H/V astigmatism compensation is scaled for each individual, suggesting that it is actively determined by a fine-tuning process. Spherical aberration shows no individual compensation, suggesting that is a passive result of genetically determined physiology. Lateral coma shows individually scaled compensation, some of which may be attributable to eccentricity of the fovea.

The allometry and scaling of the size of vertebrate eyes.

We compiled data from the literature and colleagues to examine the relationship between eye axial length and body weight for vertebrates as well as birds, mammals, reptiles, and fishes independently. After fitting the data to logarithmic and semi-logarithmic models, we found that axial length of vertebrate eyes does obey a conventional logarithmic relationship with body weight rather than a semi-logarithmic relationship as suggested by the results of previous studies. The regression slopes and intercepts appear to be characteristic of various animal groups. The axial length of the eye is largest in birds and primates, smaller in other mammals (especially rodents) and reptiles, and widely varying in fishes.

Measurement of refractive state and deprivation myopia in two strains of mice.

PURPOSE: The mouse eye has a bright retinal image (f/number <1) but low optical quality (visual acuity about 0.5 cpd) that may render emmetropization unnecessary. However, this species is potentially a powerful model to study eye growth and myopia because its genome can be readily manipulated and has been completely sequenced. We have investigated how precisely eyes of mice can be refracted and tested whether deprivation myopia can be induced by frosted diffusers. METHODS: An automated eccentric infrared photorefractor was adapted to refract eyes of two mouse strains--C57BL/6 (B6) and DBA/2 (D2)--during Tropicamide cycloplegia without anesthesia. Axial lengths were measured in highly magnified video images of freshly excised eyes. Plastic hemispherical diffusers were applied between postnatal days and 29 and left attached for 7 or 14 days. RESULTS: (1) Trial lenses ranging from +10 to -10 D produced high correlations between the brightness slope in the pupil and applied lens power (r = 0.81 and r = 0.87), demonstrating reliable refraction. Five repeated measures in 12 eyes showed an average standard deviation of 3.0 D, equivalent to an axial length change <10 microm (derived from schematic eye modeling). (2) Deprivation produced a significant shift toward myopia, relative to untreated eyes, but only after 14 days and only in B6 mice (p = 0.02 with or p = 0.00038 without one outlier; N = 9). In contrast, DBA/2J were unaffected by occlusion, perhaps due to mutations that target eye, lens, or anterior segment. (3) Both eyes of untreated animals often had axial lengths that differed markedly. Surprisingly, we detected no significant correlation between refractive error and axial length after treatment. CONCLUSIONS: The infrared refraction technique is sufficiently sensitive to resolve equivalent changes in axial length of only +/- 10 microm in alert mice. Prolonged occlusion produces a significant myopic shift in B6 mice, but not in D2 mice. Even among isogenic B6 mice, the response is variable for reasons that presumably trace back to subtle developmental, environmental, and technical factors.

The effects of constant and diurnal illumination of the pineal gland and the eyes on ocular growth in chicks.

PURPOSE: To investigate the effects of constant or 12-hour cyclic illumination of the pineal gland and the eyes on the growth of the chick eye. METHODS: Chicks (Gallus gallus, Cornell K Strain) were raised either under a 12-hour light-dark cycle of normal light or under constant light, with or without opaque removable hoods that covered the top of the head for 12 hours each day. A second group of chicks was raised under constant light with opaque eye covers that were worn on either both eyes or only the right eye for 12 hours each day. Chicks were placed in the experimental conditions on the third day after hatching and raised for 3 weeks. RESULTS: Pineal gland hoods and eye covers worn 12 hours a day significantly (P < 0.0001) protected the chicks from hyperopia under constant-light conditions. They also reduced the flattening of the cornea caused by constant light. Most striking was the protection afforded the uncovered eye from constant light's effects by the periodic covering of the opposite eye. CONCLUSIONS: A diurnal light-dark rhythm presented to one of three photosensitive organs (the pineal gland and both eyes) can protect the eyes from the effects of constant light. This is most probably due to the maintenance of a melatonin rhythm in the organ receiving the diurnal light rhythm.

Corneal power and underwater accommodation in great cormorants (Phalacrocorax carbo sinensis).

In great cormorants (Phalacrocorax carbo sinensis), corneal refractive powers, determined by photokeratometry, ranged between 52.1 diopters (52.1 D) and 63.2 D. Photorefractive reflexes, determined by infrared video photorefraction, indicated that in voluntary dives the cormorants accommodate within 40-80 ms of submergence and with myopic focusing relative to the photorefractor attained when prey was approximately one bill length from the plane of the eye. Underwater, the pupils were not constricted and retained diameters similar to those in air. These results support previously reported capacities of lenticular changes in amphibious birds yet do not fully correspond with earlier reports in terms of the coupling of iris constriction with accommodation, and time course.

High order wave aberration of eyes.

This paper is concerned with the means and distribution of the wavefront variances and their root mean squares (RMS) of normal eyes, as measured by various techniques and computed for various pupil sizes. Using data from a subjective crossed cylinder aberroscope [Howland and Howland J. Opt. Soc. Am. A 67(1977)1508] it was found that the logarithms of the wavefront variances are approximately normally distributed. A comparison of data from this subjective study with that of six other studies using a variety of techniques of wavefront measurement showed that the relationship between RMS wavefront deviations of high order aberrations (third order polynomial terms and above) obeyed the relationship: log (RMS) (microm) = -1.918 +/- 0.048 SE + 3.023 +/- 0.111 x log (pupil radius) (mm), with R2 = 0.971.

Normal values and standard deviations for pupil diameter and interpupillary distance in subjects aged 1 month to 19 years.

Normal values of pupil diameters and interpupillary distances (PDs) were measured in a population of 1311 subjects (in 4294 visits) ranging from 1 month of age to slightly over 19 years of age. Subjects in this study were recruited from birth announcements in a local newspaper for a developmental vision project. Pupil sizes were measured photographically when the corneas were illuminated by 15.9 +/- 0.5 lux ambient illumination (i.e. under mesopic conditions). Interpupillary distance was measured with an interocular distance rule while the subject fixated an object at 0.66 m distance. These PD measurements were corrected for systematic measurement errors and to an infinite viewing distance using radii of ocular rotation based on age-dependent axial lengths. Means and S.D. were calculated for age, pupil diameter and PD for each 1-year group of male and female subjects. The second order regression equation for average pupil size as a function of age was determined: [males pupil diameter (in mm) = 5.83 +/- 0.181*age in years - 0053*age in years2, r2 = 0.897; female pupil diameter = 5.40 + 0.285*age in years - 0.0109*age in years2, r2 = 0.945]. The dierence between male and female pupil sizes (mean male - female = 0.13 mm) was marginally not significant (p < 0.054). The average corrected PDs as a function of age were found to approximate another second-order regression equation: (males PD = 43.36 + 1.663*age in years - 0.034*age in years2, r2 = 0.986; females PD = 41.76 + 1.891 *age in years - 0.052*age in years2, r2 = 0.986). Male PD was wider than female PD by an average of 1.58 mm (p < 0.0003). As expected, the results of this study were similar to a preliminary investigation conducted by Thunyalukul et al. [Invest. Ophthalmol. Vis. Sci. 37 (1996) S731] on a portion of the present data set, and also very similar to data from another study of comparable racial composition using a different measurement method [Pryor, Pediatrics 44 (1969) 973]. It was concluded that pupil diameter and PD increase more gradually than axial length of the eye in the first few years of life. The normal values and S.D. for both pupil size and PD determined in this study have important clinical implications as well as applications in the optical industry.