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One of the least studied eyes of any beetle taxon are those of the scarabaeoid family Passalidae. Some members of this family of around 600 species worldwide are known to have superposition eyes (Aceraius grandis; A. hikidai) while others have apposition eyes (Cylindrocaulus patalis; Ceracupes yui). In C. yui of nearly 3 cm body length (this paper) the retinal layer is very thin and occupies approximately half of an ommatidium's total length, the latter amounting to 284 and 266 μm in the respective dorsal and ventral eye regions. The two eye regions are almost completely separated by a prominent cuticular canthus, a feature usually associated with the presence of a tracheal tapetum, a clear-zone between dioptric and light-perceiving structures and a regular array of smooth facets. In C. yui the facets are smooth (but not very regular) and a tracheal tapetum and a clear-zone are absent. The rhabdoms, formed by 8-9 retinula cells, are complicated, multilobed structures with widths and lengths of around 15 and 80 μm, respectively. The combination of some superposition and mostly apposition eye features, e.g., extensive corneal exocones, relatively small number of ommatidia, absence of a clear-zone and tracheal bush, suggest an adaptation of this species' eye to the fossorial lifestyle of C. yui, and, thus, a manifestation of the passalid eye's plasticity., Competing Interests: Declaration of competing interest The authors declare that there is no conflict of interest., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Only a few studies have examined the central visual system of Solifugae until now. To get new insights suitable for phylogenetic analysis we studied the R-cell (or retinula cell) projections and visual neuropils of Galeodes granti using various methods. G. granti possesses large median eyes and rudimentary lateral eyes. In this study, only the R-cells and neuropils of the median eyes were successfully stained. The R-cells terminate in two distinct visual neuropils. The first neuropil is located externally to the protocerebrum directly below the retina, the second neuropil lies in the cell body rind of the protocerebrum, and immediately adjacent is the arcuate body. This layout of the median eye visual system differs from Arachnopulmonata (Scorpiones + Tetrapulmonata). However, there are several similarities with Opiliones. In both, (1) the R-cells are connected to a first and second visual neuropil and not to any other region of the brain, (2) the first neuropil is not embedded in the cell body rind of the protocerebrum, it is rather external to the protocerebrum, (3) the second visual neuropil is embedded in the cell body rind, and (4) the second neuropil abuts the arcuate body. These findings may provide important new characters for the discussion on arachnid phylogeny., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Despite decades of attempts to create a model for predicting gaze locations by using saliency maps, a highly accurate gaze prediction model for general conditions has yet to be devised. In this study, we propose a gaze prediction method based on head direction that can improve the accuracy of any model. We used a probability distribution of eye position based on head direction (static eye-head coordination) and added this information to a model of saliency-based visual attention. Using empirical data on eye and head directions while observers were viewing natural scenes, we estimated a probability distribution of eye position. We then combined the relationship between eye position and head direction with visual saliency to predict gaze locations. The model showed that information on head direction improved the prediction accuracy. Further, there was no difference in the gaze prediction accuracy between the two models using information on head direction with and without eye-head coordination. Therefore, information on head direction is useful for predicting gaze location when it is available. Furthermore, this gaze prediction model can be applied relatively easily to many daily situations such as during walking., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The eye is an important sense organ for teleost species but can vary greatly depending on the adaption to the habitat, environment during ontogeny and developmental stage of the fish. The eye and retinal morphology of eight commonly caught trawl bycatch species were described: Lepidotrigla mulhalli; Lophonectes gallus; Platycephalus bassensis; Sillago flindersi; Neoplatycephalus richardsoni; Thamnaconus degeni; Parequula melbournensis; and Trachurus declivis. The cone densities ranged from 38 cones per 0.01 mm(2) for S. flindersi to 235 cones per 0.01 mm(2) for P. melbournensis. The rod densities ranged from 22800 cells per 0.01 mm(2) for L. mulhalli to 76634 cells per 0.01 mm(2) for T. declivis and potential visual acuity (based on anatomical measures) ranged from 0.08 in L. gallus to 0.31 in P. melbournensis. Higher rod densities were correlated with maximum habitat depths. Six species had the regular pattern of four double cones arranged around a single cone in the photoreceptor mosaic, while T. declivis had only rows of double cones. P. melbournensis had the greatest potential ability for detecting fine detail based on eye anatomy. The potential visual acuity estimates and rod densities can be applied to suggest the relative detection ability of different species in a commercial fishing context, since vision is a critical sense in an illuminated environment for perceiving an oncoming trawl., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Among the oldest fossil crustaceans are those of the Late Cambrian (Furongian 499 ± 0.3-488.3 ± 1.7 Ma) of Västergötland, central Sweden and the lower Ordovician (Tremadocian 488.3 and 478.6 Ma) of the island of Őland. These are three-dimensionally preserved in nodules from the so called 'stinkstone' ('Orsten') limestone. 'Orsten'-like fossils represent tiny, often meiobenthic organsisms (Haug, Maas, & Waloszek, 2009) smaller than 2mm, which mainly were arthropods, especially crustaceans close to the stemline. As a result of phosphatisation, hairs, bristles and even cellular structures up to 0.3 μm are preserved (Walossek, 1993), especially compound eyes, as typical for all visually orientated crustaceans (Schoenemann et al., 2011). We show a miniscule prototype of a compound eye (∼40 μm) in a small crustacean, which lived almost half a billion years ago. The eye is close to but comfortably established above being limited in its resolving power by diffraction, but it is too small to be an apposition eye, normally regarded as the basal form of all compound eyes, as is found in bees, dragonflies, crustaceans and many other arthropods still living today. The facets of this compound eye are ∼8 μm in size, the surface structure indicates the relicts of a tiny lens covering each facet. In order to work functionally and to ensure that that diffraction and waveguide problems were avoided, it seems reasonable to suppose that the compound eye consisted of visual units, each with a single photoreceptor cell directly below a weak lens for capturing and slightly focusing the light. The entire unit has a diameter similar to that of a normal sensory cell as found in compound eyes. Thus, the early compound eye analysed here may be interpreted as a prototype representing the earliest stages of the evolution of crustacean compound eyes., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

This study aimed to investigate the changes in ocular shape and relative peripheral refraction during the recovery from myopia produced by form deprivation (FD) and hyperopic defocus. FD was imposed in six monkeys by securing a diffuser lens over one eye; hyperopic defocus was produced in another six monkeys by fitting one eye with -3D spectacle. When unrestricted vision was re-established, the treated eyes recovered from the vision-induced central and peripheral refractive errors. The recovery of peripheral refractive errors was associated with corresponding changes in the shape of the posterior globe. The results suggest that vision can actively regulate ocular shape and the development of central and peripheral refractions in infant primates., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

The time course of elongation and recovery of axial length associated with a 30min accommodative task was studied using optical low coherence reflectometry in a population of young adult myopic (n=37) and emmetropic (n=22) subjects. Ten of the 59 subjects were excluded from analysis either due to inconsistent accommodative response, or incomplete anterior biometry data. Those subjects with valid data (n=49) were found to exhibit a significant axial elongation immediately following the commencement of a 30min, 4 D accommodation task, which was sustained for the duration of the task, and was evident to a lesser extent immediately following task cessation. During the accommodation task, on average, the myopic subjects exhibited 22±34μm, and the emmetropic subjects 6±22μm of axial elongation, however the differences in axial elongation between the myopic and emmetropic subjects were not statistically significant (p=0.136). Immediately following the completion of the task, the myopic subjects still exhibited an axial elongation (mean magnitude 12±28μm), that was significantly greater (p<0.05) than the changes in axial length observed in the emmetropic subjects (mean change -3±16μm). Axial length had returned to baseline levels 10min after completion of the accommodation task. The time for recovery from accommodation-induced axial elongation was greater in myopes, which may reflect differences in the biomechanical properties of the globe associated with refractive error. Changes in subfoveal choroidal thickness were able to be measured in 37 of the 59 subjects, and a small amount of choroidal thinning was observed during the accommodation task that was statistically significant in the myopic subjects (p<0.05). These subfoveal choroidal changes could account for some but not all of the increased axial length during accommodation., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Vision plays a major role in many spiders, being involved in prey hunting, orientation or substrate choice, among others. In Misumena vatia, which experiences morphological color changes, vision has been reported to be involved in substrate color matching. Electrophysiological evidence reveals that at least two types of photoreceptors are present in this species, but these data are not backed up by morphological evidence. This work analyzes the functional structure of the eyes of this spider and relates it to its color-changing abilities. A broad superposition of the visual field of the different eyes was observed, even between binocular regions of principal and secondary eyes. The frontal space is simultaneously analyzed by four eyes. This superposition supports the integration of the visual information provided by the different eye types. The mobile retina of the principal eyes of this spider is organized in three layers of three different types of rhabdoms. The third and deepest layer is composed by just one large rhabdom surrounded by dark screening pigments that limit the light entry. The three pairs of secondary eyes have all a single layer of rhabdoms. Our findings provide strong support for an involvement of the visual system in color matching in this spider., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Purpose: Defocus imposed to the periphery of the visual field can affect the development of foveal/central refractive errors. To make use of this observation, lenses can be designed to reduce myopia progression, but it is important to know which power profiles of the lenses are most effective. We have studied this question in chickens., Methods: Sixty male white leghorn chickens were used. From day 7 after hatching, they were treated for 5 days either with full field -7D or +7D lenses, with -7D lenses with a 4mm central hole, with hemi-field lenses of the same power, or with two different types of radial refractive gradient (RRG) lenses with increasing positive power from the center to the periphery, which were designed by Rodenstock GmbH, Munich, Germany. A macro file was written for "ImageJ" to trace and average the outlines of several excised eyes after treatment. Shapes of fellow control eyes and lens-treated eyes were compared in the horizontal and vertical meridians. Refractions were determined at -45°, 0°, and 45° over the horizontal visual field, at the beginning and at the end of experiments, using automated infrared photoretinoscopy., Results: (1) Eye length, as determined by the new automated eye shape tracing technique, was well correlated with A-scan ultrasound data. (2) The effects of previously tested lens designs were reproduced with the new tracing technique. Full field lenses were by far the most effective (-7D: external axial length +0.24mm with an increase in eye volume of about 6%, +7D: -0.08 mm, with a decrease in eye volume of about 2%). Hemi-field lenses and negative lenses with a 4mm central hole induced conspicuous local changes in eye shape. (3) The first type of RRG lenses with a plano zone of about 4mm (equivalent to about ± 12.52° in the visual field for a vertex distance of 5mm) had no apparent effect on central refractions but induced small hyperopic shifts in the periphery, more significant in the temporal retina (+1.70 ± 1.70 D, p<0.001, paired t-test to untreated fellow eyes). The second type of RRG lenses with a small plano zone of 2mm (equivalent to ± 6.34°) induced peripheral hyperopia but also changed the central refraction (temporal retina +1.50 ± 1.17D, p<0.001, central retina +0.77 ± 1.15 D, p<0.01, nasal retina +1.47±1.35D, p<0.001, paired t-test to untreated control eyes)., Conclusions: In the afoveate chick, RRG lenses have an effect on central refraction and eye growth only if the central plano zone is small (<4mm). For the second type of RRG lens with a central plano zone of about 2mm, inhibitory effects on eye growth were detected in both the center and periphery even though the optical power of the lenses in the periphery was low., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

We have re-investigated the organization of ocelli in honeybee workers and drones. Ocellar lenses are divided into a dorsal and a ventral part by a cusp-shaped indentation. The retina is also divided, with a ventral retina looking skywards and a dorsal retina looking at the horizon. The focal plane of lenses lies behind the retina in lateral ocelli, but within the dorsal retina in the median ocellus of both workers and drones. Ventral retinula cells are ca. 25μm long with dense screening pigments. Dorsal retinula cells are ca. 60μm long with sparse pigmentation mainly restricted to their proximal parts. Pairs of retinula cells form flat, non-twisting rhabdom sheets with elongated, straight, rectangular cross-sections, on average 8.7μm long and 1μm wide. Honeybee ocellar rhabdoms have shorter and straighter cross-sections than those recently described in the night-active bee Megalopta genalis. Across the retina, rhabdoms form a fan-shaped pattern of orientations. In each ocellus, ventral and dorsal retinula cell axons project into two separate neuropils, converging on few large neurons in the dorsal, and on many small neurons in the ventral neuropil. The divided nature of the ocelli, together with the particular construction and arrangement of rhabdoms, suggest that ocelli are not only involved in attitude control, but might also provide skylight polarization compass information., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Eyes are expected to be adapted to the physical characteristics of the visual environment, yet previous analyses failed to corroborate this observation. We demonstrate that nocturnal, crepuscular/cathemeral, and diurnal activity patterns occupy distinct areas in morphospace and are identified with high accuracies based on a discriminant analysis of visual performance features. Not only nocturnal and diurnal diel activity patterns are reflected in macroscopic morphology of the eyeball, but also the crepuscular/cathemeral patterns. The eyeball morphology of the latter was believed to be undistinguishable between diurnal and nocturnal types. We show that all three categories can be delineated with high accuracies., (Copyright 2010 Elsevier Ltd. All rights reserved.)

Vertebrate eyes are of the simple or camera type with a single optical system that creates an image on the retina in the back of the eye. There, the visual information is encoded as nervous signals by photoreceptors, processed by retinal neurons, and then sent to the brain via the optic nerve. Surprisingly at first sight, the retinal neurons are located between the lens and the light-sensitive parts of the photoreceptors. The tissue scatters some light, which leads to loss of light and image blur. The inverted retina has, therefore, long been regarded as inferior. Here, we provide evidence that the inverted retina actually is a superior space-saving solution, especially in small eyes. The inverted retina has most likely facilitated the evolution of image-forming eyes in vertebrates, and it still benefits especially small and highly visual species.

The medial eye and optic center of the first nauplius of Dactylopusia (=Dactylopodia) tisboides, a harpacticoid copepod, were reconstructed from serial EM micrographs. Axons from the eye project to a set of matching cartridges defined by glial cells processes, and input is then processed in sequence through two synaptic fields. A single class of local relay neurons provides the main pathway between these, subject to modulatory input from a class of densely stained neurons with distinctive dense terminals. The importance of other outside sources of synaptic input to the second synaptic field indicates that the latter is a major site for integrating the optic input with signals originating elsewhere in the CNS. This accords with physiological data on the shadow response in barnacles, whose visual system is also derived from a naupliar eye. With a body length of ca. 80microns, copepod larvae like that of Dactylopusia are arguably among the smallest functional metazoans with a complex nervous system. Hence they are promising subjects for full reconstruction of neural circuitry at the EM level that could, in principle, reveal where key decision-making functions are localized.

Polarization sensitivity provides animals with information not available in the intensity or spectral domains. We examined the polarotaxis reactions in the epiplanktonic copepod Pontella karachiensis. Polarotaxis reactions were intensity dependent. At intensities corresponding to ambient daylight, P. karachiensis showed an attraction to a polarized light field; while at low intensities, corresponding to nighttime illumination, it showed negative polarotaxis. P. karachiensis's eye contained two classes of photoreceptors, each class with microvilli at orthogonal orientation to the other. P. karachiensis' eye structure can provide information regarding the polarization percentage but is not sufficient to calculate the exact e-vector orientation. The threshold for polarotoxisis response was 20-30%. Animals responded similarly to horizontal and vertical polarization; and also showed negative phototaxis, affected by light polarization. Results suggest that P. karachiensis responds to polarized light analogously to changes in brightness. The dynamic pattern of polarotaxis responses suggests that polarization sensitivity may enable P. karachiensis to detect other planktonic animals.

New material of the lobopodian Luolishania longicruris has been recovered from the Lower Cambrian Chengjiang Lagerstätte, southwest China. The specimens throw new light on several morphological features of the species, including the paired antenniform outgrowths, eyes, head shield, setae and other cuticular projections, as well as the differentiated sclerites, appendages, claws, and lobopod interspaces. L. longicruris shows well developed tagmosis: a distinct head and a trunk divided into two sections. The new data allow a revised comparison with other lobopodians. Miraluolishania haikouensis Liu et al., 2004 is considered to be a junior synonym of L. longicruris Hou and Chen, 1989. Evidence from gut filling and specialized morphological characters indicates that L. longicruris may have had a filter feeding lifestyle. A new cladistic analysis suggests that fossil lobopodians are paraphyletic or even polyphyletic and L. longicruris may be an important representative of the stem lineage leading to arthropods.

Birds' eyes seem often to be about as large as head size allows and brain size is taken here as a measure of the ill-defined space that is available to accommodate them. In four data sets for non-passerines eye size relates more strongly to brain size than to body mass and most non-passerine data are consistent with eye:brain (or eye:head-space) isometry. Eye:body allometry thus seems to follow from a negative head-space:body allometry. In passerines the eye:brain size correlations seem to be secondary to strong eye:body, brain:body, and perhaps therefore head-space:body correlations, a difference attributed to the passerines' greater anatomical uniformity.

Purpose: To demonstrate that high-resolution biometry is possible in mouse eyes in vivo, using real-time OCT with focal plane advancement by a stepper motor., Methods: OCT images of eyes were taken from nine 29-day-old C57BL/6 mice(18 eyes) on two consecutive days. A custom-built real-time OCT instrument with a stepper motor was used to advance the focal plane from the corneal apex to the retina along the ocular axis. The ocular dimensions were determined by advancement of the stepper motor as it displayed on the OCT scan images., Results: OCT images of the entire eye, including the cornea, anterior chamber, lens, vitreous chamber, and retina, were successfully obtained from both eyes of all mice. The measured average corneal thickness from 18 eyes at the age of 29 days was 90.8+/-4.6microm, anterior chamber depth 707.4+/-21.4microm, lens thickness 1558.7+/-18.0microm, vitreous chamber depth 707.4+/-21.4microm and retinal thickness was 186.9+/-15.1microm. Total axial length (from the corneal apex to the nerve fiber layer of the retina) was 3003.3+/-44.1microm. None of them were significantly different if measured on two consecutive days, and no significant differences were found between measurements in the left and right eyes., Conclusion: By focal plane advancement of a real-time OCT instrument through the mouse eye, highly repeatable measurements of the ocular dimensions were obtained. This novel method may be used to study small animal models of normal and abnormal eye development.

The nauplius eye in Cyclestherida, Laevicaudata and Spinicaudata (previously collectively termed Conchostraca) consists of four cups of inverse sensory cells separated by a pigment layer and a tapetum layer. There are two lateral and two medial cups, a ventral medial cup and a posterior medial cup. The pigment and tapetum layers contain two different kinds of pigment granules, the inner pigment layer relatively large, dark (and electron dense) granules, and the outer tapetum layer light, reflective pigment granules. The presence of four cups and two different kinds of pigment granules are interpreted as autapomorphies of Phyllopoda. The position and shape of the nauplius eye in Spinicaudata is very distinct and herein interpreted as an autapomorphy of this taxon. Additional frontal eyes might be present dorsally or ventrally in varying proximity to the nauplius eye, but they have separate nerves from their sensory cells to the nauplius eye centre in the protocerebrum. Rhabdomeric structures are present in all these frontal eyes, evidencing their light sensitivity. In Lynceus biformis and L. tatei (Laevicaudata), two pairs of frontal eyes were found. In Cyclestheria hislopi (Cyclestherida), an unpaired ventral frontal eye is present. We did not find additional frontal eyes in Limnadopsis parvispinus and Caenestheriella sp. (Spinicaudata).

A survey is given on the scarce information on the visual organs (eyes or ocelli) of Tardigrada. Many Eutardigrada and some Arthrotardigrada, namely the Echiniscidae, possess inverse pigment-cup ocelli, which are located in the outer lobe of the brain, and probably are of cerebral origin. Occurrence of such organs in tardigrades, suggested as being eyeless, has never been checked. Depending on the species, response to light (photokinesis) is negative, positive or indifferent, and may change during the ontogeny. The tardigrade eyes of the two eutardigrades examined up to now comprise a single pigment cup cell, one or two microvillous (rhabdomeric) sensory cells and ciliary sensory cell(s). In the eyes of the eutardigrade Milnesium tardigradum the cilia are differentiated in an outer branching segment and an inner (dendritic) segment. Because of the scarcity of information on the tardigrade eyes, their homology with the visual organs of other bilaterians is currently difficult to establish and further comparative studies are needed. Thus, the significance of these eyes for the evolution of arthropod visual systems is unclear yet.

Stomatopods (mantis shrimps) possess apposition compound eyes that contain more photoreceptor types than any other animal described. This has been achieved by sub-dividing the eye into three morphologically discrete regions, a mid-band and two laterally placed hemispheres, and within the mid-band, making simple modifications to a commonly encountered crustacean photoreceptor pattern of eight photoreceptors (rhabdomeres) per ommatidium. Optically the eyes are also unusual with the directions of view of the ommatidia of all three eye regions skewed such that over 70% of the eye views a narrow strip in space. In order to scan the world with this strip, the stalked eyes of stomatopods are in almost continual motion. Functionally, the end result is a trinocular eye with monocular range finding capability, a 12-channel colour vision system, a 2-channel linear polarisation vision system and a line scan sampling arrangement that more resembles video cameras and satellite sensors than animal eyes. Not surprisingly, we are still struggling to understand the biological significance of stomatopod vision and attempt few new explanations here. Instead we use this special edition as an opportunity to review and summarise the structural aspects of the stomatopod retina that allow it to be so functionally complex.

Purpose: The purpose of this study was to characterize normal ocular development in infant monkeys and to establish both qualitative and quantitative relationships between human and monkey refractive development., Methods: The subjects were 214 normal rhesus monkeys. Cross-sectional data were obtained from 204 monkeys at about 3 weeks of age and longitudinal data were obtained from 10 representative animals beginning at about 3 weeks of age for a period of up to 5 years. Ocular development was characterized via refractive status, corneal power, crystalline lens parameters, and the eye's axial dimensions, which were determined by retinoscopy, keratometry, phakometry and A-scan ultrasonography, respectively., Results: From birth to about 5 years of age, the growth curves for refractive error and most ocular components (excluding lens thickness and equivalent lens index) followed exponential trajectories and were highly coordinated between the two eyes. However, overall ocular growth was not a simple process of increasing the scale of each ocular component in a proportional manner. Instead the rates and relative amounts of change varied within and between ocular structures., Conclusion: The configuration and contribution of the major ocular components in infant and adolescent monkey eyes are qualitatively and quantitatively very comparable to those in human eyes and their development proceeds in a similar manner in both species. As a consequence, in both species the adolescent eye is not simply a scaled version of the infant eye.

Adaptation paradigms highlight the dynamic nature of face coding and suggest that identity is coded relative to an average face that is tuned by experience. In low-level vision, adaptive coding can enhance sensitivity to differences around the adapted level. We investigated whether sensitivity to differences around the average face is similarly enhanced. Converging evidence from three paradigms showed no enhancement. Discrimination of small interocular spacing differences was not better for faces close to the average (Study 1). Nor was perceived similarity reduced for face pairs close to (spanning) the average (Study 2). On the contrary, these pairs were judged most similar. Maximum likelihood perceptual difference scaling (Studies 3 and 4) confirmed that sensitivity to differences was reduced, not enhanced, around the average. We conclude that adaptive face coding does not enhance discrimination around the average face.

High-order monochromatic aberrations could potentially influence vision-dependent refractive development in a variety of ways. As a first step in understanding the effects of wave aberration on refractive development, we characterized the maturational changes that take place in the high-order aberrations of infant rhesus monkey eyes. Specifically, we compared the monochromatic wave aberrations of infant and adolescent animals and measured the longitudinal changes in the high-order aberrations of infant monkeys during the early period when emmetropization takes place. Our main findings were that (1) adolescent monkey eyes have excellent optical quality, exhibiting total RMS errors that were slightly better than those for adult human eyes that have the same numerical aperture and (2) shortly after birth, infant rhesus monkeys exhibited relatively larger magnitudes of high-order aberrations predominately spherical aberration, coma, and trefoil, which decreased rapidly to assume adolescent values by about 200 days of age. The results demonstrate that rhesus monkey eyes are a good model for studying the contribution of individual ocular components to the eye's overall aberration structure, the mechanisms responsible for the improvements in optical quality that occur during early ocular development, and the effects of high-order aberrations on ocular growth and emmetropization.

Purpose: This study investigated changes in refraction, corneal curvature, axial components and weight of posterior sclera in guinea pig eyes during the normal development from birth., Methods: Sixty-four guinea pigs were assigned to eight groups (n=8 each). Each group underwent a series of ocular measurements at one of the eight time-points (0, 1, 2, 3, 5, 7, 9 and 11 weeks), including refraction (streak retinoscopy), corneal radius of curvature (CRC; keratometry), anterior segment length (AS: corneal thickness and depth of the anterior chamber), thickness of the crystalline lens (CL), vitreous chamber length (VC; all A-scan ultrasonography) and dry weight of a circular 6mm diameter punch in the posterior sclera (electronic balance). Results of all the measurements were statistically compared between right eye and left eye, male and female and among different age groups. Artifacts of retinoscopy due to small eye artifact were also estimated at different ages., Results: The refraction in guinea pig eyes was +5.22+/-0.23 D (Mean, SE) at birth. This value decreased rapidly during the first 3 weeks followed by a slow decline. The overall decrease in refraction was highly significant from birth to 11 weeks (p<0.001 one way ANOVA). The small eye artifact was approximately 4.00 D at birth, which reduced to 2.76 D at 11 weeks. The guinea pig eyes were emmetropic by 3 weeks of age when the small eye artifact was taken into account. The CRC (3.24+/-0.01 mm at birth), AS (1.20+/-0.01 mm at birth), CL (2.72+/-0.03 mm at birth) and VC (3.28+/-0.01 mm at birth) increased within the first 3 weeks despite a transient decrease in the CRC within the first week. The increase in CRC, CL and VC continued after 3 weeks, however, the AS remained constant after this age. The increase in VC was better correlated to the decline of hyperopia (R(2)=0.70) than the other components (R(2)=0.33-0.39). Dry weight of the posterior sclera increased linearly from birth (p<0.001 between any two close time-points from 3 to 9 weeks) and had a moderately linear correlation with the VC (R(2)=0.60). There were no significant differences between the right eye and left eye or between male and female in all the measurements., Conclusions: In guinea pigs, the hyperopia present at birth rapidly reduces to emmetropia within the first 3 weeks of age. The emmetropization process in guinea pigs is mainly related to the increase in the vitreous chamber length. This relationship in guinea pigs is similar to that in chickens, tree shrews, primates and humans. The axial development of the vitreous chamber in guinea pigs appears to be associated with tissue growth of the posterior sclera.

This study is designed to investigate the relationship between macular pigment optical density (MPOD) and ocular biometric parameters. The following details were recorded for 180 healthy subjects: demographic profile; best-corrected visual acuity; refractive status; ocular biometric parameters [axial length (AL), anterior chamber depth (ACD), lens thickness (LT) and vitreous chamber depth (VCD)]; ocular dominance; MPOD; serum lutein (L) and zeaxanthin (Z). The mean MPOD (+/-SD) was 0.307 (0.155) and 0.305 (0.149) in the right and left eyes, respectively. No demonstrable relationship was observed between MPOD and AL, ACD or VCD [AL: r=0.091, p=0.225; ACD: r=0.091, p=0.227; VCD: r=0.146, p=0.051]. There was a significant and inverse relationship between LT and MPOD (r=-0.204; p=0.008), which was attenuated to non-significance after correction for age and height (r=-0.058; p=0.466). This study fails to identify an association between MPOD and ocular biometric parameters. This is an important negative finding, which allows investigators to study MP, and its relationship with potentially important variables, without the need to correct for ocular biometric parameters.

To quantify the rate at which light in a ganzfeld produces photoisomerizations in mouse rods in situ, we measured the rate of rhodopsin bleaching in eyes of recently euthanized mice with fully dilated pupils. The amount of rhodopsin declined as a first-order (exponential) function of the duration of the exposure at the luminance of 920 scot cd m(-2): the rate constants of bleaching were 8.3 x 10(-6) and 2.8 x 10(-5) s(-1) (scot cd(-1)m2)(-1) for C57B1/6 and 129P3/J mice, respectively. When the approximately 3-fold difference in effective areas of the pupils of the mice are taken into consideration, the bleaching rates for both strains become essentially the same, 2.6 x 10(-6) fraction rhodopsin (scot Td s)(-1). Assuming 7 x 10(7) rhodopsin molecules per rod, this bleaching rate yields the result that a flash of 1 scot Td s produces 181 photoisomerizations per rod, a value close to that derived from analysis of the collecting area of the rod for axially propagating light. We measured the electroretinograms of mice of the two strains reared under controlled illumination conditions (2 and 100 lux), and compared their properties, using the calibrations to determine the absolute sensitivities of the b-wave and a-waves. The intensity that produces a half-saturating rod b-wave response is 0.3-0.6 photoisomerizations rod(-1), and the amplification constant of the rod a-wave is 5-6 s(-2) photoisomerization(-1), with little dependence on the strain.

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.

An essential feature of the superposition type of compound eye is the presence of a wide zone, which is transparent and devoid of pigment and interposed between the distal array of dioptric elements and the proximally placed photoreceptive layer. Parallel rays, collected by many lenses, must (through reflection or refraction) cross this transparent clear-zone in such a way that they become focused on one receptor. Superposition depends mostly on diameter and curvature of the cornea, size and shape of the crystalline cone, lens cylinder properties of cornea and cone, dimensions of the receptor cells, and width of the clear-zone. We examined the role of the latter by geometrical, geometric-optical, and anatomical measurements and concluded that a minimal size exists, below which effective superposition can no longer occur. For an eye of a given size, it is not possible to increase the width of the clear-zone cz=dcz/R1 and decrease R2 (i.e., the radius of curvature of the distal retinal surface) and/or c=dc/R1 without reaching a limit. In the equations 'cz' is the width of the clear-zone dcz relative to the radius R1 of the eye and c is the length of the cornea-cone unit relative to R1. Our results provide one explanation as to why apposition eyes exist in very small scarabaeid beetles, when generally the taxon Scarabaeoidea is characterized by the presence of superposition eyes. The results may also provide the answer for the puzzle why juveniles or the young of species, in which the adults possess superposition (=clear-zone) eyes, frequently bear eyes that do not contain a clear zone, but resemble apposition eyes. The eyes of the young and immature specimens may simply be too small to permit superposition to occur.

Schematic eye models have typically been used to explain the average monochromatic and chromatic imaging properties of the eye. Both monochromatic aberrations and transverse chromatic aberration are known to vary widely across subjects. However, to our knowledge, the ability of schematic eye models to predict these individual variations has not been tested experimentally. We used a spatially resolved refractometer to measure the monochromatic aberrations and the optical transverse chromatic aberration (oTCA) in a group of 15 eyes. By recording the 1st and 4th Purkinje images for five directions of gaze, we also estimated the tilt, misalignment of ocular surfaces (front surface of the cornea and back surface of the lens) and off-axis position of the fovea (angle alpha), as well as pupil centration. We conclude that, contrary to expectations none of those factors are major contributors to the variability in monochromatic aberrations and oTCA in this group of eyes. Simulations show that corneal curvature and corneal conicity are also unlikely to account for the observed relation between monochromatic aberrations and oTCA. Our results suggest an important contribution of corneal irregularities to those aberrations.

Cephalopods and fish have had no common ancestor since the Cambrian, and their eyes are a classic example of convergent evolution. The octopus has no cornea, and immerson renders the trout cornea optically ineffective. As a result, the nearly spherical lens is responsible for all refraction in these eyes. In spite of the fact that the octopus lens consists of two joined parts, while the trout lens consists of one part, we show here that their optical properties are very similar. An index gradient bends rays within these lenses, adding power and correcting spherical aberration. High spherical symmetry in both lenses strongly reduces other monochromatic aberrations and yields a wide field of vision, advantageous in attack and evasion. The octopus Mattheissen's ratio, 2.83, an inverse measure of light-gathering power, lies above the trout value of 2.38 but within the range of values reported for fish. Strong uncorrected longitudinal chromatic aberration is nearly identical in both animals as a result of similar lens protein optical properties, and will limit resolution. We discuss how animal lifestyle requirements and lens material properties influence the design of these eyes.

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).

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.

To determine the extent to which the visual experience of one eye may influence the refractive development of its fellow eye, we analyzed the data of untreated (UT) eyes of monkeys that received different types of unilateral pattern deprivation. Subjects were 15 juvenile rhesus monkeys, with five monkeys in each of three treatment groups: aphakic eyes with optical correction (AC), aphakic eyes with no correction (ANC), and eyes that were occluded with an opaque contact lens (OC). Under general anaesthesia, refractive error (D) was determined by cycloplegic retinoscopy and axial length (mm) was determined with A-scan ultrasonography. For measurements of refractive error of the UT eyes, there was a significant main effect of groups according to the treatment of the fellow eyes, F(2, 12) = 6.6. While UT eyes paired with AC fellow eyes (mean = +4.2 D) were significantly more hyperopic than the eyes of age-matched normal monkeys (mean = +2.4 D), t(25), = 2.5, UT eyes paired with OC fellow eyes (mean = -0.5 D) were significantly more myopic than the eyes of normal monkeys, t(25) = -9. UT eyes paired with ANC fellow eyes (mean = +1.9 D) were not significantly different from normal eyes. For measurements of axial length there was also a significant main effect of groups, F(2, 12) = 6.9. While UT eyes paired with AC fellow eyes (mean = 16.9 mm) were significantly shorter than the eyes of age-matched normal monkeys (mean = 17.5 mm), t(25) = 2.3, UT eyes paired with OC fellow eyes (mean = 18.1 mm) were significantly longer than the eyes of normal monkeys, t(25) = 2.3. UT eyes paired with ANC fellow eyes (mean = 17.5 mm) were not significantly different from the eyes of normal monkeys. The measurements of axial length and of refractive error of the UT eyes were also significantly correlated with one another, probably indicating that the differences in refractive error were due to differences in axial length, r = -0.8. The present data reveal that despite normal visual experience, UT eyes can have their refractive development altered, systematically, simply as a function of the type of pattern deprivation received by their fellow eyes. These data add to the growing evidence that there is an interocular mechanism that is active during emmetropization. As a consequence, future models of eye growth will need to consider both: (1) the direct influence of visual input on the growing eye; as well as (2) the indirect influence coming from the fellow eye.

Matthiessen's ratio (distance from centre of lens to retina:lens radius) was measured in developing black bream, Acanthopagrus butcheri (Sparidae, Teleostei). The value decreased over the first 10 days post-hatch from 3.6 to 2.3 along the nasal and from four to 2.6 along temporal axis. Coincidentally, there was a decrease in the focal ratio of the lens (focal length:lens radius). Morphologically, the accommodatory retractor lentis muscle appeared to become functional between 10-12 days post-hatch. The results suggest that a higher focal ratio compensates for the relatively high Matthiessen's ratio brought about by constraints of small eye size during early development. Combined with differences in axial length, this provides a means for larval fish to focus images from different distances prior to the ability to accommodate.

Refractive state and ocular dimensions were studied longitudinally in nine normal marmosets. Animals were anaesthetised and examined (with some exceptions) at 4, 6, 7, 8, 10, 15, 24 and 39 weeks of age. Cycloplegic retinoscopy showed that hyperopia early in life rapidly diminished. Refraction corrected for the artefact of retinoscopy stabilised by 8 weeks of age, but at a slightly myopic value, rather than at emmetropia. The ocular components continued to change throughout the period studied. Corneal radius, measured by photokeratometry, increased slightly during development. Anterior segment depth and vitreous chamber depth (VCD), measured by A-scan ultrasonography, increased throughout development while lens thickness initially increased and then decreased. Data from the eyes of these normal animals were compared with that from the contralateral eyes of animals which received short periods of monocular deprivation early in life (Troilo, D., & Judge S.J. (1993). Ocular development and visual deprivation myopia in the common marmoset (Callithrix jacchus jacchus). Vision Research, 33, 1311-24); eyes which viewed through no lens or a plano lens (Graham, B. & Judge, S.J. (1999)). The effects of spectacle wear in infancy on eye growth and refractive error in the marmoset (Callithrix jacchus). Vision Research, 39, 189-206), and eyes of normal animals in another colony. There were no significant differences between the first two groups and the normal animals in our colony while age-matched animals from the other colony were slightly but significantly less myopic than our animals.

This study examined the contrast and spatial-frequency requirements for emmetropization in chicks. Chicks were form deprived from hatching either constantly or had this treatment interrupted with 20 min of "visual stimulation" each day. Visual stimulation comprised exposure to either a normal cage environment (i.e., normal vision) or environments that were restricted in either their spatial contrast or spatial-frequency composition. Constant form deprivation resulted in high myopia (e.g. -11.8 D after 5 days), with refractive changes being much smaller in chicks allowed 20 min of normal vision each day (e.g. -3.4D). The restricted contrast environments (contrast range: 9-78%) were generally only slightly less effective than the normal cage environment in preventing form-deprivation myopia. However, in the case of restricted spatial-frequency environments, both the intermediate (0.86 cycles deg-1) and mixed spatial-frequency environments significantly reduced the form deprivation response, while both the high (4.3 cycles deg-1) and low spatial-frequency (0.086 cycles deg-1) stimuli, as well as the composites of these, were less effective in preventing form-deprivation myopia. This spatial-frequency dependence did not vary when, instead of white light, monochromatic illumination was used to eliminate chromatic aberration, although all groups showed more myopia under this condition. It is assumed that the observed inhibitory effects on form-deprivation myopia reflect the adequacy of the visual information presented during the period of visual stimulation for emmetropization in chicks. In this context, the data imply a mid-spatial-frequency tuning in the current study and a low contrast threshold which was not reached for this emmetropization process. Finally, the data hint that chromatic aberration may have some role as a cue to defocus in emmetropization.

We employed neutralizing infrared videophotorefraction and photokeratometry to examine the manifest refractions and corneal curvatures of 21 species of anurans (frogs and toads) in five families (Dendrobatidae, Bufonidae, Centrolenidae, Leptodactylidae, and Hylidae) resident in Central America. We found that all of the anurans exhibited hyperopic refractions in air, but that the observed hyperopia was not totally explained by the small eye artefact (Glickstein & Millodot, 1970). An allometric comparison of the corneal radii of these small anurans with those of a large number of other vertebrates, inferred from ocular axial lengths, showed that their corneal radii increased significantly more rapidly with increasing body size than that of other vertebrates generally (allometric slope constants: anurans: 0.270 +/- 0.032; other vertebrates: 0.151 +/- 0.004). Among the anurans examined, nocturnal Hylids had significantly larger eyes than diurnal Dendrobatid frogs and Bufonid toads.

Trout lens external shape and internal refractive index gradient structure were measured and used to construct an optical lens model that predicts by ray tracing the average longitudinal spherical and chromatic aberration, focal length and image quality. The nearly spherical shape of the lens was measured from photographs, and the internal refractive gradient structure was measured directly with a special Pulfrich areal refractometer. Longitudinal spherical aberration and back focal length were measured using a simplified Hartmann test using laser beams and a Schlieren test which additionally made refractive index gradient fine structure visible and detected scattering, axial symmetry and structural irregularity. Axial focus shift caused by longitudinal chromatic aberration was measured using a star test. The model lens was then incorporated into a model trout eye based on vertical and horizontal eye frozen sections. Calculated model function yields insight into the relation between eye and lens structure and optical behaviour. Semi-random secondary structural features act as perturbations on the basic model, and will result in point image fine structure.

In each of five monkeys, one eye was fitted with a diffuser lens at birth. This lens allowed pattern vision, but also reduced contrast by about 1 log unit. In four out of five monkeys, the treated eyes were shorter and more hyperopic than the untreated fellow eyes. At 25 weeks of age, interocular differences (OD -- OS) of the experimental group were significantly greater than interocular differences of age-matched normal monkeys for both axial length (P < 0.05) and refractive error (P < 0.02). In addition, while the treated eyes were significantly different from normal eyes for both axial length measurements (P < 0.01) and refractive error (P < 0.01), there were no significant differences between the untreated fellow eyes and normal eyes. In primates less severe pattern deprivation appears to produce an effect on eye growth that is opposite to that of severe pattern deprivation (little or no pattern vision), which typically results in axial myopia.

Schematic eyes for four Macaca fascicularis monkeys were constructed from measurements of the positions and curvatures of the anterior and posterior surfaces of the cornea and lens. All of these measurements were obtained from Scheimpflug photography through the use of a ray-tracing analysis. Some of these measurements were also checked (and confirmed) by keratometry and ultrasound. Gaussian lens equations were applied to the measured dimensions of each individual eye in order to construct schematic eyes. The mean total power predicted by the schematic eyes agreed closely with independent measurements based on retinoscopy and ultrasound results, 74.2 +/- 1.3 (SEM) vs 74.7 +/- 0.3 (SEM) diopters. The predicted magnification of 202 microns/deg in one eye was confirmed by direct measurement of 205 microns/deg for a foveal laser lesion. The mean foveal retinal magnification calculated for our eight schematic eyes was 211 +/- (SEM) microns/deg, slightly less than the value obtained by application of the method of Rolls and Cowey [Experimental Brain Research, 10, 298-310 (1970)] to our eight eyes but just 4% more than the value obtained by application of the method of Perry and Cowey [Vision Research, 12, 1795-1810 (1985)].

The clearnose skate (Raja eglanteria) develops in an almost opaque eggcase and lays its eggs in pairs. One sibling from each of eight pairs of skates was removed from its eggcase during embryonic development, while the other sibling developed inside the eggcase. The refractive development of the eyes at hatching was examined to see if ambient light exposure during embryonic development could influence the refractive states of hatchlings. Measurements included refractive states, ocular dimensions and lens focal properties. The differences in measurements between the two groups were not significant, which would indicate that environmental light does not influence the refractive development of the embryonic skate eye.

Fish were reared in 6 conditions: broad spectrum white light, total darkness, scotopic illumination, and 3 monochromatic colors matched to the absorption spectra of the three cone types to study the influence of the light regime on the regulation of eye growth in the African cichlid fish Haplochromis burtoni. Fish reared in total darkness showed high variability in naso-temporal diameter and axial length of the eye. Animals reared in darkness and in scotopic illumination had significantly larger eyes relative to lens size in comparison to fish reared in white light. Eye size and shape was nearly identical in fish reared in monochromatic and in white light. Because of overlap in the absorption spectra of the three cone types of H. burtoni it could not be resolved whether the regulatory mechanism receives input from all three cone types or only from the green (523 nm) sensitive cones. It is clear from our results, however, that neither the blue (455 nm) nor the yellow sensitive (562 nm) cones alone are responsible for eye size regulation. It seems equally unlikely that all three cone types have to act in concert for normal growth of the eye.

The modulation transfer function (MTF) of well-mixed unlighted and diffusely lighted samples of clear natural waters for path lengths up to 4 m was measured. The measuring conditions simulated the situation for horizontal aquatic vision. In unlighted water, the MTF decreased relatively slowly with increasing path length and spatial frequency up to 150 c/deg. We interpret this as the result of low-angle forward scattering of light from the target. For diffusely lighted water, the MTF fell much more rapidly with path length, but was nearly independent of spatial frequency. Here, scattering of ambient light into the light path contributes an additional veiling glare or path radiance, which is independent of spatial frequency but degrades the MTF strongly with increasing path length. These scattering processes are independent of wavelength in the visible spectrum. The modulation transfer properties of these waters do not preclude high aquatic visual acuity. However, the useful range of high acuity vision in diffusely lighted water is much less than for unlighted water. This places special requirements upon the design of high-acuity aquatic eyes.

The common marmoset (Callithrix jacchus) is a small, diurnal, New World monkey amenable to vision research. In this paper we describe the visual optics and cone photoreceptor topography of the normal adult marmoset. Paraxial optical ray-tracing shows that the marmoset eye is well represented as a scaled-down version of the human eye. The density of foveal and perifoveal cone photoreceptors in the marmoset is as high, and in peripheral retina higher, than those reported in humans and macaques. The foveal acuity predicted by the Nyquist limits set by the cone mosaic (30 c/deg) is in agreement with behavioral measures of visual acuity. Foveal depth of focus is remarkably small (< 0.2 D) for an eye of this size (axial length about 11 mm). Estimates of the amplitude of accommodation using infrared photorefraction indicate that the marmoset is capable of more than 20 D of accommodation.

The normal development of refractive state, ocular components and simple visually-guided behaviors was examined in maternally-reared tree shrews. Six groups consisting of 5 animals each were anesthetized and examined after 0, 15, 30, 45, 60 and 75 days of normal binocular visual exposure. Measures in the 75-day group provided values for an improved schematic eye of the tree shrew. Cycloplegic refraction showed a marked hyperopia (+25 D) at eye opening which decreased rapidly during the first 15 days of visual exposure and stabilized near the value (+5 D) expected in an eye of this axial length (approx. 7.8 mm). Corneal radius increased slightly during development. Anterior segment depth, measured by A-scan ultrasonography, seemed to complete most of its development at an earlier age (15-30 days of visual exposure) than did other ocular parameters. Lens thickness increased steadily throughout development. Vitreous chamber depth increased rapidly until 15 days of visual exposure, and then decreased because the lens thickness increased more rapidly than axial length. Crude orienting to, and following of, large objects developed shortly after eye opening (median age at onset, 5 and 6 days, respectively). Triggered visual placing responses developed at about the same time that the refractive state completed the rapid drop from highly hyperopic values. The slowed rate of ocular development after 15 days of visual exposure may be related to increased retinal activity that is permitted by neural maturation and by the presence of a relatively well-focussed retinal image. The increased activity may influence the final dimensions of the eye to coordinate the axial length with the focal length of the eye.

Measurements of angular position and velocity vectors of the eye in three human and three monkey subjects showed that: (1) position vectors lie roughly in a single plane, in accordance with Listing's law, between and during saccades; (2) primary position of the eye is often far from the centre of the oculomotor range. (3) saccades have nearly-fixed rotation axes, which tilt out of Listing's plane in a systematic way depending on current eye position. Findings 1 and 3 show that saccadic control signals accurately reflect the properties of three-dimensional rotations, as predicted by a new quaternion model of the saccadic system; models that approximate rotational kinematics using vectorial addition and integration do not predict these findings.

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.