Binocular vision: the eyes add and subtract

Our two eyes' views of the outside world are slightly different, providing the basis for stereopsis. A new study has found evidence that the human visual system has separately adaptable channels for adding and subtracting the neural signals from the two eyes, supporting an unconventional view of the initial stages of stereopsis.     

Motion and vision: why animals move their eyes

Nearly all animals with good vision have a repertoire of eye movements. The majority show a pattern of stable fixations with fast saccades that shift the direction of gaze. These movements may be made by the eyes themselves, or the head, or in some insects the whole body. The main reason for keeping gaze still during fixations is the need to avoid the blur that results from the long response time of the photoreceptors. Blur begins to degrade the image at a retinal velocity of about 1 receptor acceptance angle per response time. Some insects (e.g. hoverflies) stabilise their gaze much more rigidly than this rule implies, and it is suggested that the need to see the motion of small objects against a background imposes even more stringent conditions on image motion. A third reason for preventing rotational image motion is to prevent contamination of the translational flow-field, by which a moving animal can judge its heading and the distances of objects. Some animals do let their eyes rotate smoothly, and these include some heteropod molluscs, mantis shrimps and jumping spiders, all of which have narrow linear retinae which scan across the surroundings. Hymenopteran insects also rotate during orientation flights at speeds of 100–200° s⁻¹. This is just consistent with a blur-free image, as are the scanning speeds of the animals with linear retinae. Accepted: 29 April 1999     

"X-ray vision" and the evolution of forward-facing eyes

Why do our eyes face forward, and why do many mammals have eyes facing sideways? Here, we describe results suggesting that the degree of binocular convergence is selected to maximize how much the mammal can see in its environment. Mammals in non-cluttered environments can see the most around them with panoramic, laterally directed eyes. Mammals in cluttered environments, however, can see best when their eyes face forward, for binocularity has the power of "seeing through" clutter out in the world. Evidence across mammals closely fits the predictions of this "X-ray" hypothesis.     

Visual navigation in starfish: first evidence for the use of vision and eyes in starfish

Most known starfish species possess a compound eye at the tip of each arm, which, except for the lack of true optics, resembles an arthropod compound eye. Although these compound eyes have been known for about two centuries, no visually guided behaviour has ever been directly associated with their presence. There are indications that they are involved in negative phototaxis but this may also be governed by extraocular photoreceptors. Here, we show that the eyes of the coral-reef-associated starfish Linckia laevigata are slow and colour blind. The eyes are capable of true image formation although with low spatial resolution. Further, our behavioural experiments reveal that only specimens with intact eyes can navigate back to their reef habitat when displaced, demonstrating that this is a visually guided behaviour. This is, to our knowledge, the first report of a function of starfish compound eyes. We also show that the spectral sensitivity optimizes the contrast between the reef and the open ocean. Our results provide an example of an eye supporting only low-resolution vision, which is believed to be an essential stage in eye evolution, preceding the high-resolution vision required for detecting prey, predators and conspecifics.     

A spitting image: specializations in archerfish eyes for vision at the interface between air and water

Archerfish are famous for spitting jets of water to capture terrestrial insects, a task that not only requires oral dexterity, but also the ability to detect small camouflaged prey against a visually complex background of overhanging foliage. Because detection of olfactory, auditory and tactile cues is diminished at air–water interfaces, archerfish must depend almost entirely on visual cues to mediate their sensory interactions with the aerial world. During spitting, their eyes remain below the water''s surface and must adapt to the optical demands of both aquatic and aerial fields of view. These challenges suggest that archerfish eyes may be specially adapted to life at the interface between air and water. Using microspectrophotometry to characterize the spectral absorbance of photoreceptors, we find that archerfish have differentially tuned their rods and cones across their retina, correlated with spectral differences in aquatic and aerial fields of view. Spatial resolving power also differs for aquatic and aerial fields of view with maximum visual resolution (6.9 cycles per degree) aligned with their preferred spitting angle. These measurements provide insight into the functional significance of intraretinal variability in archerfish and infer intraretinal variability may be expected among surface fishes or vertebrates where different fields of view vary markedly.     

One eye but no vision: cave fish with induced eyes do not respond to light

One of the most intriguing questions in evolutionary biology is the degree to which behavior is a necessary consequence of morphology. We explore this issue by examining phototactic behavior in epigean (eyed surface-dwelling) and troglomorphic (blind cave) forms of the teleost Astyanax fasciatus whose eyes were modified during embryogenesis by removing one or both lens vesicles from the epigean form or by transplanting the lens vesicle from an epigean fish into the optic cup of a blind cave form. Lens removal results in eye degeneration and blindness in adult epigean fish, whereas lens transplantation stimulates growth of the eye, inducing the development of optic tissues in the normally eyeless adult cave fish. Photoresponsiveness was examined by placing fish in an aquarium with one half illuminated and the other half dark and scoring their presence in the illuminated or dark half. Both the eyeless epigean fish and cave fish with induced eyes are indifferent to the illumination whereas the surface forms are scotophilic, suggesting that optic development and phototactic behavior are decoupled.     

Evolving eyes

Despite the incredible diversity among extant eyes, laws of physics constrain how light can be collected resulting in only eight known optical systems in animal eyes. Surprisingly, all animal eyes share a common molecular strategy using opsin for catching photons, but there are a diverse collection of mechanisms with proteins unrelated to each other used to focus light for vision. However, opsin is expressed in either one of two types of photoreceptor that differ fundamentally in their structure and tissue of origin. Taken together, this collection of observations strongly suggests that eyes have had multiple origins with remarkable convergence due to physics and molecular conservation of the opsin protein. Yet recent work has shown that a family of conserved genes are involved in eye formation despite substantial differences in their structure and origin, leading to a controversy over whether eyes evolved once or repeatedly. A likely resolution of this discussion is that particular genes and genetic programs have become associated with specific features needed for eyes and such suites of genes have been recruited as new eyes evolve. Since specific genes and their products are used repeatedly, it is somewhat difficult to conceptualize their causal relationships relative to evolutionary processes. However, detailed comparison of developmental programs may offer clues about multiple origins.     

Warm brain and eye temperatures in sharks

Temperatures in the brain and eyes of mako and porbeagle sharks (Lamnidae) are 5 degrees C warmer than the water while the brain and eye temperatures in six other species of pelagic sharks are within 0.1 degrees C of water temperature. An orbital rete mirabile is present in the porbeagle and mako sharks but absent in the cranial vasculature of eleven other species of pelagic sharks. The orbital rete in the head of the porbeagle and mako sharks acts as a heat exchanger which conserves metabolic heat and raises the local tissue temperatures. This brain and eye warming system should buffer the central nervous system from the effects of rapid temperature change. Warming of the retina may improve the visual sensitivity of these active predators.     

Scleral thickness in human eyes

Purpose

To obtain information about scleral thickness in different ocular regions and its associations.

Methods

The histomorphometric study included 238 human globes which had been enucleated because of choroidal melanomas or due to secondary angle-closure glaucoma. Using light microscopy, anterior-posterior pupil-optic nerve sections were measured.

Results

In the non-axially elongated group (axial length ≤26 mm), scleral thickness decreased from the limbus (0.50±0.11 mm) to the ora serrata (0.43±0.14 mm) and the equator (0.42±0.15 mm), and then increased to the midpoint between posterior pole and equator (0.65±0.15 mm) and to the posterior pole (0.94±0.18 mm), from where it decreased to the peri-optic nerve region (0.86±0.21 mm) and finally the peripapillary scleral flange (0.39±0.09 mm). Scleral thickness was significantly lower in the axially elongated group (axial length >26 mm) than in the non-axially elongated group for measurements taken at and posterior to the equator. Scleral thickness measurements of the posterior pole and of the peripapillary scleral flange were correlated with lamina cribrosa thickness measurements. Scleral thickness measurements at any location of examination were not significantly (all P>0.10) correlated with corneal thickness measurements. Scleral thickness was statistically independent of age, gender and presence of glaucoma.

Conclusions

In non-axially elongated eyes, the sclera was thickest at the posterior pole, followed by the peri-optic nerve region, the midpoint between posterior pole and equator, the limbus, the ora serrata, the equator and finally the peripapillary scleral flange. In axially elongated eyes, scleral thinning occurred at and posterior to the equator, being more marked closer to the posterior pole and the longer the axial length was. Within the anterior and posterior segment respectively, scleral thickness measurements were correlated with each other. Posterior scleral thickness was correlated with lamina cribrosa thickness. Scleral thickness measurements at any location of examination were not significantly correlated with corneal thickness or with age, gender and presence of absolute secondary angler-closure glaucoma.     

Arginine phosphate in compound eyes

Summary The steady state levels of various phosphate esters were measured in the compound eyes of two species of decapod crustaceans (Astacus andEupagurus) and of the blowfly (Calliphora). In the crustaceans, free nucleotides—especially ATP—contribute only little to the soluble phosphate fraction. The adenylate charges of all three species are in the range of about 0.8, though the proportion of adenine nucleotides to other organic phosphates differs considerably. The main energy rich phosphate compound in crustacean eyes was found to be arginine phosphate; the ratio ATP/Arg-P is 1:10 inAstacus, and 1:4 inEupagurus; in the blowfly it is near 1:1. Arg-P is considered to play a role as a phosphagen.This investigation was supported by Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 114 (Bionach)     

Origins of superior dynamic visual acuity in baseball players: superior eye movements or superior image processing

Dynamic visual acuity (DVA) is defined as the ability to discriminate the fine parts of a moving object. DVA is generally better in athletes than in non-athletes, and the better DVA of athletes has been attributed to a better ability to track moving objects. In the present study, we hypothesized that the better DVA of athletes is partly derived from better perception of moving images on the retina through some kind of perceptual learning. To test this hypothesis, we quantitatively measured DVA in baseball players and non-athletes using moving Landolt rings in two conditions. In the first experiment, the participants were allowed to move their eyes (free-eye-movement conditions), whereas in the second they were required to fixate on a fixation target (fixation conditions). The athletes displayed significantly better DVA than the non-athletes in the free-eye-movement conditions. However, there was no significant difference between the groups in the fixation conditions. These results suggest that the better DVA of athletes is primarily due to an improved ability to track moving targets with their eyes, rather than to improved perception of moving images on the retina.