Reverse lateralization of visual discriminative abilities in the European starling

Previous experiments on visual feature discrimination abilities have consistently shown a right-eye system lateralization in pigeons, Columba livia, and young domestic chickens, Gallus gallus domesticus, both nonpasserine species. Recently, however, it has been shown that photoreceptor distribution in the left and right retinas are asymmetrical in the European starling, Sturnus vulgaris, a passerine species. Single cone receptors are significantly more abundant in the left retina, which suggests that starlings should perform visual discrimination tasks more proficiently with the left eye, in contrast to previous findings with nonpasserines. We tested this hypothesis using the technique of monocular occlusion. In the first experiment, starlings were tested on a simultaneous visual discrimination task in three conditions: binocular (both eyes), left monocular (left eye only) and right monocular (right eye only). Subjects in the binocular and left-monocular conditions achieved significantly higher performance scores on the discrimination task than birds in the right-monocular condition. A second experiment found similar results, with birds in the left-monocular condition learning the discrimination task more than twice as quickly as those in the right-monocular condition. Subsequent tests with the alternative eye for both groups indicated no interocular transfer. These findings suggest that visual discriminative abilities in starlings are asymmetrical, and that they are lateralized in the opposite eye system than has been reported for all other species tested to date.     

Lateralized visual behavior in bottlenose dolphins (Tursiops truncatus) performing audio-visual tasks: the right visual field advantage

Analyzing cerebral asymmetries in various species helps in understanding brain organization. The left and right sides of the brain (lateralization) are involved in different cognitive and sensory functions. This study focuses on dolphin visual lateralization as expressed by spontaneous eye preference when performing a complex cognitive task; we examine lateralization when processing different visual stimuli displayed on an underwater touch-screen (two-dimensional figures, three-dimensional figures and dolphin/human video sequences). Three female bottlenose dolphins (Tursiops truncatus) were submitted to a 2-, 3- or 4-, choice visual/auditory discrimination problem, without any food reward: the subjects had to correctly match visual and acoustic stimuli together. In order to visualize and to touch the underwater target, the dolphins had to come close to the touch-screen and to position themselves using monocular vision (left or right eye) and/or binocular naso-ventral vision. The results showed an ability to associate simple visual forms and auditory information using an underwater touch-screen. Moreover, the subjects showed a spontaneous tendency to use monocular vision. Contrary to previous findings, our results did not clearly demonstrate right eye preference in spontaneous choice. However, the individuals' scores of correct answers were correlated with right eye vision, demonstrating the advantage of this visual field in visual information processing and suggesting a left hemispheric dominance. We also demonstrated that the nature of the presented visual stimulus does not seem to have any influence on the animals' monocular vision choice.     

Fixational Eye Movements in the Earliest Stage of Metazoan Evolution

All known photoreceptor cells adapt to constant light stimuli, fading the retinal image when exposed to an immobile visual scene. Counter strategies are therefore necessary to prevent blindness, and in mammals this is accomplished by fixational eye movements. Cubomedusae occupy a key position for understanding the evolution of complex visual systems and their eyes are assumedly subject to the same adaptive problems as the vertebrate eye, but lack motor control of their visual system. The morphology of the visual system of cubomedusae ensures a constant orientation of the eyes and a clear division of the visual field, but thereby also a constant retinal image when exposed to stationary visual scenes. Here we show that bell contractions used for swimming in the medusae refresh the retinal image in the upper lens eye of Tripedalia cystophora. This strongly suggests that strategies comparable to fixational eye movements have evolved at the earliest metazoan stage to compensate for the intrinsic property of the photoreceptors. Since the timing and amplitude of the rhopalial movements concur with the spatial and temporal resolution of the eye it circumvents the need for post processing in the central nervous system to remove image blur.     

Interspecific differences in the visual system and scanning behavior of three forest passerines that form heterospecific flocks

Little is known as to how visual systems and visual behaviors vary within guilds in which species share the same micro-habitat types but use different foraging tactics. We studied different dimensions of the visual system and scanning behavior of Carolina chickadees, tufted titmice, and white-breasted nuthatches, which are tree foragers that form heterospecific flocks during the winter. All species had centro-temporally located foveae that project into the frontal part of the lateral visual field. Visual acuity was the highest in nuthatches, intermediate in titmice, and the lowest in chickadees. Chickadees and titmice had relatively wide binocular fields with a high degree of eye movement right above their short bills probably to converge their eyes while searching for food. Nuthatches had narrower binocular fields with a high degree of eye movement below their bills probably to orient the fovea toward the trunk while searching for food. Chickadees and titmice had higher scanning (e.g., head movement) rates than nuthatches probably due to their wider blind areas that limit visual coverage. The visual systems of these three species seem tuned to the visual challenges posed by the different foraging and scanning strategies that facilitate the partitioning of resources within this guild.     

A test of the visual acuity of the locust eye

Targets consisting of solid rectangles (bright on a dark ground), of a range of widths, or pairs of thin lines of separation equal to the width of each rectangle, were used to evoke on, or movement responses in, the DCMD and other visual interneurones in the locust (Schistocerca gregaria). It was found that, judged from the discharges in these interneurones, both types could distinguish solid from paired-line figures of identical subtense, down to an angle of 0.4 degrees. Such an angle is well within the values of the eye parameters DeltaP and Deltaphi. Similar results were obtained with figures of the reversed contrast, showing that acuity was maintained even in the presence of a bright surrounding.     

Eye structure and amphibious foraging in albatrosses

Anterior eye structure and retinal visual fields were determined in grey-headed and black-browed albatrosses, Diomedea melanophris and D. chrysostoma (Procellariiformes, Diomedeidae), using keratometry and an ophthalmoscopic reflex technique. Results for the two species were very similar and indicate that the eyes are of an amphibious optical design suggesting that albatross vision is well suited to the visual pursuit of active prey both on and below the ocean surface. The corneas are relatively flat (radius ca. 14.5 mm) and hence of low absolute refractive power (ca. 23 dioptres). In air the binocular fields are relatively long (vertical extent ca. 70 degrees) and narrow (maximum width in the plane of the optic axes 26–32 degrees), a topography found in a range of bird species that employ visual guidance of bill position when foraging. The cyclopean fields measure approximately 270 degrees in the horizontal plane, but there is a 60 degrees blind sector above the head owing to the positioning of the eyes below the protruding supraorbital ridges. Upon immersion the monocular fields decrease in width such that the binocular fields are abolished. Anterior eye structure, and visual field topography in both air and water, show marked similarity with those of the Humboldt penguin.     

Ecological constraints on sensory systems: compound eye size in Daphnia is reduced by resource limitation

Eye size is an indicator of visual capability, and macroevolutionary patterns reveal that taxa inhabiting dim environments have larger eyes than taxa from bright environments. This suggests that the light environment is a key driver of variation in eye size. Yet other factors not directly linked with visual tasks (i.e., non-sensory factors) may influence eye size. We sought to jointly investigate the roles of sensory (light) and non-sensory factors (food) in determining eye size and ask whether non-sensory factors could constrain visual capabilities. We tested environmental influences on eye size in four species of the freshwater crustacean Daphnia, crossing bright and dim light levels with high and low resource levels. We measured absolute eye size and eye size relative to body size in early and late adulthood. In general, Daphnia reared on low resources had smaller eyes, both absolutely and relatively. In contrast to the dominant macroevolutionary pattern, phenotypic plasticity in response to light was rarely significant. These patterns of phenotypic plasticity were true for overall diameter of the eye and the diameter of individual facets. We conclude that non-sensory environmental factors can influence sensory systems, and in particular, that resource availability may be an important constraint on visual capability.     

In the eye of the beholder: visual mate choice lateralization in a polymorphic songbird

Birds choose mates on the basis of colour, song and body size, but little is known about the mechanisms underlying these mating decisions. Reports that zebra finches prefer to view mates with the right eye during courtship, and that immediate early gene expression associated with courtship behaviour is lateralized in their left hemisphere suggest that visual mate choice itself may be lateralized. To test this hypothesis, we used the Gouldian finch, a polymorphic species in which individuals exhibit strong, adaptive visual preferences for mates of their own head colour. Black males were tested in a mate-choice apparatus under three eye conditions: left-monocular, right-monocular and binocular. We found that black male preference for black females is so strongly lateralized in the right-eye/left-hemisphere system that if the right eye is unavailable, males are unable to respond preferentially, not only to males and females of the same morph, but also to the strikingly dissimilar female morphs. Courtship singing is consistent with these lateralized mate preferences; more black males sing to black females when using their right eye than when using their left. Beauty, therefore, is in the right eye of the beholder for these songbirds, providing, to our knowledge, the first demonstration of visual mate choice lateralization.     

Perceptual grouping of biological motion promotes binocular rivalry

Investigation of perceptual rivalry between conflicting stimuli presented one to each eye can further understanding of the neural underpinnings of conscious visual perception. During rivalry, visual awareness fluctuates between perceptions of the two stimuli. Here, we demonstrate that high-level perceptual grouping can promote rivalry between stimulus pairs that would otherwise be perceived as nonrivalrous. Perceptual grouping was generated with point-light walker stimuli that simulate human motion, visible only as lights placed on the joints. Although such walking figures are unrecognizable when stationary, recognition judgments as complex as gender and identity can accurately be made from animated displays, demonstrating the efficiency with which our visual system can group dynamic local signals into a globally coherent walking figure. We find that point-light walker stimuli presented one to each eye and in different colors and configurations results in strong rivalry. However, rivalry is minimal when the two walkers are split between the eyes or both presented to one eye. This pattern of results suggests that processing animated walker figures promotes rivalry between signals from the two eyes rather than between higher-level representations of the walkers. This leads us to hypothesize that awareness during binocular rivalry involves the integrated activity of high-level perceptual mechanisms in conjunction with lower-level ocular suppression modulated via cortical feedback.     

Ontogeny of visual acuity in two species of deermice (Peromyscus)

Development of visual acuity in Peromycsus maniculatus and P. leucopus was measured by optokinetic responses. Mice were suspended within a rolating drum lined with black and white stripes of equal width. Stripe width was varied to determine the minimum visual angle which could be seen by each mouse. At 20-cm stimulus distance both species reached a mean visual angle of 21′ of arc by 2 days after eye opening. Development was slower at 40-cm stimulus distace, both species reaching a mean angle of 18′ of arc by 8 days after eye opening. Mean visual angles at three illuminance levels (4·3 lux, 86·1 lux and 861·1 lux) were not diferent. Deermice have better visual acuity and visual range than previously tested rodents and exhibit rapid visual maturation.     

Morphological differentiation of the central visual cells R7/8 in various regions of the blowfly eye

The central rhabdomeres in the retina of the blowfly Calliphora erythrocephala and the house fly Musca domestica are not structurally uniform. In Calliphora, four classes of central rhabdomeres were found; they are formed by a total of seven types of central visual cells, clearly distinguished by the following structural features: length of the rhabdomeres R7 or R8, position of the nucleus, rhabdomere twist, fine structure in the R7/R8 transition region, and cross-sectional area of the rhabdomeres. In the lateral part of the eye only the most common central-rhabdomere class, ‘sl.’ is present, whereas in the frontal and dorsal parts classes ‘sl’ and ‘ls’ are found in a particular numerical ratio. Near the frontal eye margin the rare class ‘per’ also appears, with two separate rhabdomeres, R7per and R8s; the morphological properties of R7per are midway between those of peripheral and central visual cells. The special ommatidia at the dorsal margin of the eye are characterized by the central rhabdomeres ‘marg’. The known functional properties of the visual cells in the fly eye can be readily assigned to these classes (Table 1, Fig. 12). The non-uniform distribution of the various kinds of central rhabdomeres suggests functional differentiation of the eye region.     

Social recognition and approach in the chick: lateralization and effect of visual experience

Chicks, Gallus gallus domesticus, tested monocularly on day 3 after hatching recognize familiar versus unfamiliar conspecifics and choose to approach one or other when they use the left eye, whereas they approach familiar and unfamiliar chicks at random when they use the right eye. In experiment 1 we investigated the effects of light exposure of embryos prior to hatching on this particular form of lateralization. Irrespective of whether they hatched from eggs incubated in the dark or from eggs exposed to light during the final days of incubation, chicks using the left eye had higher choice scores (meaning they chose to approach either a familiar or an unfamiliar chick) than chicks using the right eye or both eyes. Therefore, light experience prior to hatching did not influence the lateralization of individual recognition or choice behaviour, although it did affect latency to move out of, and time spent in, the centre of the runway. Experiment 2 showed that visual/social experience posthatching influences choice behaviour: chicks housed in a group in the light for 12 h on day 1 posthatching made a clear choice between familiar and unfamiliar chicks when tested on day 3, but chicks kept in a group in the dark on day 1 did not make a choice, instead alternating between the two stimuli. In experiment 3 we found that posthatching visual/social experience increased the choice scores of chicks using the right eye and thereby removed any lateralization of choice behaviour. The results suggest that visual experience of a social group is required before chicks using their right eye (and left hemisphere) will pay attention to the cues that distinguish one chick from another. Chicks using their left eye (and right hemisphere) recognize the difference between individuals without requiring visual experience with other chicks. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Blue light and the regeneration of human rhodopsin in situ

Hubbard has found that the photoisomerization of retinene was important for the regeneration of rhodopsin in vitro, and the object of the present investigation was to find whether this was also true for regeneration in the living human eye. In the Appendix is described a device which permits the rhodopsin density to be measured by analysing the light reflected from the fundus oculi in an ophthalmoscopic arrangement, the measurement taking about 5 seconds. Now if a blue and a yellow light viewed scotopically are adjusted in intensity so as to appear identical, they must bleach rhodopsin equally, but the blue will be more than 10 times as effective in isomerizing retinene. Therefore if retinene isomerization is important for rhodopsin regeneration, blue light should cause a more rapid regeneration after bleaching, and during bleaching the equilibrium level attained should be less profound. But, as the figures show, the course of bleaching and regeneration is identical for the matched yellow or blue bleaching lights, therefore isomerization of retinene is not important for rhodopsin regeneration in the living human eye.     

Drosophila Dyrk2 Plays a Role in the Development of the Visual System

The DYRKs (dual-specificity tyrosine phosphorylation-regulated kinases) are a conserved family of protein kinases that are associated with a number of neurological disorders, but whose biological targets are poorly understood. Drosophila encodes three Dyrks: minibrain/Dyrk1A, DmDyrk2, and DmDyrk3. Here we describe the creation and characterization of a DmDyrk2 null allele, DmDyrk2^1w17. We provide evidence that the smell impaired allele smi35A¹, is likely to encode DmDyrk2. We also demonstrate that DmDyrk2 is expressed late in the developing third antennal segment, an anatomical structure associated with smell. In addition, we find that DmDyrk2 is expressed in the morphogenetic furrow of the developing eye, that loss of DmDyrk2 in the eye produced a subtle but measurable defect, and that ectopic DmDyrk2 expression in the eye produced a strong rough eye phenotype characterized by increased secondary, tertiary and bristle interommatidial cells. This phenotype was dependent on DmDyrk2 kinase activity and was only manifest when expressed in post-mitotic non-neuronal progenitors. Together, these data indicate that DmDyrk2 is expressed in developing sensory systems, that it is required for the development of the visual system, and that the eye is a good model to identify DmDyrk2 targets.     

THE VISUAL ACUITY OF THE FIDDLER-CRAB,UCA PUGNAX

The visual acuity of the fiddler-crab can be measured at various illuminations by means of its response to a moving visual pattern. The method, although similar to that used by Hecht and Wolf for the bee and Hecht and Wald for Drosophila, must be modified to give consistent results. An explanation of the response to a visual pattern is given in terms of the structure of the eye. Visual acuity of the crab varies with log I as in man, the bee, and Drosophila. Hecht and Wolf''s explanation of the varying visual acuity with illumination in terms of the distribution of functional ommatidia in the eye is supported to that extent. In the fiddler-crab as in man, monocular and binocular visual acuity is similar with a maximum of 0.0042 for the fiddler-crab. This agrees fairly well with visual acuities of 0.0041, 0.0038, and 0.0032 as found in the field. In man and the bee, the minimum visual angle corresponds to the minimum angle of two adjacent receptors; in Drosophila and the fiddler-crab the minimum visual angle corresponds to approximately twice the minimum angle between two adjacent receptors.     

Modelling the visual world of a velvet worm

In many animal phyla, eyes are small and provide only low-resolution vision for general orientation in the environment. Because these primitive eyes rarely have a defined image plane, traditional visual-optics principles cannot be applied. To assess the functional capacity of such eyes we have developed modelling principles based on ray tracing in 3D reconstructions of eye morphology, where refraction on the way to the photoreceptors and absorption in the photopigment are calculated incrementally for ray bundles from all angles within the visual field. From the ray tracing, we calculate the complete angular acceptance function of each photoreceptor in the eye, revealing the visual acuity for all parts of the visual field. We then use this information to generate visual filters that can be applied to high resolution images or videos to convert them to accurate representations of the spatial information seen by the animal. The method is here applied to the 0.1 mm eyes of the velvet worm Euperipatoides rowelli (Onychophora). These eyes of these terrestrial invertebrates consist of a curved cornea covering an irregular but optically homogeneous lens directly joining a retina packed with photoreceptive rhabdoms. 3D reconstruction from histological sections revealed an asymmetric eye, where the retina is deeper in the forward-pointing direction. The calculated visual acuity also reveals performance differences across the visual field, with a maximum acuity of about 0.11 cycles/deg in the forward direction despite laterally pointing eyes. The results agree with previous behavioural measurements of visual acuity, and suggest that velvet worm vision is adequate for orientation and positioning within the habitat.     

Relating Lateralization of Eye Use to Body Motion in the Avoidance Behavior of the Chameleon (Chamaeleo chameleon)

Lateralization is mostly analyzed for single traits, but seldom for two or more traits while performing a given task (e.g. object manipulation). We examined lateralization in eye use and in body motion that co-occur during avoidance behaviour of the common chameleon, Chamaeleo chameleon. A chameleon facing a moving threat smoothly repositions its body on the side of its perch distal to the threat, to minimize its visual exposure. We previously demonstrated that during the response (i) eye use and body motion were, each, lateralized at the tested group level (N = 26), (ii) in body motion, we observed two similar-sized sub-groups, one exhibiting a greater reduction in body exposure to threat approaching from the left and one – to threat approaching from the right (left- and right-biased subgroups), (iii) the left-biased sub-group exhibited weak lateralization of body exposure under binocular threat viewing and none under monocular viewing while the right-biased sub-group exhibited strong lateralization under both monocular and binocular threat viewing. In avoidance, how is eye use related to body motion at the entire group and at the sub-group levels? We demonstrate that (i) in the left-biased sub-group, eye use is not lateralized, (ii) in the right-biased sub-group, eye use is lateralized under binocular, but not monocular viewing of the threat, (iii) the dominance of the right-biased sub-group determines the lateralization of the entire group tested. We conclude that in chameleons, patterns of lateralization of visual function and body motion are inter-related at a subtle level. Presently, the patterns cannot be compared with humans'' or related to the unique visual system of chameleons, with highly independent eye movements, complete optic nerve decussation and relatively few inter-hemispheric commissures. We present a model to explain the possible inter-hemispheric differences in dominance in chameleons'' visual control of body motion during avoidance.     

The Measurement and Treatment of Suppression in Amblyopia

Amblyopia, a developmental disorder of the visual cortex, is one of the leading causes of visual dysfunction in the working age population. Current estimates put the prevalence of amblyopia at approximately 1-3%^1-3, the majority of cases being monocular². Amblyopia is most frequently caused by ocular misalignment (strabismus), blur induced by unequal refractive error (anisometropia), and in some cases by form deprivation.Although amblyopia is initially caused by abnormal visual input in infancy, once established, the visual deficit often remains when normal visual input has been restored using surgery and/or refractive correction. This is because amblyopia is the result of abnormal visual cortex development rather than a problem with the amblyopic eye itself^4,5 . Amblyopia is characterized by both monocular and binocular deficits^6,7 which include impaired visual acuity and poor or absent stereopsis respectively. The visual dysfunction in amblyopia is often associated with a strong suppression of the inputs from the amblyopic eye under binocular viewing conditions⁸. Recent work has indicated that suppression may play a central role in both the monocular and binocular deficits associated with amblyopia^9,10 .Current clinical tests for suppression tend to verify the presence or absence of suppression rather than giving a quantitative measurement of the degree of suppression. Here we describe a technique for measuring amblyopic suppression with a compact, portable device^11,12 . The device consists of a laptop computer connected to a pair of virtual reality goggles. The novelty of the technique lies in the way we present visual stimuli to measure suppression. Stimuli are shown to the amblyopic eye at high contrast while the contrast of the stimuli shown to the non-amblyopic eye are varied. Patients perform a simple signal/noise task that allows for a precise measurement of the strength of excitatory binocular interactions. The contrast offset at which neither eye has a performance advantage is a measure of the "balance point" and is a direct measure of suppression. This technique has been validated psychophysically both in control^13,14 and patient^6,9,11 populations.In addition to measuring suppression this technique also forms the basis of a novel form of treatment to decrease suppression over time and improve binocular and often monocular function in adult patients with amblyopia^12,15,16 . This new treatment approach can be deployed either on the goggle system described above or on a specially modified iPod touch device¹⁵.     

Ciliary muscle contraction force and trapezius muscle activity during manual tracking of a moving visual target

Previous studies have shown an association of visual demands during near work and increased activity of the trapezius muscle. Those studies were conducted under stationary postural conditions with fixed gaze and artificial visual load. The present study investigated the relationship between ciliary muscle contraction force and trapezius muscle activity across individuals during performance of a natural dynamic motor task under free gaze conditions. Participants (N = 11) tracked a moving visual target with a digital pen on a computer screen. Tracking performance, eye refraction and trapezius muscle activity were continuously measured. Ciliary muscle contraction force was computed from eye accommodative response. There was a significant Pearson correlation between ciliary muscle contraction force and trapezius muscle activity on the tracking side (0.78, p < 0.01) and passive side (0.64, p < 0.05). The study supports the hypothesis that high visual demands, leading to an increased ciliary muscle contraction during continuous eye–hand coordination, may increase trapezius muscle tension and thus contribute to the development of musculoskeletal complaints in the neck–shoulder area. Further experimental studies are required to clarify whether the relationship is valid within each individual or may represent a general personal trait, when individuals with higher eye accommodative response tend to have higher trapezius muscle activity.