Gadi Katzir

Visual Fields in Short-Toed Eagles, Circaetus gallicus (Accipitridae), and the Function of Binocularity in Birds

Visual fields were determined in alert restrained birds using an ophthalmoscopic reflex technique. The region of binocular overlap is relatively small: maximum width of 20° occurs approximately 15° below the horizontal, and the field extends vertically through 80° with the bill tip placed close to the centre. Monocular field width in the horizontal plane is 139°, and the field is asymmetric about the optic axis. The cyclopean field extends through 260°, and the blind area above and behind the head reaches maximum width of 100° close to the horizontal. At the frontal margins of the monocular field the retinal and optical fields do not coincide; the retinal field margin lies approximately 10° inside the optical margin. This gives rise to an apparent binocular field that is twice the width of the functional binocular field. Interspecific comparisons show that the binocular field of Short-toed Eagles is similar in shape and size to those of bird species that differ markedly in phylogeny, ecology, foraging technique, and eye size. This suggests that these relatively narrow binocular fields are a convergent feature of birds whose foraging is guided by visual cues irrespective of whether items are taken directly in the bill or in the feet, as in eagles, and irrespective of the size and shape of the monocular and cyclopean visual fields. It is argued that binocular vision in birds results from the requirement for each monocular field to extend contralaterally to embody a portion of the optical flow field which is radially symmetrical about the direction of travel. This is in contrast to functional explanations of binocularity, such as those concerned with stereopsis, which present it as a means of extracting higher order information through the combination of two monocular images of the same portion of a scene.

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

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

Multifocal lenses in coral reef fishes

SUMMARY The optical properties of crystalline lenses were studied in eleven species of coral reef fish from the Red Sea in Eilat, Israel. Three species each of diurnal planktivores, nocturnal planktivores and diurnal herbivores constituted three groups of animals with little within-group variability. In addition we studied two predators, which differed with respect to body size, prey preference, hunting method and diel activity period. All species studied have multifocal lenses. There were statistically significant differences in the optical properties of the lenses between the first three groups and between the predatory species. The properties of the lenses correlate well with known complements of visual pigments and feeding habits. Lenticular zones focusing ultraviolet light were found in two diurnal planktivores. The optical properties of the lens seem to be specifically adapted to the visual needs of each species.

Cormorants keep their power: visual resolution in a pursuit-diving bird under amphibious and turbid conditions.

Cormorants (Aves; Phalacrocoracidae) are active fliers, yet they forage by pursuit diving and capture of fish with the bill. In air, the cormorant’s cornea provides most of the total refractive power of the eye [1]. Underwater, however, corneal power is lost, as the cornea is now bathed in liquids of similar refractive index. The retention of a sharp image, while performing precise visual tasks underwater, requires that the cormorant’s optical system compensates for the loss of refractive power of the cornea. In addition, the underwater photic environment differs markedly from the aerial one, with the image quality undergoing a rapid deterioration through scatter and absorption [2,3]. Upon submergence, cormorants compensate for the loss of corneal power (>55 dioptres, D) and rapidly (>1000 D/sec) attain a state of emmetropia, i.e. they are well focussed [4], by marked changes in the shape of their very flexible lens [1,5]. However, the visual capacities of pursuit-diving birds under the optical demands imposed by moving from one medium to another and the respective differences in the photic environments have not been determined to date. We tested the aerial and underwater visual resolution of the great cormorant (Phalacrocorax carbo sinensis) for high contrast, square wave gratings. Stimuli were presented in a forced choice situation (ymaze) under high levels of natural illumination. Visual resolution was calculated from gratings of given bar widths at given distances from the y-junction to the stimuli. In clear water, the cormorants’ resolution was 8.9′ ± 0.5′ (minutes of arc, mean ± s.e, n = 5, range 10.4′ – 7.8′; Figure 1A), while in air it was 3.8′ ± 0.3′ (n = 3, range 4.3′ – 3.3′; Figure 1B). The birds’ choice underwater was performed while they swam at ~1.7 m/sec so that the testing of resolution replicated the naturally occurring dynamic discrimination of underwater events. The position at which an actual choice was made could be determined from the abrupt change in the orientation of the head toward the stimulus. In all tests, the choice was made between ~60 and 90 cm before the y-junction. Calculating underwater visual resolution for the positions of choice provided a value of 6.3′ ± 0.4′ (n = 5) underwater and 3.1′ ± 0.3′ (n = 3) in air. While vision is regarded as the major modality used in the detection and capture of prey by pursuit-diving birds, this is the first quantitative estimate of the amphibious visual capacity in any bird. Compared with other bird species, the cormorants’ resolution in air (Supplemental Table 1) was relatively low in both absolute terms (Figure 1C) and when body mass [6,7] or eye height above the ground [8] were considered. The cormorants’ resolution underwater was comparable with the higher values reported for fishes [9], Pinnipeds (e.g., seals, sea lions) [10] and Cetaceans (e.g., killer whales, dolphins) [11] (Figure 1C). Because visual resolution in single chambered eyes tends to increase with eye size [6,7], it was expected that cormorants, with corneo-scleral diameters of ~18–19 mm will have a lower resolution than aquatic mammals having eye diameters of ~20 mm (dolphins) and ~50 mm (baleen whales). The requirements to perform precise visuo-motor tasks in two optically different media, and the uniqueness of the lenticular system of these birds [1,5] make the vision of pursuitdiving birds a model of vertebrate capacities at the extreme. Performing visual tasks underwater is hindered by the rapid degradation of image brightness and contrast due to scatter and absorption of light by water molecules and by suspended particles (turbidity) [2]. We tested the visual resolution of cormorants (n = 5) under controlled, low levels of water turbidity (< 3 nephlometric turbidity units; NTU). The minimal resolvable stripe width was linearly correlated with turbidity (y = 3.71x + 7.6; R2 = 0.98; p < 0.001), hence, resolution declined with increasing turbidity within the tested range. The results obtained for clear water (~0.2 NTU) fitted the regression line well. Experimental results on the effects of turbidity on visually guided behavior of aquatic vertebrates are uncommon and, in fishes, are confined mostly to turbidity levels higher than 3 NTU and to the use of reactive distance as a behavioral measure [12]. Our results show that turbidity levels lower than 1 NTU have a clear effect on image formation underwater and consequently on the underwater visual environment in general. Low turbidity levels are commonly encountered in natural water bodies and thus are of crucial importance in our understanding of the evolution, sensory ecology, and microhabitat selection in aquatic organisms [13–15].

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

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

Sun shades and eye size in birds.

Visual field width above the head is significantly correlated (rs = 0.92, n = 11, p < 0.001) with eye size in a sample of terrestrial birds that differ in their phylogeny and ecology. These species can be divided into two groups. Smaller-eyed sun-observers (axial length <18 mm) have comprehensive or near comprehensive visual coverage of the celestial hemisphere and are thus unable to avoid viewing the sun. Larger-eyed sun-avoiders (axial length >18 mm) have restricted visual fields and various types of optical adnexa (enlarged brows, hair like feathers on the eye lids and around the eye) which can prevent solar illumination of the cornea. We suggest that these differences relate to visual rather than pathological problems and argue that the reduction of disability glare, produced by sunlight falling directly upon the eye, becomes increasingly significant as eye size increases. We propose that the reduced visual fields and optical adnexa of the larger-eyed birds are primarily concerned with the maintenance of high spatial resolution.

Ostrich ocular optics

The optical structure of the eyes of ostriches (Struthio camelus; Struthionidae; Struthioniformes) was determined by the construction of a schematic eye model for paraxial optics. The eye is large (axial length = 38 mm) and of globose shape with an anterior focal length (posterior nodal distance) of 21.8 mm. The optical design of the eye is such that the lens and cornea contribute equally to its total optical power. Interspecific comparison shows that optically the ostrich eye is a larger scaled version of the eyes of common starlings (Sturnus vulgaris) and an owl (Strix aluco).

Visual fields in herons (Ardeidae) — panoramic vision beneath the bill

keep such myrmecophiles as Homoptera and Lepidoptera for harvesting honeydew or some secretion [2, 3]. The interaction between Indonesian Myrmecina and the myrmecophilous oribatid mite is the first case of ant-myrmecophile relationships. The morphological and behavioral characteristics of the oribatid mite differ from free-living oribatid mites. To understand the evolution of the unique habit of the mites and the ants, comparative studies of the interactions in several Myrmecina ants are important.

Scanning and tracking with independent cameras--a biologically motivated approach based on model predictive control

Abstract This paper presents a framework for visual scanning and target tracking with a set of independent pan-tilt cameras. The approach is systematic and based on Model Predictive Control (MPC), and was inspired by our understanding of the chameleon visual system. We make use of the most advanced results in the MPC theory in order to design the scanning and tracking controllers. The scanning algorithm combines information about the environment and a model for the motion of the target to perform optimal scanning based on stochastic MPC. The target tracking controller is a switched control combining smooth pursuit and saccades. Min-Max and minimum-time MPC theory is used for the design of the tracking control laws. We make use of the observed chameleon’s behavior to guide the scanning and the tracking controller design procedures, the way they are combined together and their tuning. Finally, simulative and experimental validation of the approach on a robotic chameleon head composed of two independent Pan-Tilt cameras is presented.

Visually Guided Avoidance in the Chameleon (Chamaeleo chameleon): Response Patterns and Lateralization

The common chameleon, Chamaeleo chameleon, is an arboreal lizard with highly independent, large-amplitude eye movements. In response to a moving threat, a chameleon on a perch responds with distinct avoidance movements that are expressed in its continuous positioning on the side of the perch distal to the threat. We analyzed body-exposure patterns during threat avoidance for evidence of lateralization, that is, asymmetry at the functional/behavioral levels. Chameleons were exposed to a threat approaching horizontally from the left or right, as they held onto a vertical pole that was either wider or narrower than the width of their head, providing, respectively, monocular or binocular viewing of the threat. We found two equal-sized sub-groups, each displaying lateralization of motor responses to a given direction of stimulus approach. Such an anti-symmetrical distribution of lateralization in a population may be indicative of situations in which organisms are regularly exposed to crucial stimuli from all spatial directions. This is because a bimodal distribution of responses to threat in a natural population will reduce the spatial advantage of predators.