Aspects of eye accommodation evaluated by finite elements

Axisymmetric nonlinear finite models of accommodation incorporating the posteriorly sloped force and vitreous effects have been studied by means of their effectiveness in mechanical and optical performances. All materials were assumed to be linearly elastic, vitreous and lens matrices were incompressible. The present model is subjected to certain indicated shortcomings, however, the behavior of the model is predictable, reasonable and favourably consistent with different published data, supporting the Helmholtz theory of accommodation.     

The refractive significance of the nictitating membrane of the bird eye

Summary Previous authors have suggested that in certain aquatic birds a transparent nictitating membrane of high refractive index could compensate for the refractive loss of the cornea in water. Measurements of refractive indices, refractive state and curvature carried out on a number of aquatic birds indicate that the nictitating membrane does not have a refractive function; the refractive indices of the cornea and nictitating membrane are very similar (approximately 1.37), the nictitating membrane does not alter the refractive state of the eye, and its curvature, when it is in place in front of the globe, is virtually the same as that of the cornea.This research was supported by grants from the National Research Council of Canada and the Canadian National Sportsmen's Fund. The assistance of Mote Marine Laboratory, The Mount Desert Island Biological Laboratory, the Niska Wildlife Foundation and the Felicadades Wildlife Foundation is gratefully acknowledged. The herring gulls and black guillemots were provided by Dr. W.B. Kinter. In addition, the authors are grateful for the assistance provided by Drs. J.L. Lincer and D. Peakall and Mr. R. Orleib.     

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

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

Mechanisms controlling the sensitivity of the Limulus lateral eye in natural lighting

Electroretinograms were recorded from the horseshoe crab compound eye using a high-intensity light-emitting diode and a whole-eye seawater electrode. Recordings were made from both lateral eyes in natural daylight or in continuous darkness with the optic nerve intact or cut. Recordings from two eyes of the same animal in different conditions facilitated direct comparisons of the effects of diurnal lighting and circadian efferent activity on the daily patterns of sensitivity of the eye. Structural changes appear to account for about half of the total electroretinogram excursion. Circadian input begins about 45 min in advance of sunset and the nighttime sensitivity returns to the daytime values 20 min after sunrise. When the optic nerve is cut, the nighttime sensitivity shows exponential decay over the next 5 or 6 days, consistent with a light-triggered structural light adaptation process unopposed by efferent input. Our results suggest that two mechanisms mediate the increase in lateral eye sensitivity at night—physiological dark adaptation and circadian efferent input. Three mechanisms appear to be involved in mediating the decrease in lateral eye sensitivity during daylight—physiological light adaptation, a continuous structural light adaptation process, and a separate light-triggered, efferent-primed structural light adaptation process.     

Mechanisms regulating bird predation on a herbivorous Larva guild in boreal coniferous forests

Bird predation was previously found to considerably reduce the abundance of the herbivorous insect larva guild feeding on bilberry Vacanium myrtillus within boreal coniferous forests In this follow-up study, interest was focused on determining whether female pied flycatchers Ficedula hypoleuca tend to use the field layer more frequently than other types of feeding niches In addition, the degree to which the conspicuousness of a larva increases the risk of its being preyed upon by birds was evaluated in order to further understand the mechanisms regulating the bird predation effect Larval predation risks were measured by presenting larvae with contrasting characteristics to seven captured female flycatchers
Female flycatchers hunted more frequently in the tree and field layers than in the air However, the relative degree of utilization of the tree and field layer varied considerably between years, with use of field layer being positively related to the abundance of insect larvae on bilberry
Large larvae did not suffer higher bird predation than small larvae when both size classes were presented to the birds on bilberry Larvae that crawl on leaves and stems during feeding (high degree of exposure) were preyed upon more than larvae that feed between leaves they have spun together (low degree of exposure) No difference in larval predation risk was found between sawflies and geometrids, the two taxa represented in the exposed feeder group However, dark geometrids suffered higher predation than green sawflies and green geometrids
My findings suggest that the herbivorous larva guild constitutes an abundant food resource frequently utilized by small bird predators However, certain members of the guild appeared to be preyed upon more than other members Thus the risk for predation seems to be highest for dark geometnds followed in decreasing order by green geometrids. sawflies pyralids, and tortricids     

In vitro clonal propagation of bird eye chilli (Capsicum frutescens Mill.)

An efficient and highly reproducible protocol for micropropagation of bird eye chilli Capsicum frutescens was attempted. Murashige and Skoog (MS) medium containing 0.5-3.0 mgl(-1) of 6-benzyladenine (BA), 2-isopentenyl adenine (2iP), kinetin and 0.5-2.0 mg l(-1) of indole-3-acetic acid (IAA), indole-3-butyric acid (IBA) along with 1 gl(-1) activated charcoal (AC) were used for shoot regeneration from both shoot tip and nodal explants. Shoot tip explants (100%) grew well on medium containing 1 mgl(-1) of kinetin and 1 mgl(-1) of IBA. Shoot proliferation (1-3) from nodal explants was effective on this medium. The regenerated shoots with 4-7 nodes had further growth upon sub-culturing onto kinetin (1 mgl(-1)) and IBA (1 mgl(-1)) and rooted simultaneously. The rooted plants were transferred to pots after hardening under controlled conditions. The survival percentage in pots was 80-90%.     

Adaptive Mechanisms in the compound eye of Squilla mantis (Crustacea,Stomatopoda)

Summary Light and dark adaptations were studied in the eye of Squilla mantis. Light adaptation is characterized by (1) a proximal shift of the distal pigment sheath (DPS) surrounding the proximal portion of the crystalline cone above its zone of contact with the rhabdom; (2) flattening of the distal pigment sheath; (3) lengthening of the crystalline cone correlated with shortening of the rhabdom; (4) a migration of screening pigment granules in retinula cells in the protoplasmic bridges crossing the perirhabdomal space. In animals kept in constant darkness, longitudinal displacements of the distal pigment sheath were found to be subject to a circadian rhythm characterized by a maximal light adaptation state at about 5 p.m. and a minimal one at 5 a.m. Screening pigment granule translocation in retinula cells does not show such rhythmic activity.Abbreviations a, b maximal incidence angles in L.A., and D.A., respectively - Cc crystalline cone - Dps distal pigment sheath - I extreme incident light beam - Prs perirhabdomal space - Rh rhabdom - Rp reflecting pigment This research has been supported by grant 3.012-76 of the Swiss National Science Foundation     

Mechanisms of immune privilege in the anterior segment of the eye: what we learn from corneal transplantation

The eye, like the brain and reproductive organs, possesses inherent immune privilege, and inflammation is self-regulated so as to preserve the organ functions. Studies over the past 30 years have provided insights of the multiple mechanisms of immune privilege. At present, three major lines of thought prevail regarding the molecular mechanisms of immune privilege in the eye: there are (1) anatomical, cellular, and molecular barriers in the eye; (2) eye-derived immunological tolerance, the so-called anterior chamber-associated immune deviation; and (3) immune suppressive intraocular microenvironment. In this review, the mechanisms of immune privilege that have been learned from ocular inflammation animal models, especially corneal transplantation, are described. Roles of new B7 family molecules on local immune regulation within the cornea are also introduced.     

Adaptation and accommodation in the squid axon

Current clamp data of the squid axon indicate that there is a qualitative change in the adaptive response as the magnitude of the current step is increased. Large stimulus currents have a strong inhibitory effect on spike generation and on active responses in general. Such currents always lead to only one action-potential and to the elimination of post-spike subthreshold oscillation. In view of a direct connection between stimulus current and potassium current I _K, the potassium channel of the Hodgkin-Huxley model is reinterpreted in a natural way such that the K⁺ conductance is directly dependent on I _K in addition to a voltage dependence. The I-_Kdependence seems to dominate whenever the stimulus current is greater than approximately 35 μA/cm². For current ramps, and large current steps, such a current formulation leads to good agreement with the data.     

Integral control in accommodation.

Models of the accommodation system available inthe literature may be classified as proportional controller, optimal bang-bang controller and pure integral controller types. Each of these models explains only some aspects of the experimental behavior of the system and conflicts with other experimental observations. Based on two different experiments in which the built-in phyiological feedback loop has been opened, a leaky-integrator element is proposed for the accommodation controller. This element appears to resolve the conflict between the proportional and integral controller models and also explains the experimental observations better than any of the existing models.