Polarization sensitivity as a contrast enhancer in pelagic predators: lessons from in situ polarization imaging of transparent zooplankton

Because light in the pelagic environment is partially polarized, it has been suggested that the polarization sensitivity found in certain pelagic species may serve to enhance the contrast of their transparent zooplankton prey. We examined its potential during cruises in the Gulf of Mexico and Atlantic Ocean and at a field station on the Great Barrier Reef. First, we collected various species of transparent zooplankton and micronekton and photographed them between crossed polarizers. Many groups, particularly the cephalopods, pelagic snails, salps and ctenophores, were found to have ciliary, muscular or connective tissues with striking birefringence. In situ polarization imagery of the same species showed that, while the degree of underwater polarization was fairly high (approx. 30% in horizontal lines of sight), tissue birefringence played little to no role in increasing visibility. This is most likely due to the low radiance of the horizontal background light when compared with the downwelling irradiance. In fact, the dominant radiance and polarization contrasts are due to unpolarized downwelling light that has been scattered from the animal viewed against the darker and polarized horizontal background light. We show that relatively simple algorithms can use this negative polarization contrast to increase visibility substantially.     

Celestial orientation in bees: the use of spectral cues

Summary The spectral cues used in the bee's celestial compass are investigated by presenting bees dancing on a horizontal comb with unpolarized (or polarized) spectral stimuli. Where appropriate, the use of e-vector information is prevented by painting out the specialized dorsal margin of the bee's eye (POL area, Fig. 1). This area has been shown to mediate e-vector information (Fig. 3; Wehner 1982), whereas the remainder of the dorsal retina is sufficient for mediating spectral information (Fig. 4).Spectral cues are used by the bees to discriminate between sun and sky (Fig. 4). According to physical reality (Fig. 2), a long-wavelength stimulus is taken as the sun, whereas a short-wavelength stimulus is expected by the bee to lie anywhere within the antisolar half of the sky (Figs. 5 and 6). This is in accord with the bee's e-vector compass in which e-vectors are confined to the antisolar half of the sky (Fig. 9).In general, spectral cues do not provide precise compass information except when a full celestial colour gradient is available including the solar and the antisolar meridian (Figs. 7 and 8).     

The lumbosacral transition: morphologic variations and their functional significance

The articular facet of a superior articular process of the sacrum is directed backward, inward, and upward with marked variations. 4 angles characterize the orientation of this facet: a) The relative angle of tilt: i.e. the angle between the articular facet and the upper end-plate of the sacrum, measured in a sagittal plane. b) The absolute angle of tilt: i.e. the angle between the articular facet and the horizontal plane, measured in a sagittal plane. c) The tilted part-angle of opening: i.e. the angle between the articular facet and the sagittal plane, measured in a plane parallel to the upper end-plate of the sacrum. d) The horizontal part-angle of opening: i.e. the angle between the articular facet and the sagittal plane, measured in a horizontal plane. These 4 angles are determined by characteristic straights within the articular facet and certain reference planes (upper end-plate of the sacrum, horizontal plane, sagittal plane). Only 2 intersecting straights suffice for an adequate determination of a geometrical plane; therefore, if we know the relative angle of tilt and the tilted part-angle of opening, we are able to construct or to calculate the absolute angle of tilt as well as the horizontal part-angle of opening by using the range of inclination of the sacrum. The shape as well as the orientation of the articular facets at the superior articular processes of the sacrum do not depend on the inclination of the pelvis nor on the inclination of the sacrum nor on the range of the lumbosacral angle. Only the absolute angle of tilt shows a reference to the inclination of the sacrum because the relative angle of tilt shows a certain constancy. The orientation of the articular facets is slightly influenced by static moments, but considerably determined by dynamical requirements. At spines with irregular numbers of praesacral vertebrae, the orientation of the lumbosacral articular facets do not differ from the orientation of these facets at spines with the regular number of 24 praesacral vertebrae. This, however, does not prove right at spines, that have a lumbosacral "transitional vertebra". Such lumbosacral transitional vertebrae detract much from the stability of the lumbosacral region of the spine.     

The effect of wire plane tilt and olive wires on proximal tibia fracture fragment stability and fracture site motion

To analyze the effect of the tilt angle relationship between the crossed wire plane and the bone axis on the stiffness of fine wire external fixation, load-deformation behavior was compared across different tilt angles (0 degree, 10 degrees, and 20 degrees) of the plane containing crossed smooth or olive wires under identical conditions of central axial compression, medial compression-bending, posterior compression-bending, posteromedial compression-bending, and torsion. Stiffness values were calculated from the load-deformation and torque-angle curves. A tilt angle of 20 degrees with olive wires provided significantly greater stiffness compared to smooth wires at any angle in any loading condition (p < 0.05). A tilt angle of 20 degrees with olive wires was also significantly more stiff than a tilt angle of 0 degree with olive wires in any loading condition. In torsion, olive wires with 10 degrees and 20 degrees tilt were not significantly different, while in posterior bending olive wires with 10 degrees tilt were significantly stiffer than olive wires with 0 degree or 20 degrees tilt. With smooth wires, tilting the wire plane caused a decrease in stiffness in posterior bending, posteromedial bending, and torsion. Overall, the use of olive wires in conjunction with tilting the wire plane enhances the fixation stiffness for proximal tibia fractures while allowing more options for wire configurations that avoid neurovascular and musculotendinous structures, and wounds.     

Polarized skylight orientation in the desert antCataglyphis

Summary The desert antCataglyphis bicolor is able to use the pattern of polarized light in the sky as compass. By confronting the ant to single spots of artificially and naturally polarized light it is shown howCataglyphis uses the polarization pattern.When exposed to a horizontal e-vector,Cataglyphis was always oriented correctly. Orientation errors occurred, however, when other e-vector directions were presented. This indicates that the e-vector positions assumed by the ant do not coincide with the e-vector positions actually realized in the sky. From this it is concluded thatCataglyphis has no detailed knowledge of the actual azimuthal positions of the e-vectors. Instead, it is relying on a simplified celestial map of the polarization patterns in the sky (Fig. 7).Usually, the ant did not confuse celestial spots with identical e-vector directions. Even at sunset when the polarization pattern is completely ambiguous, correct orientation occurred. This suggests that the ant uses additional celestial cues such as the degree of polarization, the color or the intensity to find its way home when the sun is obscured.     

Effect of body tilt on calf muscle performance and blood flow in humans.

To explore the effect of posture on muscle performance, we tested the effects of body tilt angle on the strength, endurance, and fatigue of, and blood flow into, the plantar flexors. Human subjects were fixed to a tilt table that could tilt them from the horizontal (0 degrees ) to upright (90 degrees ) position and enabled force to be applied to a footplate through isometric action of the right calf muscle. In experiment 1, six subjects performed a strength test and graded test (intermittent contractions) to the point of failure at three tilt angles (0, 47, and 90 degrees ). In Experiment 2, seven subjects performed a strength test and constant-force test [70% maximum force (F(max)); intermittent contractions] to the point of failure in the horizontal and three inclined positions (32, 47, and 67 degrees ). In experiment 3, leg blood flow was assessed during constant-force exercise at two intensities (30 and 70% F(max)) and two tilt angles (0 and 67 degrees ) in six subjects. Strength was not affected (P > 0.05) by tilt angle. Time to failure during the graded test was significantly higher at 47 degrees (25.9 +/- 2.0 min) and 90 degrees (25.1 +/- 3.0 min) than 0 degrees (22.2 +/- 2.6 min). Time to failure during the constant-force test was also significantly higher at 32 degrees (7.1 +/- 3.6 min), 47 degrees (8.0 +/- 5.2 min), and 67 degrees (8.6 +/- 5.6 min) compared with 0 degrees (4.0 +/- 2.6 min). When graded or constant-force exercise was performed with arterial flow to the leg eliminated, there were no differences in exercise time between the horizontal and an inclined position. During nonischemic exercise, leg blood flow was significantly higher during exercise in the inclined position. These results demonstrate that head-up tilt improves endurance of the plantar flexors, that this effect occurs in the absence of an effect on strength, and that it depends on an intact peripheral circulation. Moreover, the postural effect on muscle endurance appears to be due to a greater blood flow into the leg, an effect that is established during the initial contractions.     

Correlation between Lamina Cribrosa Tilt Angles,Myopia and Glaucoma Using OCT with a Wide Bandwidth Femtosecond Mode-Locked Laser

Purpose

To measure horizontal and vertical lamina cribrosa (LC) tilt angles and investigate associated factors using prototype optical coherence tomography (OCT) with a broad wavelength laser light source.

Design

Cross sectional study.

Methods

Twenty-eight no glaucoma eyes (from 15 subjects) and 25 glaucoma eyes (from 14 patients) were enrolled. A total of 300 optic nerve head B-scans were obtained in 10 µm steps and the inner edge of Bruch''s membrane opening (BMO) was identified as the reference plane. The vertical and horizontal angles between BMO line and approximate the best-fitting line for the surface of the LC were measured and potential associated factors were estimated with univariate and multivariate logistic regression analyses.

Results

The median (interquartile range) horizontal and vertical tilt angles were 7.10 (2.43–11.45) degrees and 4.15 (2.60–6.85) degrees in eyes without glaucoma and 8.50 (4.40–14.10) degrees and 9.30 (6.90–14.15) degrees in glaucoma eyes, respectively. The refractive errors had a statistically significant association with horizontal LC tilt angles (coefficients, −1.53 per diopter) and glaucoma had a significant correlation with vertical tilt angles (coefficients, 6.56) using multiple logistic regression analysis (p<0.001).

Conclusions

OCT allowed evaluation of the internal tilting of the LC compared with the BMO. The horizontal internal LC tilt angle was correlated with refractive errors, corresponding to myopic physiological changes, and vertical internal LC tilt was correlated with glaucoma, corresponding to glaucomatous pathological changes. These parameters have important implications for investigation of the correlation between myopia, glaucoma and LC morphological features.     

AXIAL ROTATION OF ERYTHRINA HERBACEA LEAFLETS

The leaflets of Erythrina herbacea, a leguminous plant native to subtropical North America, undergo daily movements. During the night, leaflets are oriented with their tips pointing downwards and show no rotation around the midrib of each leaflet. In the morning, leaflets rise, passing through a horizontal orientation and, somewhat before noon, reach a maximum average midrib elevation of approximately sixty degrees above the horizontal plane. Simultaneously, each leaflet rotates about the midrib by an average angle of forty to fifty degrees. These leaflet movements result in an initial increase in the relative leaflet surface area presented to the sun during the morning, a decrease in interception of direct solar radiation around noon, followed by a second maximum in light interception in early afternoon.     

Imported fire ant (Hymenoptera: Formicidae) mound shape characteristics along a north-south gradient

The nests of some mound-building ants are thought to serve an important function as passive solar collectors. To test this hypothesis, imported fire ant (Solenopsis invicta Buren, S. richteri Forel, and their hybrid) mound shape characteristics (south facing slope angle and area, mound height, and basal elongation in the plane of the ground) were quantified in 2005 and 2006 at a number of locations from approximately 30 degrees 25' N (Long Beach, MS) to 35 degrees 3' N (Fayetteville, TN). Insolation (w*h/m2), maximum sun angle (sun elevation in degrees above the horizon at noon, dependent on date and latitude), cumulative rainfall (7 and 30 d before sampling), and mean ambient temperature (7 d before sampling) for each site x date combination were used as predictive variables to explain mound shape characteristics. Steepness of south-facing mound slopes was negatively associated with maximum sun angle at higher temperatures, with predicted values falling from approximately 36 degrees at sun angle=40 degrees to 26 degrees at sun angle=70 degrees; at lower temperatures, slope remained relatively constant at 28 degrees. On average, mound height was negatively correlated with maximum sun angle. Rainfall had a net negative effect on mound height, but mound height increased slightly with maximum sun angle when rainfall was high. Mound elongation generally increased with increased mound building activity. Under favorable temperature conditions and average rainfall, imported fire ant mounds were tallest, most eccentric, and had the steepest south facing slopes during periods of low maximum sun angle. Mound shape characteristics are discussed with regard to season and their potential usefulness for remote sensing efforts.     

Slant-tilt: The visual encoding of surface orientation

A specific form for the internal representation of local surface orientation is proposed, which is similar to Gibson's (1950) “amount and direction of slant”. Slant amount is usually quantifed by the angle σ between the surface normal and the line of sight (0°≦σ≦90°). Slant direction corresponds to the direction of the gradient of distance from the viewer to the surface, and may be defined by the image direction τ to which the surface normal would project (0°≦τ≦360°). Since the direction of slant is specified by the tilt of the projected surface normal, it is referred to as surface tilt (Stevens, 1979; Marr, 1982). The two degrees of freedom of orientation are therefore quantified by slant, an angle measured perpendicular to the image plane, and tilt, an angle measured in the image plane. The slanttilt form provides several computational advantages relative to some other proposals and is consistent with various psychological phenomena. Slant might be encoded by various means, e.g. by the cosine of the angle, by the tangent, or linearly by the angle itself. Experimental results are reported that suggest that slant is encoded by an internal parameter that varies linearly with slant angle, with resolution of roughly one part in 100. Thus we propose that surface orientation is encoded in human vision by two quantities, one varying linearly with slant angle, the other varying linearly with tilt angle.     

Orientation of the tyrosyl D, pheophytin anion, and semiquinone Q(A)(*)(-) radicals in photosystem II determined by high-field electron paramagnetic resonance

The radical forms of two cofactors and an amino acid in the photosystem II (PS II) reaction center were studied by using high-field EPR both in frozen solution and in oriented multilayers. Their orientation with respect to the membrane was determined by using one-dimensionally oriented samples. The ring plane of the stable tyrosyl radical, Y(D)(*), makes an angle of 64 degrees +/- 5 degrees with the membrane plane, and the C-O direction is tilted by 72 degrees +/- 5 degrees in the plane of the radical compared to the membrane plane. The semiquinone, Q(A)(*)(-), generated by chemical reduction in samples lacking the non-heme iron, has its ring plane at an angle of 72 degrees +/- 5 degrees to the membrane plane, and the O-O axis is tilted by 21 degrees +/- 5 degrees in the plane of the quinone compared to the membrane plane. This orientation is similar to that of Q(A)(*)(-) in purple bacteria reaction centers except for the tilt angle which is slightly bigger. The pheophytin anion was generated by photoaccumulation under reducing conditions. Its ring plane is almost perpendicular to the membrane with an angle of 70 degrees +/- 5 degrees with respect to the membrane plane. This is very similar to the orientation of the pheophytin in purple bacteria reaction centers. The position of the g tensor with respect to the molecule is tentatively assigned for the anion radical on the basis of this comparison. In this work, the treatment of orientation data from EPR spectroscopy applied to one-dimensionally oriented multilayers is examined in detail, and improvements over previous approaches are given.     

Effect of eccentric exercise on position sense at the human forearm in different postures.

This is a study of the ability of blindfolded human subjects to match the position of their forearms before and after eccentric exercise. The hypothesis tested was that the sense of effort contributed to forearm position sense. The fall in force after the exercise was predicted to alter the relationship between effort and force and thereby induce position errors. In the arms-in-front posture, subjects had their unsupported reference arm set to one of two angles from the horizontal, 30 or 60 degrees , and they matched its position by voluntary placement of their other arm. Matching errors were compared with a task where the arms were counterweighted, so could be moved in the vertical plane with minimal effort, and where the arms were moved in the horizontal plane. In these latter two tasks, the intention was to test whether removal of an effort sensation from holding the arm against gravity influenced matching performance. It was found that, although absolute errors for counterweighted and horizontal matching were no larger than for unsupported matching, their standard deviations, 6.1 and 6.8 degrees , respectively, were significantly greater than for unsupported matching (4.6 degrees ), indicating more erratic matching. The eccentric exercise led, the next day, to a fall in maximum voluntary muscle torque of >or=15%. This was accompanied by a significant increase in matching errors for the unsupported matching task from 2.7 +/- 0.5 to 0.8 +/- 0.7 degrees but not for counterweighted (1.4 +/- 0.2 to -0.2 degrees +/- 1.1 degrees ) or horizontal matching (-1.3 +/- 0.7 degrees to -1.8 +/- 0.7 degrees ). This, it is postulated, is because the reduced voluntary torque after exercise was accompanied by a greater effort required to support the arms, leading to larger matching errors. However, effort is only able to provide positional information for unsupported matching where gravity plays a role. In gravity-neutral tasks like counterweighted or horizontal matching, a change in the effort-force relationship after exercise leaves matching accuracy unaffected.     

Navigation by light polarization in clear and turbid waters

Certain terrestrial animals use sky polarization for navigation. Certain aquatic species have also been shown to orient according to a polarization stimulus, but the correlation between underwater polarization and Sun position and hence the ability to use underwater polarization as a compass for navigation is still under debate. To examine this issue, we use theoretical equations for per cent polarization and electric vector (e-vector) orientation that account for the position of the Sun, refraction at the air-water interface and Rayleigh single scattering. The polarization patterns predicted by these theoretical equations are compared with measurements conducted in clear and semi-turbid coastal sea waters at 2 m and 5 m depth over sea floors of 6 m and 28 m depth. We find that the per cent polarization is correlated with the Sun's elevation only in clear waters. We furthermore find that the maximum value of the e-vector orientation angle equals the angle of refraction only in clear waters, in the horizontal viewing direction, over the deeper sea floor. We conclude that navigation by use of underwater polarization is possible under restricted conditions, i.e. in clear waters, primarily near the horizontal viewing direction, and in locations where the sea floor has limited effects on the light's polarization.     

Risk management in optimal foragers: the effect of sightlines and predator type on patch use,time allocation,and vigilance in gerbils

In the foraging game between gerbils and their predators, gerbils manage risk of predation using the tools of time allocation (where, when and for how long to forage) and vigilance. The optimal level of a forager's vigilance should be affected by its encounter rate with predators and the effectiveness of its vigilance in reducing mortality risk. The physical structure of the environment can alter the effectiveness of its vigilance and therefore alter its foraging behaviour. We tested this for gerbils at risk of predation from barn owls or foxes in a large vivarium. In particular, we reduced the effectiveness of vigilance by placing obstructions around feeding trays that blocked sight lines along either the vertical (vigilance directed against owls) or horizontal axis (vigilance directed against foxes), thereby changing the physical structure of the environment. In addition, we manipulated the presence of foxes and owls. In general, gerbils harvested fewer seeds, allocated less time to foraging in dangerous patches, and used more vigilance while foraging where and when risks were higher (i.e. in the presence of predators and in bright moonlight). Vertical and horizontal sightline treatments interacted synergistically to further raise perceived risk. These results imply that blocking sight lines reduces the effectiveness of vigilance, causing gerbils to use it less. Moreover, in the presence of a predator, the gerbils’ response to the blocked sightlines was more severe – harvesting less food and spending less time and vigilance – in the patches with the increased risk. This was especially so in the presence of the predator that was expected to most benefit from blocking that particular type of sight line: cover that blocked vertical sight lines was riskiest in the presence of owls, and cover that blocked horizontal sight lines was riskiest in the presence of foxes. These results strongly indicate the importance of sightlines and landscape features such as bushes in the risk management and forging decisions of gerbils, demonstrating that bush cover provides mixed blessing to gerbils by providing cover, but making vigilance ineffective.     

Accurate determination of local defocus and specimen tilt in electron microscopy

Accurate knowledge of defocus and tilt parameters is essential for the determination of three-dimensional protein structures at high resolution using electron microscopy. We present two computer programs, CTFFIND3 and CTFTILT, which determine defocus parameters from images of untilted specimens, as well as defocus and tilt parameters from images of tilted specimens, respectively. Both programs use a simple algorithm that fits the amplitude modulations visible in a power spectrum with a calculated contrast transfer function (CTF). The background present in the power spectrum is calculated using a low-pass filter. The background is then subtracted from the original power spectrum, allowing the fitting of only the oscillatory component of the CTF. CTFTILT determines specimen tilt parameters by measuring the defocus at a series of locations on the image while constraining them to a single plane. We tested the algorithm on images of two-dimensional crystals by comparing the results with those obtained using crystallographic methods. The images also contained contrast from carbon support film that added to the visibility of the CTF oscillations. The tests suggest that the fitting procedure is able to determine the image defocus with an error of about 10nm, whereas tilt axis and tilt angle are determined with an error of about 2 degrees and 1 degrees, respectively. Further tests were performed on images of single protein particles embedded in ice that were recorded from untilted or slightly tilted specimens. The visibility of the CTF oscillations from these images was reduced due to the lack of a carbon support film. Nevertheless, the test results suggest that the fitting procedure is able to determine image defocus and tilt angle with errors of about 100 nm and 6 degrees, respectively.     

Hemodynamic adjustments to head-up posture in the partly arboreal snake, Elaphe obsoleta

Radioactively-labeled microspheres were used to quantify adjustments of regional blood flows in 15 snakes (Elaphe obsoleta) subjected to 45 degrees head-up tilt. Heart rate and peripheral vascular resistance increased during tilt to compensate for the passive drop of pressure at the head. Two snakes failed to regulate blood pressure, but in 13 others arterial pressure increased at midbody (where passive changes in pressure are unexpected due to tilt alone) and arterial pressure at the head averaged 67% of the pretilt value. Tissue blood flow was reduced significantly in visceral organs, posterior skin and posterior skeletal muscle, but was maintained at pretilt levels in brain, heart, lung and anterior tissues. Ventricular systemic output averaged 24 ml/min X kg in horizontal posture and 9.4 ml/min X kg during tilt. Comparable values for pulmonary output were 4 and 6.5 ml/min X kg. Patterns of intraventricular shunting of blood acted to maintain pulmonary flow during tilt. A large right-to-left shunt (mean 76%) was present in horizontal snakes, but the shunted fraction declined during tilt (mean 54%). Left-to-right shunt increased during tilt from 7% to 14%.     

The underwater and airborne horizontal localization of sound by the northern fur seal

The accuracy of the underwater and airborne horizontal localization of different acoustic signals by the northern fur seal was investigated by the method of instrumental conditioned reflexes with food reinforcement. For pure-tone pulsed signals in the frequency range of 0.5-25 kHz the minimum angles of sound localization at 75% of correct responses corresponded to sound transducer azimuth of 6.5-7.5 degrees +/- 0.1-0.4 degrees underwater (at impulse duration of 3-90 ms) and of 3.5-5.5 degrees +/- 0.05-0.5 degrees in air (at impulse duration of 3-160 ms). The source of pulsed noise signals (of 3-ms duration) was localized with the accuracy of 3.0 degrees +/- 0.2 degrees underwater. The source of continuous (of 1-s duration) narrow band (10% of c.fr.) noise signals was localized in air with the accuracy of 2-5 degrees +/- 0.02-0.4 degrees and of continuous broad band (1-20 kHz) noise, with the accuracy of 4.5 degrees +/- 0.2 degrees.     

Diel Acid Fluctuations in C4 Amphibious Grasses

Orcuttieae is a small tribe of C4 grasses endemic to seasonal pools in the southwestern U.S., comprising the basal genus Neostapfia, Tuctoria, and the most derived group, Orcuttia. Growth is initiated underwater, and when pools dry, species undergo a metamorphosis replacing aquatic foliage with terrestrial foliage. O. californica and O. viscida exhibit CAM-like diel fluctuations in acidity in the aquatic foliage. Pulse-chase studies showed that although CO2 was fixed into malic acid in the dark, an overnight chase in the dark revealed that most label was not retained in organic acids, indicating a role other than CAM. Terrestrial foliage exhibited a very different diel fluctuation; acids accumulated during the day, and diminished overnight. Malic acid predominated and was secreted on the surface of the leaf in a manner similar to another arid land species. This terrestrial daytime acid accumulation may not be related to photosynthetic pathway but may play an anti-herbivore function. No acid fluctuations were observed in either N. colusana or T. greenei.     

Membrane insertion of Rhodopseudomonas acidophila light harvesting complex 2 investigated by high resolution AFM

Light harvesting complexes 2 (LH2) are the peripheral antenna proteins in the bacterial photosynthetic apparatus and are built of alpha/beta-heterodimers containing three bacteriochlorophylls and two carotenoids each. Previously, we have found in 2D-crystals that the complexes could be inserted within the membrane with a tilt with respect to the membrane plane (Rhodobacter sphaeroides) or without tilt (Rubrivivax gelatinosus). To investigate whether the tilted insertion represents the native state or if it is due to specific 2D-crystal contacts, we have used atomic force microscopy to investigate LH2 from Rhodopseudomonas acidophila reconstituted at different lipid to protein ratios. High-resolution topographs could be acquired of two types of 2D-crystals or of densely packed membranes. Interestingly, in type 2 2D-crystals and in non-crystalline densely packed membranes, cylinders are integrated with their symmetry axis normal to the membrane plane, while in type 1 2D-crystals LH2 cylinders are integrated with a tilt of approximately 4 degrees with respect to the membrane plane. Therefore, we present strong evidence that the tilt of LH2 does not represent the native membrane state and is due to protein-protein contacts in specific 2D-crystals.     

Orientation by polarized light in the crayfish dorsal light reflex: behavioral and neurophysiological studies

In decapod crustaceans, the dorsal light reflex rotates the eyestalk so that the dorsal retina faces the brightest segment of dorsal visual space. Stepwise displacements of white stripes elicit eyestalk rotations in the same direction as that of the stripe. Conversely, stepwise displacements of black stripes on a white background elicit eyestalk rotations in the opposite direction as that of the stripe. The reversal of the response with contrast inversion distinguishes the dorsal light reflex from an optokinetic reflex. When the visual scene is composed of polarized light, segmented by variations in e-vector orientation, displacement of segments containing near vertical e-vectors elicit responses similar to those elicited by a white stripe. Displacement of polarized stripes containing near horizontal e-vectors elicit eyestalk rotations similar to those elicited by a black stripe. The results are consistent with the use of polarized light in orientation. The stimulus conditions described above were also applied to visual interneurons (sustaining fibers) and oculomotor neurons and the results were generally in accord with the behavior. In the neural studies, it was possible to show that responses to polarized stripe displacements are predictable from the receptive field location and the neuron’s polarization tuning function. John P. Schroeter deceased on September 14, 2006.