The information content of texture gradients

Texture gradients are systematic variations in projected surface texture. It is expected that image variables such as texture density or size carry information about the surface orientation and distance. This article reexamines the information content of texture gradients and reviews the relevant psychophysics. Slant is shown to be difficult to infer from a texture gradient, but tilt (the direction of slant) is easily and reliably determined. Regarding distance, a new texture measure is introduced that has a simple geometric definition, and from which distance can be readily computed. Variables affecting the precision and the accuracy of the distance computation are discussed.     

Transient and linear dichroism studies on bacteriorhodopsin: determination of the orientation of the 568 nm all-trans retinal chromophore

The orientation of the 568 nm transition dipole moment of the retinal chromophore of bacteriorhodopsin has been determined in purple membranes from Halobacterium halobium and in reconstituted vesicles. The angle between the 568 nm transition dipole moment and the normal to the plane of the membrane was measured in two different ways.In the first method the angle was obtained from transient dichroism measurements on bacteriorhodopsin incorporated into large phosphatidylcholine vesicles. Following flash excitation with linearly polarized light, the anisotropy of the 568 nm ground-state depletion signal first decays but then reaches a time-independent value. This result, obtained above the lipid phase transition, is interpreted as arising from rotational motion of bacteriorhodopsin which is confined to an axis normal to the plane of the membrane. It is shown that the relative amplitude of the time-independent component depends on the orientation of the 568 nm transition dipole moment. From the data an angle of 78 ° ± 3 ° is determined.In the second method the linear dichroism was measured as a function of the angle of tilt between the oriented purple membranes and the direction of the light beam. The results were corrected for the angular distribution of the membranes within the oriented samples, which was determined from the mosaic spread of the first-order lamellar neutron diffraction peak. In substantial agreement with the results of the transient dichroism method, linear dichroism measurements on oriented samples lead to an angle of 71 ° ± 4 °.No significant wavelength dependence of the dichroic ratio across the 568 nm band was observed, implying that the exciton splitting in this band must be substantially smaller than the recently suggested value of 20 nm (Ebrey et al., 1977).The orientation of the 568 nm transition dipole moment, which coincides with the direction of the all-trans polyene chain of retinal, is not only of interest in connection with models for the proton pump, but can also be used to calculate the inter-chromophore distances in the purple membrane.     

Orientation of the pigments in Photosystem II: A low-temperature linear dichroism and polarized fluorescence emission study of chlorophyll-protein complexes isolated from Chlamydomonas reinhardtii

The orientation of the various absorbing and fluorescing dipoles in Photosystem II have been investigated by linearly polarized light spectroscopy at 5 K, performed on macroscopically oriented PS II complexes derived from Chlamydomonas reinhardtii. Linear dichroism and absorption spectra show that the Q_Y transitions of the chlorophyll molecules are mostly tilted at less than 35° from the plane of largest cross-section of the particle (which in vivo coincides with the plane of the thylakoid membrane). The chlorophyll forms absorbing at 676 and 683 nm are oriented closer to the membrane than the forms absorbing at 665 and 670 nm which are tilted at approximately 35° from the plane. A dip observed around 680 nm in the LD/absorption spectra indicates a component tilted at a larger angle away from the membrane plane than the 676 nm- and 683 nm-absorbing species. A component weakly absorbing around 693 nm and exhibiting a negative LD (tilt larger than 35°) is clearly resolved. The amplitude of the LD at 693 nm relative to that observed at the maximum (676 nm) varies from sample to sample. In the blue spectral region, two populations of carotenoids are observed; one absorbs around 460 and 490 nm, while the other absorbs around 510 nm. They are oriented out of and near to the thylakoid plane, respectively. Comparison of polarized absorption and fluorescence spectra from the same oriented samples allows the assignment of the 695 nm fluorescence emission to the dipoles responsible for the LD signal at 693 nm.     

Reinterpretation of Linear Dichroism of Chromatin Supports a Perpendicular Linker Orientation in the Folded State

Abstract

We present a reinterpretation of linear dichroism data for the salt induced condensation of chromatin. A conflict between electric and flow linear dichroism data for identical chromatin samples, studied at varying degrees of Mg²⁺ induced folding, can be solved if the orientation in electric fields is mainly determined through the polarization of counter ions along the linker parts, whereas the orientation in flow is governed by the hydrodynamical response of the entire chromatin fiber. The orientation of a chromatin fiber in an electric field would then depend on the linker tilt angle so that at an angle larger than 55° the fiber would tend to orient perpendicular to the applied field. The different orientation distributions obtained with the two methods of alignment may in this way provide extra information about the structure and folding of chromatin.     

Comparing non-invasive scapular tracking methods across elevation angles,planes of elevation and humeral axial rotations

Altered scapular motions premeditate shoulder impingement and other musculoskeletal disorders. Divergent experimental conditions in previous research precludes rigorous comparisons of non-invasive scapular tracking techniques. This study evaluated scapular orientation measurement methods across an expanded range of humeral postures. Scapular medial/lateral rotation, anterior/posterior tilt and protraction/retraction was measured using an acromion marker cluster (AMC), a scapular locator, and a reference stylus. Motion was captured using reflective markers on the upper body, as well as on the AMC, locator and stylus. A combination of 5 arm elevation angles, 3 arm elevation planes and 3 arm axial rotations was examined. Measurement method interacted with elevation angle and plane of elevation for all three scapular orientation directions (p < 0.01). Method of measurement interacted with axial rotation in anterior/posterior tilt and protraction/retraction (p < 0.01). The AMC had strong agreement with the reference stylus than the locator for the majority of humeral elevations, planes and axial rotations. The AMC underestimated lateral rotation, with the largest difference of ∼2° at 0° elevation. Both the locator and AMC overestimated posterior tilt at high arm elevation by up to 7.4°. Misestimations from using the locator could be enough to potentially obscure meaningful differences in scapular rotations.     

Interfacial layering and orientation ordering of ionic liquid around single-walled carbon nanotube: a molecular dynamics study

Single-walled carbon nanotubes (SWNTs) tend to aggregate to heavily tangled bundles due to the strong van der Waals attraction. Ionic liquids (ILs) are a kind of newly proposed solvents in which SWNT can be physically well dispersed. In this article, the cylindrical interface has been investigated by molecular dynamics simulation between IL of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) and an infinite long armchair (6,6) SWNT. The highly ordered structure of the cations and anions is elucidated by the simulation results. Two evident dense layers are found for both the cations and anions along the surface normal direction of the SWNT. In addition, we have observed two different orientation patterns of the cations in the first layer. In sublayer 1A, which is the nearest to the surface, the imidazolium rings of the cations prefer to be parallel to the surface, with a slight tilt angle less than 15°. In sublayer 1B, they tend to be perpendicular to the surface, with their butyl chains appearing in sublayer 1A. The [BF₄]^?  anions are found to cling to the nanotube surface with three fluoride atoms, also indicating a highly ordered orientation. The simulation results in this work provide a clue to understand the stabilisation and dispersion of SWNT bundles in ILs.     

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.     

Stress fibers of the aortic smooth muscle cells in tissues do not align with the principal strain direction during intraluminal pressurization

Stress fibers (SFs) in cells transmit external forces to cell nuclei, altering the DNA structure, gene expression, and cell activity. To determine whether SFs are involved in mechanosignal transduction upon intraluminal pressure, this study investigated the SF direction in smooth muscle cells (SMCs) in aortic tissue and strain in the SF direction. Aortic tissues were fixed under physiological pressure of 120 mmHg. First, we observed fluorescently labeled SFs using two-photon microscopy. It was revealed that SFs in the same smooth muscle layers were aligned in almost the same direction, and the absolute value of the alignment angle from the circumferential direction was 16.8° ± 5.2° (n = 96, mean ± SD). Second, we quantified the strain field in the aortic tissue in reference to photo-bleached markers. It was found in the radial-circumferential plane that the largest strain direction was − 21.3° ± 11.1°, and the zero normal strain direction was 28.1° ± 10.2°. Thus, the SFs in aortic SMCs were not in line with neither the largest strain direction nor the zero strain direction, although their orientation was relatively close to the zero strain direction. These results suggest that SFs in aortic SMCs undergo stretch, but not maximal and transmit the force to nuclei under intraluminal pressure.

     

Recovery of surface pose from texture orientation statistics under perspective projection

In a seminal paper, Witkin (1981) derived a model of surface slant and tilt recovery based on the statistics of the orientations of texture elements (texels) on a planar surface. This model made use of basic mathematical properties of probability distributions to formulate a posterior distribution on slant and tilt given a set of image orientations under orthographic projection. One problem with the Witkin model was that it produced a posterior distribution with multiple maxima, reflecting the inherent ambiguity in scene reconstruction under orthographic projection. In the present article, we extend Witkin’s method to incorporate the effects of perspective projection. An identical approach is used to that of Witkin; however, the model now reflects the effects of perspective projection on texel orientation. Performance of the new model is compared against that of Witkin’s model in a basic surface pose recovery task using both a maximum a posteriori (MAP) decision rule and a rule based on the expected value of the posterior distribution. The resultant posterior of the new model is shown to have only one maximum and thereby the ambiguity in scene interpretation is resolved. Furthermore, the model performs better than Witkin’s model using both MAP and expected value decision rules. The results are discussed in the context of human slant estimation.     

Orientation of retinal in purple membrane determined by polarized Raman spectroscopy.

The orientation of the retinal molecule in the purple membrane was determined by polarized Raman spectroscopy for stacked purple membranes. The depolarization ratios of C = C stretching vibration mode were measured for three scattering geometries of purple membrane films. From the depolarization ratios we estimated the tilt angle of the transition dipole moment of retinal to the membrane normal and the rotational angle of the molecular plane along the transition dipole moment of retinal. The molecular plane of M intermediate was found to be almost perpendicular to the membrane plane. We confirmed that the tilt angle was 65 +/- 2 degrees for both bR and M intermediates.     

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.     

Difference between palpation and optoelectronics recording of scapular motion

The aim of this study is to determine the errors of scapular localisation due to skin relative to bone motion with an optoelectronic tracking system. We compared three-dimensional (3D) scapular positions obtained with skin markers to those obtained through palpation of three scapular anatomical landmarks. The scapular kinematics of nine subjects were collected. Static positions of the scapula were recorded with the right arm elevated at 0°, 40°, 80°, 120° and 160° in the sagittal plane. Palpation and subsequent digitisation of anatomical landmarks on scapula and thorax were done at the same positions. Scapular 3D orientation was also computed during 10 repeated movements of arm elevation between 0° and 180°. Significant differences in scapular kinematics were seen between static positions and palpation when considering anterior/posterior tilt and upward/downward rotation at angles over 120° of humeral elevation and only at 120° for internal/external rotation. There was no significant difference between positions computed during static positions and during the movement for the three scapular orientations. A rotation correction model is presented in order to reduce the errors between static position and palpation measurement.     

Sensing Nanometer Depth of Focused Optical Fields with Scanning Surface Plasmon Microscopy

Evanescent wave imaging has been developed in the past decades for discriminating sub-micronic structures confined on a planar surface from bulk medium. However, this imaging method assumes a correct orientation of the objective lens versus the normal of the sample plane (null tilt angle) for a uniform evanescent illumination of the sample. Here, we propose to use the $V(Z)$ response of a heterodyne scanning microscope coupled to a high numerical aperture lens to compensate the tilt, scanning images in three dimensions. This method can be used in standard glass to dielectric reflection regime or coupled to surface plasmon resonance. We show that in the second case, we can afford tilt angle corrections better than 10 $^{-4}$ radian. As an illustration, this method is applied to a patterned surface with nanometer-squared silica islets and to adsorbed nanoparticles.     

Probing depth in monocular images

It is generally expected that depth (distance) is the internal representational primitive that corresponds to much of the perception of 3D. We tested this assumption in monocular surface stimuli that are devoid of distance information (due to orthographic projection and the chosen surface shape, with perspective projection used as a control) and yet are vividly three-dimensional. Slant judgments were found to be in close correspondence with the actual geometric slant of the stimuli; the spatial orientation of the surfaces was perceived accurately. The apparent depth in these stimuli was then tested by superimposing a stereo depth probe over the monocular surface. In both the perspective and orthographic projection the gradient of perceived depth, measured by matching the apparent depth of the stereo probe with that of the monocular surface at a series of locations, was substantial. The experiments demonstrate that in orthographic projection the visual system can compute from local surface orientation a depth quantity that is commensurate with the relative depth derived from stereo disparity. The depth data suggests that, at least in the near field, the zero value for relative depth lies at the same absolute depth as the stereo horopter (locus of zero stereo disparity). Relative to this zero value, the depth-from-slant computation seems to provide an estimate of distance information that is independent of the absolute distance to the surface.Supproted by Office of Naval Research Contract N00014-K-84-0533. We gratefully acknowledge the suggestions of Jacob Beck regarding the experimental design, and the assistance provided by Cathryn Stanford     

Orientation of reaction center and antenna chromophores in the photosynthetic membrane of rhodopseudomonas viridis

Whole cells of Rhodopseudomonas viridis were oriented in a magnetic field. The degree of orientation of the cells was determined by using a photoselection technique. In order to deduce the orientation of the antennae and chromophores of the reaction centers with respect to the membrane plane, we performed linear dichroism measurements of absolute spectra and light induced difference spectra linked to states P⁺I and PI^? on oriented cells. These measurements lead to the following conclusions:The antennae bacteriochlorophyll molecular plane is nearly perpendicular to the membrane. The Q_y and Q_x transitions moments of these molecules make respectively angles of 20 and 70°ith the membrane plane. The antenna carotenoid molecules make an angle of 45°ith the membrane.The primary electron donor possesses two transition moments centered respectively at 970 and 850 nm. The 970 nm transition moment is parallel to the membrane plane, the 850 nm transition is tilted out of the plane. Upon photooxidation of this primary electron donor, a monomer-like absorption band appears at 805 nm. Its transition makes an angle smaller than 25° with the membrane. The photooxidation of the dimer also induces an absorption band shift for the two other bacteriochlorophyll molecules of the reaction center. The absorption band shifts of the two bacteriochlorophyll molecules occur in opposite direction.One bacteriopheophytin molecule is photoreduced in state PI^?. This photoreduction induces an absorption band shift for only one bacteriochlorophyll molecule. Finally, the geometry of the dimeric primary donor seems to be affected by the presence of a negative charge in the reaction center.     

Chlorophylls and carotenoids states in vivo I - A linear dichroism study of pigments orientation in spinach chloroplasts

Using a spectropolarimeter for measuring the linear dichroism of pigments in oriented spinach chloroplasts we found a composite signal that could be interpreted neither as textural dichroism nor as a sample birefringence. We detected a fairly good orientation of pigments with respect to the normal at the plane of chloroplast lamellae. We failed to show any orientation axe in this plane. We found that all the Ca 683 is oriented, its Y direction being parallel to, or lying in the lamellae plane. Ca 673 is either unoriented or is oriented with its Y direction making an angle of 55° with the normal. If Ca 673 is unoriented, then the X direction of Ca 683 could be space positionned at about 45° of the lamellae plane. Carotenoids are oriented in the lamellar plane or close to it. Cb is equally oriented.     

Evidence of partial rotational order in gel phase DPPC.

It is shown how the dichroic ratio of the symmetric methylene stretching modes depends upon both the rotational order of hydrocarbon chains about their long axis and the tilting of the long chains with respect to the bilayer normal. Use of a recent determination of the tilt angle from x-ray measurements together with recent dichroic infrared data yields a rotational order parameter g = -0.30 compared to g = 0 for complete disorder and g = +/- 1 for complete order. The negative value of g corresponds to a preference for the plane defined by the chain carbons to be more perpendicular than parallel to the plane defined by the tilt direction and the bilayer normal.     

Electrostatic interactions at charged lipid membranes. Electrostatically induced tilt.

The changes in bilayer structure induced by surface charges in the case of an ionizable lipid were studied by X-ray diffraction, Raman spectroscopy, and film-balance measurements. With increasing surface charge in the ordered phase, the X-ray results show a decrease in bilayer thickness, whereas the hydrocarbon chain packing stays essentially constant, the Raman data signify that the internal chain ordering does not change, and the monolayer studies show a lateral expansion of the bilayer. These results are interpreted in terms of a tilt of the chains caused by the surface charges on the polar heads. The tilt angle between the direction of the chains and the bilayer normal is obtained by a detailed theoretical evaluation. The tilt allows for a better understanding of the electrostatically induced shift of the phase transition temperature and of the shift induced by the binding of water in the case of lecithin in contrast ethanolamine.     

Determining the helical tilt angle and dynamic properties of the transmembrane domains of pinholin S2168 using mechanical alignment EPR spectroscopy

The lytic cycle of bacteriophage φ21 for the infected E. coli is initiated by pinholin S²¹, which determines the timing of host cell lysis through the function of pinholin (S²¹68) and antipinholin (S²¹71). The activity of pinholin or antipinholin directly depends on the function of two transmembrane domains (TMDs) within the membrane. For active pinholin, TMD1 externalizes and lies on the surface while TMD2 remains incorporated inside the membrane forming the lining of the small pinhole. In this study, spin labeled pinholin TMDs were incorporated separately into mechanically aligned POPC (1-palmitoyl-2-oleoyl-glycero-3-phosphocholine) lipid bilayers and investigated with electron paramagnetic resonance (EPR) spectroscopy to determine the topology of both TMD1 and TMD2 with respect to the lipid bilayer; the TOAC (2,2,6,6-tetramethyl-N-oxyl-4-amino-4-carboxylic acid) spin label was used here because it attaches to the backbone of a peptide and is very rigid. TMD2 was found to be nearly colinear with the bilayer normal (n) with a helical tilt angle of 16 ± 4° while TMD1 lies on or near the surface with a helical tilt angle of 84 ± 4°. The order parameters (~0.6 for both TMDs) obtained from our alignment study were reasonable, which indicates the samples incorporated inside the membrane were well aligned with respect to the magnetic field (B₀). The data obtained from this study supports previous findings on pinholin: TMD1 partially externalizes from the lipid bilayer and interacts with the membrane surface, whereas TMD2 remains buried in the lipid bilayer in the active conformation of pinholin S²¹68. In this study, the helical tilt angle of TMD1 was measured for the first time. For TMD2 our experimental data corroborates the findings of the previously reported helical tilt angle by the Ulrich group.