Nearby contours abolish the binocular advantage

That binocular viewing confers an advantage over monocular viewing for detecting isolated low luminance or low contrast objects, has been known for well over a century; however, the processes involved in combining the images from the two eyes are still not fully understood. Importantly, in natural vision, objects are rarely isolated but appear in context. It is well known that nearby contours can either facilitate or suppress detection, depending on their distance from the target and the global configuration. Here we report that at close distances collinear (but not orthogonal) flanking contours suppress detection more under binocular compared to monocular viewing, thus completely abolishing the binocular advantage, both at threshold and suprathreshold levels. In contrast, more distant flankers facilitate both monocular and binocular detection, preserving a binocular advantage up to about four times the detection threshold. Our results for monocular and binocular viewing, for threshold contrast discrimination without nearby flankers, can be explained by a gain control model with uncertainty and internal multiplicative noise adding additional constraints on detection. However, in context with nearby flankers, both contrast detection threshold and suprathreshold contrast appearance matching require the addition of both target-to-target and flank-to-target interactions occurring before the site of binocular combination. To test an alternative model, in which the interactions occur after the site of binocular combination, we performed a dichoptic contrast matching experiment, with the target presented to one eye, and the flanks to the other eye. The two models make very different predictions for abutting flanks under dichoptic conditions. Interactions after the combination site predict that the perceived contrast of the flanked target will be strongly suppressed, while interactions before the site predict the perceived contrast will be more or less veridical. The data are consistent with the latter model, strongly suggesting that the interactions take place before the site of binocular combination.

A Simple Contrast Matching Rule for OSEM Reconstructed PET Images with Different Time of Flight Resolution

Background: Time-of-Flight (TOF) is a leading technological development of Positron Emission Tomography (PET) scanners. It reduces noise at the Maximum-Likelihood solution, depending on the coincidence–timing–resolution (CTR). However, in clinical applications, it is still not clear how to best exploit TOF information, as early stopped reconstructions are generally used. Methods: A contrast-recovery (CR) matching rule for systems with different CTRs and non-TOF systems is theoretically derived and validated using (1) digital simulations of objects with different contrasts and background diameters, (2) realistic phantoms of different sizes acquired on two scanners with different CTRs. Results: With TOF, the CR matching rule prescribes modifying the iterations number by the CTRs ratio. Without TOF, the number of iterations depends on the background dimension. CR matching was confirmed by simulated and experimental data. With TOF, image noise followed the square root of the CTR when the rule was applied on simulated data, while a significant reduction was obtained on phantom data. Without TOF, preserving the CR on larger objects significantly increased the noise. Conclusions: TOF makes PET reconstructions less dependent on background dimensions, thus, improving the quantification robustness. Better CTRs allows performing fewer updates, thus, maintaining accuracy while minimizing noise.

Membrane Protein Structures in Lipid Bilayers; Small-Angle Neutron Scattering With Contrast-Matched Bicontinuous Cubic Phases

This perspective describes advances in determining membrane protein structures in lipid bilayers using small-angle neutron scattering (SANS). Differentially labeled detergents with a homogeneous scattering length density facilitate contrast matching of detergent micelles; this has previously been used successfully to obtain the structures of membrane proteins. However, detergent micelles do not mimic the lipid bilayer environment of the cell membrane in vivo. Deuterated vesicles can be used to obtain the radius of gyration of membrane proteins, but protein-protein interference effects within the vesicles severely limits this method such that the protein structure cannot be modeled. We show herein that different membrane protein conformations can be distinguished within the lipid bilayer of the bicontinuous cubic phase using contrast-matching. Time-resolved studies performed using SANS illustrate the complex phase behavior in lyotropic liquid crystalline systems and emphasize the importance of this development. We believe that studying membrane protein structures and phase behavior in contrast-matched lipid bilayers will advance both biological and pharmaceutical applications of membrane-associated proteins, biosensors and food science.

A feasibility study of inverse contrast-matching small-angle neutron scattering method combined with size exclusion chromatography using antibody interactions as model systems

Small-angle neutron scattering (SANS) and small- angle X-ray scattering (SAXS) are powerful techniques for the structural characterization of biomolecular complexes. In particular, SANS enables a selective observation of specific components in complexes by selective deuteration with contrast-matching techniques. In most cases, however, biomolecular interaction systems with heterogeneous oligomers often contain unfavorable aggregates and unbound species, hampering data interpretation. To overcome these problems, SAXS has been recently combined with size exclusion chromatography (SEC), which enables the isolation of the target complex in a multi-component system. By contrast, SEC–SANS is only at a preliminary stage. Hence, we herein perform a feasibility study of this method based on our newly developed inverse contrast-matching (iCM) SANS technique using antibody interactions as model systems. Immunoglobulin G (IgG) or its Fc fragment was mixed with 75% deuterated Fc-binding proteins, i.e. a mutated form of IgG-degrading enzyme of Streptococcus pyogenes and a soluble form of Fcγ receptor IIIb, and subjected to SEC–SANS as well as SEC–SAXS as reference. We successfully observe SANS from the non-deuterated IgG or Fc formed in complex with these binding partners, which were unobservable in terms of SANS in D2O, hence demonstrating the potential utility of the SEC–iCM–SANS approach.

Elucidation of the mechanism of subunit exchange in αB crystallin oligomers

AlphaB crystallin (αB-crystallin) is a key protein for maintaining the long-term transparency of the eye lens. In the eye lens, αB-crystallin is a “dynamical” oligomer regulated by subunit exchange between the oligomers. To elucidate the unsettled mechanism of subunit exchange in αB-crystallin oligomers, the study was carried out at two different protein concentrations, 28.5 mg/mL (dense sample) and 0.45 mg/mL (dilute sample), through inverse contrast matching small-angle neutron scattering. Interestingly, the exchange rate of the dense sample was the same as that of the dilute sample. From analytical ultracentrifuge measurements, the coexistence of small molecular weight components and oligomers was detected, regardless of the protein concentration. The model proposed that subunit exchange could proceed through the assistance of monomers and other small oligomers; the key mechanism is attaching/detaching monomers and other small oligomers to/from oligomers. Moreover, this model successfully reproduced the experimental results for both dense and dilute solutions. It is concluded that the monomer and other small oligomers attaching/detaching mainly regulates the subunit exchange in αB-crystallin oligomer.

Direct localization of detergents and bacteriorhodopsin in the lipidic cubic phase by small-angle neutron scattering

Lipidic cubic phase (LCP) crystallization methods have been essential in obtaining crystals of certain membrane proteins, particularly G-protein-coupled receptors. LCP crystallization is generally optimized across a large number of potential variables, one of which may be the choice of the solubilizing detergent. A better fundamental understanding of the behavior of detergents in the LCP may guide and simplify the detergent selection process. This work investigates the distribution of protein and detergent in LCP using the membrane protein bacteriorhodopsin (bR), with the LCP prepared from highly deuterated monoolein to allow contrast-matched small-angle neutron scattering. Contrast-matching allows the scattering from the LCP bilayer itself to be suppressed, so that the distribution and behavior of the protein and detergent can be directly studied. The results showed that, for several common detergents, the detergent micelle dissociates and incorporates into the LCP bilayer essentially as free detergent monomers. In addition, the detergent octyl glucoside dissociates from bR, and neither the protein nor detergent forms clusters in the LCP. The lack of detergent assemblies in the LCP implies that, upon incorporation, micelle sizes and protein/detergent interactions become less important than they would be in solution crystallization. Crystallization screening confirmed this idea, with crystals obtained from bR in the presence of most detergents tested. Thus, in LCP crystallization, detergents can be selected primarily on the basis of protein stabilization in solution, with crystallization suitability a lesser consideration.

Small-angle neutron scattering solution structures of NADPH-dependent sulfite reductase

Sulfite reductase (SiR), a dodecameric complex of flavoprotein reductase subunits (SiRFP) and hemoprotein oxidase subunits (SiRHP), reduces sulfur reduction for biomass incorporation. Electron transfer within SiR requires intra- and inter-subunit interactions that are mediated by the relative position of each protein, governed by flexible domain movements. Using small-angle neutron scattering, we report the first solution structures of SiR heterodimers containing a single copy of each subunit. These structures show how the subunits bind and how both subunit binding and oxidation state impact SiRFP's conformation. Neutron contrast matching experiments on selectively deuterated heterodimers allow us to define the contribution of each subunit to the solution scattering. SiRHP binding induces a change in the position of SiRFP's flavodoxin-like domain relative to its ferredoxin-NADP+ reductase domain while compacting SiRHP's N-terminus. Reduction of SiRFP leads to a more open structure relative to its oxidized state, re-positioning SiRFP's N-terminal flavodoxin-like domain towards the SiRHP binding position. These structures show, for the first time, how both SiRHP binding to, and reduction of, SiRFP positions SiRFP for electron transfer between the subunits.

Abstract 15791: Evaluation of Medicare Claims for Heart Failure Diagnostic Development: A Multisense-linkage Feasibility Analysis

Introduction: The validity of utilizing claims data for the development of worsening heart failure (HF) diagnostics has not been previously reported. Therefore, we linked data from the MultiSENSE study, in which the HeartLogic diagnostic was first reported, with claims data in order to validate claims HF events and compare diagnostic performance. Methods: Identifiers from 791 United States study patients were directly linked with Medicare claims to produce 320 patients with continuous Medicare Part A/B fee-for-service (FFS) coverage during study participation. Claims HF events were defined using primary HF diagnosis codes (DRG and ICD-9-CM) and included inpatient events as well as outpatient events with intravenous decongestive therapy. We matched events by patient and date, calculated agreement between events (Cohen’s kappa coefficient κ), and evaluated HeartLogic diagnostic performance using sensitivity and false positive rate (FPR). Results: The linked cohort was older than the remaining patients and had greater disease burden. Study deaths (N=14) matched exactly with claims. In contrast, matching of 207 study hospitalizations with 197 acute inpatient events resulted in a total of 239 events of which 193 matched with claims (81%), 32 (13%) were claims-unique, and 14 (6%) were study-unique. Claims HF events had substantial agreement with study adjudicated hospitalizations (observed = 93.3%, κ = 0.823). The HeartLogic algorithm detected claims-derived events with a sensitivity of 75.6% and an FPR of 1.539 alerts/pt-year, which was not different from performance derived using study events (sensitivity = 77.6% and FPR = 1.528, P = 0.82 and 0.92). HeartLogic detected events contributed to 91% of HF event costs used for performance evaluation ($605,000 out of $663,000). Conclusions: The agreement between claims and study events, and equivalence of HeartLogic diagnostic performance suggest that claims data may have utility for future diagnostic development/enhancement. Additional data are needed to demonstrate safety, efficacy, and cost-effectiveness of HeartLogic-directed interventions.