Search goal tunes visual features optimally

How does a visual search goal modulate the activity of neurons encoding different visual features (e.g., color, direction of motion)? Previous research suggests that goal-driven attention enhances the gain of neurons representing the target's visual features. Here, we present mathematical and behavioral evidence that this strategy is suboptimal and that humans do not deploy it. We formally derive the optimal feature gain modulation theory, which combines information from both the target and distracting clutter to maximize the relative salience of the target. We qualitatively validate the theory against existing electrophysiological and psychophysical literature. A surprising prediction is that it is sometimes optimal to enhance nontarget features. We provide experimental evidence toward this through psychophysics experiments on human subjects, thus suggesting that humans deploy the optimal gain modulation strategy.     

Bayesian modeling of dynamic motion integration

The quality of the representation of an object's motion is limited by the noise in the sensory input as well as by an intrinsic ambiguity due to the spatial limitation of the visual motion analyzers (aperture problem). Perceptual and oculomotor data demonstrate that motion processing of extended objects is initially dominated by the local 1D motion cues, related to the object's edges and orthogonal to them, whereas 2D information, related to terminators (or edge-endings), takes progressively over and leads to the final correct representation of global motion. A Bayesian framework accounting for the sensory noise and general expectancies for object velocities has proven successful in explaining several experimental findings concerning early motion processing [Weiss, Y., Adelson, E., 1998. Slow and smooth: a Bayesian theory for the combination of local motion signals in human vision. MIT Technical report, A.I. Memo 1624]. In particular, these models provide a qualitative account for the initial bias induced by the 1D motion cue. However, a complete functional model, encompassing the dynamical evolution of object motion perception, including the integration of different motion cues, is still lacking. Here we outline several experimental observations concerning human smooth pursuit of moving objects and more particularly the time course of its initiation phase, which reflects the ongoing motion integration process. In addition, we propose a recursive extension of the Bayesian model, motivated and constrained by our oculomotor data, to describe the dynamical integration of 1D and 2D motion information. We compare the model predictions for object motion tracking with human oculomotor recordings.     

The Mycobacterium tuberculosis FAS-II condensing enzymes: their role in mycolic acid biosynthesis, acid-fastness, pathogenesis and in future drug development

Mycolic acids are very long-chain fatty acids representing essential components of the mycobacterial cell wall. Considering their importance, characterization of key enzymes participating in mycolic acid biosynthesis not only allows an understanding of their role in the physiology of mycobacteria, but also might lead to the identification of new drug targets. Mycolates are synthesized by at least two discrete elongation systems, the type I and type II fatty acid synthases (FAS-I and FAS-II respectively). Among the FAS-II components, the condensing enzymes that catalyse the formation of carbon-carbon bonds have received considerable interest. Four condensases participate in initiation (mtFabH), elongation (KasA and KasB) and termination (Pks13) steps, leading to full-length mycolates. We present the recent biochemical and structural data for these important enzymes. Special emphasis is given to their role in growth, intracellular survival, biofilm formation, as well as in the physiopathology of tuberculosis. Recent studies demonstrated that phosphorylation of these enzymes by mycobacterial kinases affects their activities. We propose here a model in which kinases that sense environmental changes can phosphorylate the condensing enzymes, thus representing a novel mechanism of regulating mycolic acid biosynthesis. Finally, we discuss the attractiveness of these enzymes as valid targets for future antituberculosis drug development.     

Structural similarity between the DnaA-binding proteins HobA (HP1230) from Helicobacter pylori and DiaA from Escherichia coli

In prokaryotes, DNA replication is initiated by the binding of DnaA to the oriC region of the chromosome to load the primosome machinery and start a new replication round. Several proteins control these events in Escherichia coli to ensure that replication is precisely timed during the cell cycle. Here, we report the crystal structure of HobA (HP1230) at 1.7 A, a recently discovered protein that specifically interacts with DnaA protein from Helicobacter pylori (HpDnaA). We found that the closest structural homologue of HobA is a sugar isomerase (SIS) domain containing protein, the phosphoheptose isomerase from Pseudomonas aeruginosa. Remarkably, SIS proteins share strong sequence homology with DiaA from E. coli; yet, HobA and DiaA share no sequence homology. Thus, by solving the structure of HobA, we unexpectedly discovered that HobA is a H. pylori structural homologue of DiaA. By comparing the structure of HobA to a homology model of DiaA, we identified conserved, surface-accessible residues that could be involved in protein-protein interaction. Finally, we show that HobA specifically interacts with the N-terminal part of HpDnaA. The structural homology between DiaA and HobA strongly supports their involvement in the replication process and these proteins could define a new structural family of replication regulators in bacteria.     

Comparative morphology of cephalic exocrine glands among castes of the black ant Lasius niger

The glandular system is crucially involved in main aspects of ant social life. The function of glands has been primarily studied in the workers (the non-reproductive individuals in a colony). In contrast, little information is available on queens (the reproductive females in a colony) or males in spite of the obvious functional differences between these castes. Here we report a comparison of the general morphology of the mandibular, propharyngeal and postpharyngeal glands between the three castes of the black ant Lasius niger. The analysis clearly shows that all these cephalic glands differ in relative size between castes and suggests a link between gland structure and its behavioral role in queens, workers and males. In particular, males present a hypertrophied mandibular gland. This is consistent with the fact that these glands might be the source of the sex pheromone in this caste. By contrast, queens exhibited the most developed postpharyngeal glands. This is consistent with the production of particular cues by queens for workers to help them to distinguish between reproductive and non-reproductive females. Finally, the propharyngeal glands were most developed in the worker caste and of similar relative size in males and queens. Their function is still enigmatic.     

High synovial expression of the inhibitory FcγRIIb in rheumatoid arthritis

Activating Fc gamma receptors (FcγRs) have been identified as having important roles in the inflammatory joint reaction in rheumatoid arthritis (RA) and murine models of arthritis. However, the role of the inhibitory FcγRIIb in the regulation of the synovial inflammation in RA is less known. Here we have investigated synovial tissue from RA patients using a novel monoclonal antibody (GB3) specific for the FcγRIIb isoform. FcγRIIb was abundantly expressed in synovia of RA patients, in sharp contrast to the absence or weak staining of FcγRIIb in synovial biopsies from healthy volunteers. In addition, the expression of FcγRI, FcγRII and FcγRIII was analyzed in synovia obtained from early and late stages of RA. Compared with healthy synovia, which expressed FcγRII, FcγRIII but not FcγRI, all activating FcγRs were expressed and significantly up-regulated in RA, regardless of disease duration. Macrophages were one of the major cell types in the RA synovium expressing FcγRIIb and the activating FcγRs. Anti-inflammatory treatment with glucocorticoids reduced FcγR expression in arthritic joints, particularly that of FcγRI. This study demonstrates for the first time that RA patients do not fail to up-regulate FcγRIIb upon synovial inflammation, but suggests that the balance between expression of the inhibitory FcγRIIb and activating FcγRs may be in favour of the latter throughout the disease course. Anti-inflammatory drugs that target activating FcγRs may represent valuable therapeutics in this disease.