Symmetry Breaking Behavior Research Articles

Imaging multicellular specimens with real-time optimized tiling light-sheet selective plane illumination microscopy.

Despite the progress made in selective plane illumination microscopy, high-resolution 3D live imaging of multicellular specimens remains challenging. Tiling light-sheet selective plane illumination microscopy (TLS-SPIM) with real-time light-sheet optimization was developed to respond to the challenge. It improves the 3D imaging ability of SPIM in resolving complex structures and optimizes SPIM live imaging performance by using a real-time adjustable tiling light sheet and creating a flexible compromise between spatial and temporal resolution. We demonstrate the 3D live imaging ability of TLS-SPIM by imaging cellular and subcellular behaviours in live C. elegans and zebrafish embryos, and show how TLS-SPIM can facilitate cell biology research in multicellular specimens by studying left-right symmetry breaking behaviour of C. elegans embryos.

Comparison of current disruption and magnetic reconnection

We examine similarities and differences between the concepts of current disruption (CD) and magnetic reconnection (MR). Both concepts have been invoked to account for explosive phenomena that involve energy transformation from magnetic field to charged particles. Similarities of these two concepts include (1) the occurrence of breakdown in magnetic connectivity, (2) magnetic energy as the primary energy source, and (3) plasma energization as a product of the process. Differences include (1) plasma flow across separatrix surfaces in an X-type magnetic field geometry being essential for MR but not for CD, (2) plasma flow ordered by the magnetic field geometry for MR but not for CD, (3) field line topology change essential for MR but not necessary for CD, and (4) CD exhibiting multifractal and symmetry breaking behavior while no such behavior has been investigated for MR. Overall, CD can be viewed as a form of generalized MR (GMR) in which the requirement for a specific magnetic field geometry and its constraint on the plasma flow pattern are removed. Therefore, the CD concept has a broader scope in applications than the MR concept alone.

Bifurcation mechanism of bursting oscillations in parametrically excited dynamical system

The evolution of bursting oscillations in a parametrically excited dynamical system with order gap between the excited frequency and the natural frequency is investigated in this paper. By regarding the periodic excited term as a slow-varying parameter, different forms of bifurcations of the system are obtained. Base on the overlap between the bifurcation diagram and the phase portrait, the mechanism of different types of bursting oscillations are obtained. Furthermore, some phenomena in bursting oscillations such as symmetry breaking behavior are explained through the bifurcations occurring at the transitions between the quiescent state (QS) and spiking state (SP).

Electronic Structure of LaNiGa2

The electronic structure of a centrosymmetric Ni-based superconductor LaNiGa 2 is calculated by the full-potential augmented plane-wave method. Comparing with LaNiC 2 , which is a non-centrosymmetric superconductor, there are two significant features in LaNiGa 2 . One is the lack of asymmetric spin–orbit interaction, which is required from space inversion symmetry. The other is the complexity of Fermi surfaces. In LaNiC 2 there is essentially one Fermi surface, where in LaNiGa 2 there are five Fermi surfaces. Based on the multiband nature of LaNiGa 2 , we propose a type of superconducting gap symmetry “Δ 1 + iΔ 2 ”, which can explain both of the full-gap behavior and the time-reversal symmetry breaking behavior.

Experimental and theoretical study on generalized oscillator strengths of the valence-shell electronic excitations in CF4

We report an angle-resolved electron energy loss spectroscopy (EELS) study on the valence-shell electronic excitations in CF(4). Experimentally momentum-transfer-dependent generalized oscillator strengths (GOSs) or GOS profiles for low-lying electronic excitations at 12.6, 13.8, and 14.8 eV are derived from EELS spectra measured at an incident electron energy of 3 keV. We also calculate GOS profiles using theoretical wave functions at the equation-of-motion coupled cluster singles and doubles level. There are good agreements between experiment and theory except for a significant discrepancy at small momentum transfer for the 1t(l) → 3s Rydberg excitation at 12.6 eV. The experimental GOS profile for 1t(l) → 3s exhibits a shape that is typical of a dipole allowed transition, while the excitation is formally dipole forbidden. This symmetry breaking behavior is rationally accounted for by qualitatively analyzing the nature of vibronic coupling effects. For the excitation band at 13.8 eV, a shoulder and extrema are observed in the GOS profile and are then found to be mainly due to the 2(1)T(2) transition. Furthermore, the theoretical GOS profile for the 2(1)T(2) transition exhibits a remarkable oscillatory pattern; its origin is discussed by considering multicenter interference effects. For the 14.8 eV excitation band, the predominant nondipole nature of the underlying transitions are revealed and comparisons with the theoretical calculations show that major contributions to this band come from the 4t(2) → 3p excitation.

Theory and applications of swarm intelligence

Swarm intelligence is the discipline that deals with natural and artificial systems composed of many individuals that coordinate their activities using decentralized control and self-organization. In particular, the discipline focuses on the behavior of social insects such as fish schools and bird flocks and colonies of ants, termites, bees, and wasps. The most well-known examples of systems studied by swarm intelligence are particle swarm optimization (PSO) and ant colony optimization (ACO). Particle swarm optimization mimics the behavior of fish schooling and bird flocking. PSO is a population-based stochastic optimization strategy with fast convergent speed than general evolutionary algorithms (EAs). Different from EAs, each particle employs not only position information but also the velocity information. They communicate good positions to each other and adjust their own positions according to their decision. In PSO, a number of simple entities—the particles—are placed in the search space of some problem or function, and each evaluates the objective function at its current location. Each particle then determines its movement through the search space by combining some aspect of the history of its own current and best (best-fitness) locations with those of one or more members of the swarm, with some random perturbations. The next iteration takes place after all particles have been moved. Eventually, the swarm as a whole, like a flock of birds collectively foraging for food, moves close to an optimum of the fitness function. Ant colony optimization is a probabilistic technique for solving computational hard problems which can be reduced to finding optimal paths. The main idea is the indirect communication between the ants by means of chemical pheromone trails. The special issue on the swarm intelligence is dedicated to the latest work in the theory and applications in this exciting area. Our aim is to provide a useful reference for understanding new trends on swarm intelligence. After a detailed review process, a total of four papers were selected to reflect the call thematic vision. The contents of these studies are briefly described as follows. The first paper entitled ‘Analysis of emergent symmetry breaking in collective decision making’ by Heiko Hamann, Thomas Schmickl, Heinz Worn, and Karl Crailsheim investigates a simulated multi-agent system (MAS) that collectively decides to aggregate at an area of high utility. The agents’ control algorithm is based on random agent– agent encounters and is inspired by the aggregation behavior of honey bees. In this article, symmetry breaking is defined, several symmetry-breaking measures are proposed, and phenomenon of emergent symmetry breaking within authors’ observed system is reported. The ability of the MAS to successfully break the symmetry depends significantly on a local-neighborhood-based threshold of the agents’ control algorithm that determines at which number of neighbors the agents stop. This dependency is analyzed, and two macroscopic features are determined that significantly influence the symmetry-breaking behavior. In addition, the connection between the ability of the Z. Cui (&) Complex System and Computational Intelligence Laboratory, Taiyuan University of Science and Technology, 030024 Shanxi, Taiyuan, China e-mail: cuizhihua@gmail.com

Analysis of emergent symmetry breaking in collective decision making

We investigate a simulated multi-agent system (MAS) that collectively decides to aggregate at an area of high utility. The agents’ control algorithm is based on random agent–agent encounters and is inspired by the aggregation behavior of honeybees. In this article, we define symmetry breaking, several symmetry breaking measures, and report the phenomenon of emergent symmetry breaking within our observed system. The ability of the MAS to successfully break the symmetry depends significantly on a local-neighborhood-based threshold of the agents’ control algorithm that determines at which number of neighbors the agents stop. This dependency is analyzed and two macroscopic features are determined that significantly influence the symmetry breaking behavior. In addition, we investigate the connection between the ability of the MAS to break symmetries and the ability to stay flexible in a dynamic environment.

Metastable states and space-time phase transitions in a spin-glass model

We study large deviations of the dynamical activity in the random orthogonal model. This is a fully connected spin-glass model with one-step replica symmetry-breaking behavior, consistent with the random first-order transition scenario for structural glasses. We show that this model displays dynamical (space-time) phase transitions between active and inactive phases, as demonstrated by singularities in large deviation functions. We argue that such transitions are generic in systems with long-lived metastable states.

Directional decisions during neutrophil chemotaxis inside bifurcating channels.

The directional migration of human neutrophils in classical chemotaxis assays is often described as a "biased random walk" implying significant randomness in speed and directionality. However, these experiments are inconsistent with in vivo observations, where neutrophils can navigate effectively through complex tissue microenvironments towards their targets. Here, we demonstrate a novel biomimetic assay for neutrophil chemotaxis using enclosed microfluidic channels. Remarkably, under these enclosed conditions, neutrophils recapitulate the highly robust and efficient navigation observed in vivo. In straight channels, neutrophils undergo sustained, unidirectional motion towards a chemoattractant source. In more complex maze-like geometries, neutrophils are able to select the most direct route over 90% of the time. Finally, at symmetric bifurcations, neutrophils split their leading edge into two sections and a "tug of war" ensues. The competition between the two new leading edges is ultimately resolved by stochastic, symmetry-breaking behavior. This behavior is suggestive of directional decision-making localized at the leading edge and a signaling role played by the cellular cytoskeleton.

On the Nature of Unrestricted Orbitals in Variational Active Space Wave Functions

Active space coupled cluster methods exhibit unusual, nonsmooth spin symmetry-breaking behavior where the unrestricted minimum lies higher in energy at short bond distances and crosses below the restricted solution at longer distances. The restricted solution is also observed to be a stable minimum slightly beyond the symmetry-breaking point. This behavior arises due to differences in the optimal active spaces defining the restricted and unrestricted wave functions and results in unrestricted wave functions that are not strictly size consistent. We suggest a new, size-consistent model that allows the orbitals to break spin symmetry only within the active space.

Bosonic stimulation as a natural realization of the Pólya urn problem

The kinematics of Bose statistics for the scattering of incoming bosons into a set of non-overlapping, degenerate single-particle states, is shown to manifest a metrically non-transitive and symmetry-breaking behaviour similar to the Pólya urn problem, familiar in diverse situations ranging from population biology to economics. It is a purely statistical consequence of bosonic stimulation, which favours scattering into a state with an already higher occupation number. A possible experimental realization is suggested.

Symmetry‐breaking in mammalian cell cohort migration during tissue pattern formation: Role of random‐walk persistence

Coordinated, cohort cell migration plays an important role in the morphogenesis of tissue patterns in metazoa. However, individual cells intrinsically move in a random walk-like fashion when studied in vitro. Hence, in the absence of an external orchestrating influence or template, the emergence of cohort cell migration must involve a symmetry-breaking event. To study this process, we used a novel experimental system in which multiple capillary endothelial cells exhibit spontaneous and robust cohort migration in the absence of chemical gradients when cultured on micrometer-scale extracellular matrix islands fabricated using microcontact printing. A computational model suggested that directional persistence of random-walk and dynamic mechanical coupling of adjacent cells are the critical control parameters for this symmetry-breaking behavior that is induced in spatially-constrained cell ensembles. The model predicted our finding that fibroblasts, which exhibit a much shorter motility persistence time than endothelial cells, failed to undergo symmetry breaking or produce cohort migration on the matrix islands. These findings suggest that cells have intrinsic motility characteristics that are tuned to match their role in tissue patterning. Our results underscore the importance of studying cell motility in the context of cell populations, and the need to address emergent features in multicellular organisms that arise not only from cell-cell and cell-matrix interactions, but also from properties that are intrinsic to individual cells.

Scaffold-mediated symmetry breaking by Cdc42p.

Cell polarization generally occurs along a single well-defined axis that is frequently determined by environmental cues such as chemoattractant gradients or cell-cell contacts, but polarization can also occur spontaneously in the apparent absence of such cues, through a process called symmetry breaking. In Saccharomyces cerevisiae, cells are born with positional landmarks that mark the poles of the cell and guide subsequent polarization and bud emergence to those sites, but cells lacking such landmarks polarize towards a random cortical site and proliferate normally. The landmarks employ a Ras-family GTPase, Rsr1p, to communicate with the conserved Rho-family GTPase Cdc42p, which is itself polarized and essential for cytoskeletal polarization. We found that yeast Cdc42p was effectively polarized to a single random cortical site even in the combined absence of landmarks, microtubules and microfilaments. Among a panel of Cdc42p effectors and interacting proteins, we found that the scaffold protein Bem1p was uniquely required for this symmetry-breaking behaviour. Moreover, polarization was dependent on GTP hydrolysis by Cdc42p, suggesting that assembly of a polarization site involves cycling of Cdc42p between GTP- and GDP-bound forms, rather than functioning as a simple on/off switch.

Spontaneous appearance of chirally asymmetric steady states in a reaction model including Michaelis–Menten type catalytic reactions

Based on a chemical oscillation model which includes both an autocatalytic and a Michaelis–Menten type catalytic reaction, a chiral symmetry breaking reaction model was constructed and its symmetry breaking transition resulting in the dominance of one enantiomer and the disappearance of the other was studied. Symmetry breaking behavior of the system depends not only on the stereoselectivity of the autocatalytic reaction but also on the stereospecificity of the Michaelis–Menten type catalytic reaction, and it was shown that a chiral symmetry breaking transition can take place even though the selectivities of these reactions are not so high. For example, when the Michaelis–Menten type catalytic reaction is perfectly stereospecific, a chirally asymmetric steady state appears even though the enantiomeric excess of the stereoselective autocatalytic reaction is less than 40%. That is, a stereoselective autocatalytic reaction is always needed for any chiral symmetry breaking transition, but its stereoselectivity need not to be high. Until now, many chemical oscillation systems, which always include an autocatalytic process, have been reported already. Accordingly, if the autocatalyst possesses optical isomers and each isomer catalyzes its own production, spontaneous appearance of a chiral symmetry breaking state can be observed in the chemical oscillation system under an appropriate experimental condition, even though the stereoselectivity in the autocatalytic reaction is not so high.

On-line learning of a mixture-of-experts neural network

The on-line learning of a mixture-of-experts system is studied in the framework of statistical physics. The time dependence of the overlap-order parameters during training is calculated analytically in the thermodynamic limit. When the number of training examples is small each expert is in a symmetric state. As the number of time steps approaches a critical point, the symmetric state begins to disintegrate. This symmetry-breaking behaviour is accounted for by means of a gating network. In the symmetric state the gating network has little effect on the learning, but when the symmetry is broken the gating network assigns the experts to appropriate subspaces in the input space. A generalization curve shows a plateau between the symmetric- and broken-symmetry states. We also find that the learning curves show different behaviours depending on the stiffness of the gating function.

Cooperative symmetry-breaking by actin polymerization in a model for cell motility.

Polymerizing networks of actin filaments are capable of exerting significant mechanical forces, used by eukaryotic cells and their prokaryotic pathogens to change shape or to move. Here we show that small beads coated uniformly with a protein that catalyses actin polymerization are initially surrounded by symmetrical clouds of actin filaments. This symmetry is broken spontaneously, after which the beads undergo directional motion. We have developed a stochastic theory, in which each actin filament is modelled as an elastic brownian ratchet, that quantitatively accounts for the observed emergent symmetry-breaking behaviour. Symmetry-breaking can only occur for polymers that have a significant subunit off-rate, such as the biopolymers actin and tubulin.

Excitonic-dimer model with oscillatory Fano damping

The dynamical properties of an excitonic dimer coupled to a harmonic oscillator are analysed as a simple model for the problem of self-localization of excitons in crystalline systems. If the oscillator is treated classically and a damping term is included in a phenomenological way, the system relaxes to its lowest-energy state, which is either a symmetrical or a site-trapped, symmetry-broken state depending on the values of the system parameters. By quantum mechanical analysis, employing the accurate eigensolutions of the system obtained in previous work, it is possible to establish a semi-quantitative argument suggesting that this symmetry-breaking behaviour is an artifact of the semiclassical approximation. This argument is based, on the one hand, on group theoretical requirements and, on the other, on the exploitation of a Fano-type model for the coupling of the local oscillator to a bath. An illustration is made for the down-cascading of the excitonic energy to the final state.

Dynamics of the dimer-oscillator model with Fano damping

The dynamical properties of an excitonic dimer coupled to a harmonic oscillator are analyzed as a simple model for the problem of self-localization of excitons in crystalline systems. If the oscillator is treated classically and a damping term is included in a phenomenological way, the system relaxes to its lowest-energy state, which is either a symmetrical or a site-trapped, symmetry-broken state depending on the values of the system parameters. When the system is treated quantum mechanically, and the accurate eigenstates of the system obtained in previous work are used, it is possible to establish a semiquantitative argument suggesting that this symmetry-breaking behavior is an artifact of the semiclassical approximation.

Dynamical symmetry breaking and space-time topology.

Vacuum polarization and spontaneous symmetry breaking in a two-dimensional massless fermion field theory with quartic interactions (the Gross-Neveu model) are studied in a cylindrical (${R}^{1}$\ifmmode\times\else\texttimes\fi{}${S}^{1}$) space-time topology. In the case of untwisted fermion fields the symmetry-breaking behavior is similar to the model in Minkowski space-time except that the location of the effective potential minima depends upon the size of the space. In the case of a twisted fermion field there exists a critical size of the space such that the symmetry breaking occurs only for space larger than the critical one.