Defining and identifying communities in networks

The investigation of community structures in networks is an important issue in many domains and disciplines. This problem is relevant for social tasks (objective analysis of relationships on the web), biological inquiries (functional studies in metabolic and protein networks), or technological problems (optimization of large infrastructures). Several types of algorithms exist for revealing the community structure in networks, but a general and quantitative definition of community is not implemented in the algorithms, leading to an intrinsic difficulty in the interpretation of the results without any additional nontopological information. In this article we deal with this problem by showing how quantitative definitions of community are implemented in practice in the existing algorithms. In this way the algorithms for the identification of the community structure become fully self-contained. Furthermore, we propose a local algorithm to detect communities which outperforms the existing algorithms with respect to computational cost, keeping the same level of reliability. The algorithm is tested on artificial and real-world graphs. In particular, we show how the algorithm applies to a network of scientific collaborations, which, for its size, cannot be attacked with the usual methods. This type of local algorithm could open the way to applications to large-scale technological and biological systems.

National estimates of racial segregation in rural and small-town America

The objective of this paper is to provide, for the first time, comparative estimates of racial residential segregation of blacks, Hispanics, and Native Americans in nonmetropolitan and metropolitan places in 1990 and 2000. Analyses are based on block data from the 1990 and 2000 U.S. decennial censuses. The results reveal a singularly important and perhaps surprising central conclusion: levels and trends in recent patterns of racial segregation in America's small towns are remarkably similar to patterns observed in larger metropolitan cities. Like their big-city counterparts, nonmetropolitan blacks are America's most highly segregated racial minority--roughly 30% to 40% higher than the indices observed for Hispanics and Native Americans. Finally, baseline ecological models of spatial patterns of rural segregation reveal estimates that largely support the conclusions reached in previous metropolitan studies. Racial residential segregation in rural places increases with growing minority percentage shares and is typically lower in "new" places (as measured by growth in the housing stock), while racially selective annexation and the implied "racial threat" at the periphery exacerbate racial segregation in rural places. Our study reinforces the need to broaden the spatial scale of segregation beyond its traditional focus on metropolitan cities or suburban places, especially as America's population shifts down the urban hierarchy into exurban places and small towns.

Acute ACE inhibition causes plasma extravasation in mice that is mediated by bradykinin and substance P

The use of angiotensin-converting enzyme (ACE) has been associated with the occurrence of adverse effects, including cough and angioneurotic edema. Accumulation of kinins has been suggested to play a major role in these adverse effects of ACE inhibitor, although conclusive evidence for such a role is lacking. We investigated whether ACE inhibition increases plasma extravasation in mice (Swiss, C57Bl/6J, and J129Sv/Ev strains) via inhibition of bradykinin metabolism and stimulation of neurogenic inflammatory mechanisms. Intravenous captopril and enalapril increased the extravasation of Evans blue dye in all tissues examined (trachea, stomach, duodenum, and pancreas). This effect was evident 15 minutes after drug administration. The particulate dye Monastral blue identified the sites of captopril-induced leakage in the microvasculature. Pretreatment with the bradykinin B2 receptor antagonist Hoe 140 or with the tachykinin NK1 receptor antagonist SR 140333 inhibited captopril-evoked increase in plasma extravasation. In mice in which the gene encoding the bradykinin B2 receptor was disrupted by gene targeting, neither bradykinin nor captopril increased plasma extravasation. Pretreatment with Hoe 140 did not reduce the hypotensive response induced by captopril. The present findings suggest that ACE inhibition increases kinin levels in tissues and/or plasma. These increased kinin levels increase microvascular leakage in mouse airways and digestive tract via the release of tachykinins from terminals of primary sensory neurons. Exaggerated kinin production and the subsequent stimulation of peptide release from sensory nerves may be involved in adverse effects of ACE inhibitors.

TRoPICALS: a computational embodied neuroscience model of compatibility effects

Perceiving objects activates the representation of their affordances. For example, experiments on compatibility effects showed that categorizing objects by producing certain handgrips (power or precision) is faster if the requested responses are compatible with the affordance elicited by the size of objects (e.g., small or large). The article presents a neural-network architecture that provides a general framework to account for compatibility effects. The model was designed with a methodological approach (computational embodied neuroscience) that aims to provide increasingly general accounts of brain and behavior (4 sources of constraints are used: neuroscientific data, behavioral data, embodied systems, reproduction of learning processes). The model is based on 4 principles of brain organization that we claim underlie most compatibility effects. First, visual perception and action are organized in the brain along a dorsal neural pathway encoding affordances and a ventral pathway encoding goals. Second, the prefrontal cortex within the ventral pathway gives a top-down bias to action selection by integrating information on stimuli, context, and goals. Third, reaction times depend on dynamic neural competitions for action selection that integrate bottom-up and top-down information. The congruence or incongruence between affordances and goals explains the different reaction times found in the experiments. Fourth, as words trigger internal simulations of their referents, they can cause compatibility effects as objects do. We validated the model by reproducing and explaining 3 types of compatibility effects and showed its heuristic power by producing 2 testable predictions. We also assessed the explicative power of the model by comparing it with related models and showed how it can be extended to account for other compatibility effects.

Integrating reinforcement learning, equilibrium points, and minimum variance to understand the development of reaching: a computational model

Despite the huge literature on reaching behavior, a clear idea about the motor control processes underlying its development in infants is still lacking. This article contributes to overcoming this gap by proposing a computational model based on three key hypotheses: (a) trial-and-error learning processes drive the progressive development of reaching; (b) the control of the movements based on equilibrium points allows the model to quickly find the initial approximate solution to the problem of gaining contact with the target objects; (c) the request of precision of the end movement in the presence of muscular noise drives the progressive refinement of the reaching behavior. The tests of the model, based on a two degrees of freedom simulated dynamical arm, show that it is capable of reproducing a large number of empirical findings, most deriving from longitudinal studies with children: the developmental trajectory of several dynamical and kinematic variables of reaching movements, the time evolution of submovements composing reaching, the progressive development of a bell-shaped speed profile, and the evolution of the management of redundant degrees of freedom. The model also produces testable predictions on several of these phenomena. Most of these empirical data have never been investigated by previous computational models and, more important, have never been accounted for by a unique model. In this respect, the analysis of the model functioning reveals that all these results are ultimately explained, sometimes in unexpected ways, by the same developmental trajectory emerging from the interplay of the three mentioned hypotheses: The model first quickly learns to perform coarse movements that assure a contact of the hand with the target (an achievement with great adaptive value) and then slowly refines the detailed control of the dynamical aspects of movement to increase accuracy.

Unexpected Stretching of Entangled Ring Macromolecules

In the melt state at equilibrium, entangled nonconcatenated ring macromolecules adapt more compact conformations compared to their linear analogs and do not form an entanglement network. We show here that, when subjected to uniaxial stretching, they exhibit a unique response, which sets them apart from any other polymer. Remarkably, whereas both linear and ring polymers strain-harden, the viscosity of the rings increases dramatically (the melt thickens) at very low stretch rates due to the unraveling of their conformations along the stretching direction. At high rates, stretching leads to viscosity thinning similar to that of entangled linear polymers, albeit with subtle differences.

Comparison of voter and Glauber ordering dynamics on networks

We study numerically the ordering process of two very simple dynamical models for a two-state variable on several topologies with increasing levels of heterogeneity in the degree distribution. We find that the zero-temperature Glauber dynamics for the Ising model may get trapped in sets of partially ordered metastable states even for finite system size, and this becomes more probable as the size increases. Voter dynamics instead always converges to full order on finite networks, even if this does not occur via coherent growth of domains. The time needed for order to be reached diverges with the system size. In both cases the ordering process is rather insensitive to the variation of the degree distribution from sharply peaked to scale free.

Evolving mobile robots able to display collective behaviors

We present a set of experiments in which simulated robots are evolved for the ability to aggregate and move together toward a light target. By developing and using quantitative indexes that capture the structural properties of the emerged formations, we show that evolved individuals display interesting behavioral patterns in which groups of robots act as a single unit. Moreover, evolved groups of robots with identical controllers display primitive forms of situated specialization and play different behavioral functions within the group according to the circumstances. Overall, the results presented in the article demonstrate that evolutionary techniques, by exploiting the self-organizing behavioral properties that emerge from the interactions between the robots and between the robots and the environment, are a powerful method for synthesizing collective behavior.

How affordances associated with a distractor object affect compatibility effects: a study with the computational model TRoPICALS

Seeing an object activates both visual and action codes in the brain. Crucial evidence supporting this view is the observation of object to response compatibility effects: perception of an object can facilitate or interfere with the execution of an action (e.g., grasping) even when the viewer has no intention of interacting with the object. TRoPICALS is a computational model that proposes some general principles about the brain mechanisms underlying compatibility effects, in particular the idea that top-down bias from prefrontal cortex, and whether it conflicts or not with the actions afforded by an object, plays a key role in such phenomena. Experiments on compatibility effects using a target and a distractor object show the usual positive compatibility effect of the target, but also an interesting negative compatibility effect of the distractor: responding with a grip compatible with the distractor size produces slower reaction times than the incompatible case. Here, we present an enhanced version of TRoPICALS that reproduces and explains these new results. This explanation is based on the idea that the prefrontal cortex plays a double role in its top-down guidance of action selection producing: (a) a positive bias in favour of the action requested by the experimental task; (b) a negative bias directed to inhibiting the action evoked by the distractor. The model also provides testable predictions on the possible consequences of damage to volitional circuits such as in Parkinsonian patients.

Duplication of modules facilitates the evolution of functional specialization

The evolution of simulated robots with three different architectures is studied in this article. We compare a nonmodular feed-forward network, a hardwired modular, and a duplication-based modular motor control network. We conclude that both modular architectures outperform the non-modular architecture, both in terms of rate of adaptation as well as the level of adaptation achieved. The main difference between the hardwired and duplication-based modular architectures is that in the latter the modules reached a much higher degree of functional specialization of their motor control units with regard to high-level behavioral functions. The hardwired architectures reach the same level of performance, but have a more distributed assignment of functional tasks to the motor control units. We conclude that the mechanism through which functional specialization is achieved is similar to the mechanism proposed for the evolution of duplicated genes. It is found that the duplication of multifunctional modules first leads to a change in the regulation of the module, leading to a differentiation of the functional context in which the module is used. Then the module adapts to the new functional context. After this second step the system is locked into a functionally specialized state. We suggest that functional specialization may be an evolutionary absorption state.

Nonhuman gamblers: lessons from rodents, primates, and robots

The search for neuronal and psychological underpinnings of pathological gambling in humans would benefit from investigating related phenomena also outside of our species. In this paper, we present a survey of studies in three widely different populations of agents, namely rodents, non-human primates, and robots. Each of these populations offers valuable and complementary insights on the topic, as the literature demonstrates. In addition, we highlight the deep and complex connections between relevant results across these different areas of research (i.e., cognitive and computational neuroscience, neuroethology, cognitive primatology, neuropsychiatry, evolutionary robotics), to make the case for a greater degree of methodological integration in future studies on pathological gambling.

Microsurgical treatment of injury to peripheral nerves in upper and lower limbs: a critical review of the last 8 years

Nerve injuries to the upper and inferior limbs represent a common event due both to home and working accidents. Minor traumas can lead to severe disabilities if a wrong treatment is carried out. From 1997 to 2005, 920 patients were observed with a total of 1,200 major or minor nerves injuries operated. In 852 cases acute lesions treated. In 68 patients we reviewed old injuries. In 707 patients direct nerve suture and in 145 patients nerve grafts were carried out. One to 8 years follow up observed. Different protocols were adopted according to the injury: simple cut, complex lesion or surgical revision. Results were evaluated during the follow up period with different models: motor evaluation in six levels (BMRC), sensitive evaluation in five levels (HIGHET), and global evaluation with four grades (from the worst to the best result). Nerve healing is a complex biological phenomenon influenced by many parameters related both to the patient characteristics and nerve lesion. Functional and esthetical outcomes vary also according to: age, cultural and economic condition, health status, and smoking. Post operative rehabilitation is mandatory to obtain an acceptable functional result. Nerve injuries to the upper and inferior limbs represent a challenge for the microsurgeon. Ultimate success in nerve surgery is judged by functional as well as cosmetic parameters. Only patients with the appropriate indication should be operated and always by a skilled surgeon.

Penile length is a very important factor for cosmesis, function and psychosexual development in patients affected by hypospadias: Results from a long-term longitudinal cohort study

Few studies of long-term outcome of hypospadias treatment in terms of voiding, surgical complications, sexual functioning, intimate relationships and cosmetic results have been investigated and contrasting results have been obtained so far. The aim of our study is to investigate the long-term outcome of urinary and sexual function, cosmesis and the quality of intimate relationships in a series of hypospadias. In this study, 42 patients who underwent surgery for hypospadias were prospectively followed for 15 years. Medical records provided the hypospadias data, the number of reconstructive operations and the reconstruction technique that was used. Patients underwent physical examination, including penile length measurement and completed International Prostatic Symptoms Score (I-PSS), International Index Of Erectile Function (IIEF 15) and the Penile Perception Score questionnaire (PPPS). Twenty patients agreed to participate in the study. At the enrolment, the median value of HOSE was 13, as regards PPPS, 18/20 (90%) were satisfied, while in 1998 only 80% were satisfied. No significant statistical difference has been reported from the results obtained at enrolment and those obtained at follow-up, in terms of PPPS (P = 0.81), IPSS and IIEF-15. Penile length was 6.5 cm flaccid and 10.5 cm stretched. Our data show how cosmesis, function and psychosexual development for these patients are highly connected to surgical outcome, which is understood to be a decrease in penile size.

What does it take to evolve behaviorally complex organisms?

What genotypic features explain the evolvability of organisms that have to accomplish many different tasks? The genotype of behaviorally complex organisms may be more likely to encode modular neural architectures because neural modules dedicated to distinct tasks avoid neural interference, i.e. the arrival of conflicting messages for changing the value of connection weights during learning. However, if the connection weights for the various modules are genetically inherited, this raises the problem of genetic linkage: favorable mutations may fall on one portion of the genotype encoding one neural module and unfavorable mutations on another portion encoding another module. We show that this can prevent the genotype from reaching an adaptive optimum. This effect is different from other linkage effects described in the literature and we argue that it represents a new class of genetic constraints. Using simulations we show that sexual reproduction can alleviate the problem of genetic linkage by recombining separate modules all of which incorporate either favorable or unfavorable mutations. We speculate that this effect may contribute to the taxonomic prevalence of sexual reproduction among higher organisms. In addition to sexual recombination, the problem of genetic linkage for behaviorally complex organisms may be mitigated by entrusting evolution with the task of finding appropriate modular architectures and learning with the task of finding the appropriate connection weights for these architectures.

The processing of verbs and nouns in neural networks: insights from synthetic brain imaging

The paper presents a computational model of language in which linguistic abilities evolve in organisms that interact with an environment. Each individual's behavior is controlled by a neural network and we study the consequences in the network's internal functional organization of learning to process different classes of words. Agents are selected for reproduction according to their ability to manipulate objects and to understand nouns (objects' names) and verbs (manipulation tasks). The weights of the agents' neural networks are evolved using a genetic algorithm. Synthetic brain imaging techniques are then used to examine the functional organization of the neural networks. Results show that nouns produce more integrated neural activity in the sensory-processing hidden layer, while verbs produce more integrated synaptic activity in the layer where sensory information is integrated with proprioceptive input. Such findings are qualitatively compared with human brain imaging data that indicate that nouns activate more the posterior areas of the brain related to sensory and associative processing, while verbs activate more the anterior motor areas.

Modeling orienting behavior and its disorders with "ecological" neural networks

Computational modeling is a useful tool for spelling out hypotheses in cognitive neuroscience and testing their predictions in artificial systems. Here we describe a series of simulations involving neural networks that learned to perform their task by self-organizing their internal connections. The networks controlled artificial agents with an orienting eye and an arm. Agents saw objects with various shapes and locations and learned to press a key appropriate to their shape. The results showed the following: (1) Despite being able to see the entire visual scene without moving their eye, agents learned to orient their eye toward a peripherally presented object. (2) Neural networks whose hidden layers were previously partitioned into units dedicated to eye orienting and units dedicated to arm movements learned the identification task faster and more accurately than did nonmodular networks. (3) Nonetheless, even nonmodular networks developed a similar functional segregation through self-organization of their hidden layer. (4) After partial disconnection of the hidden layer from the input layer, the lesioned agents continued to respond accurately to single stimuli, wherever they occurred, but on double simultaneous stimulation they oriented toward and responded only to the right-sided stimulus, thus simulating extinction/neglect. These results stress the generality of the advantages provided by orienting processes. Hard-wired modularity, reminiscent of the distinct cortical visual streams in the primate brain, provided further evolutionary advantages. Finally, disconnection is likely to be a mechanism of primary importance in the pathogenesis of neglect and extinction symptoms, consistent with recent evidence from animal studies and brain-damaged patients.

Sedation Analgesia during Office-Based Plastic Surgery Procedures: Comparison of Two Opioid Regimens

BACKGROUND

The combination of sedative and analgesic drugs is increasingly being used during minimally invasive surgery. The authors compared the clinical efficacy of two different fentanyl regimens, in combination with midazolam, for sedation analgesia in patients undergoing office-based plastic surgery procedures under local anesthesia.

METHODS

One-hundred patients were randomized into two groups of 50 subjects each. Group F1 received a fentanyl bolus of 0.7 microg/kg before infiltration with local anaesthetics; group F2 received the same bolus plus 0.6 microg/kg fentanyl every 45 minutes. All patients received a midazolam bolus of 0.05 mg/kg plus continuous infusion 0.08 mg/kg per hour.

RESULTS

High-quality analgesia was obtained in every group, without significant differences between the two fentanyl regimens. Group F2 was associated with lower intraoperative mean blood pressure and SpO2 values compared with group F1. No differences were detected between the two groups in perioperative side effects or postoperative pain.

CONCLUSION

Higher doses of opioid did not improve the quality of perioperative patient comfort but acted synergistically with the sedative drugs, amplifying the hemodynamic and respiratory side effects.