Efficient event-driven simulation of large networks of spiking neurons and dynamical synapses

A simulation procedure is described for making feasible large-scale simulations of recurrent neural networks of spiking neurons and plastic synapses. The procedure is applicable if the dynamic variables of both neurons and synapses evolve deterministically between any two successive spikes. Spikes introduce jumps in these variables, and since spike trains are typically noisy, spikes introduce stochasticity into both dynamics. Since all events in the simulation are guided by the arrival of spikes, at neurons or synapses, we name this procedure event-driven. The procedure is described in detail, and its logic and performance are compared with conventional (synchronous) simulations. The main impact of the new approach is a drastic reduction of the computational load incurred upon introduction of dynamic synaptic efficacies, which vary organically as a function of the activities of the pre- and postsynaptic neurons. In fact, the computational load per neuron in the presence of the synaptic dynamics grows linearly with the number of neurons and is only about 6% more than the load with fixed synapses. Even the latter is handled quite efficiently by the algorithm. We illustrate the operation of the algorithm in a specific case with integrate-and-fire neurons and specific spike-driven synaptic dynamics. Both dynamical elements have been found to be naturally implementable in VLSI. This network is simulated to show the effects on the synaptic structure of the presentation of stimuli, as well as the stability of the generated matrix to the neural activity it induces.

Collective behavior of networks with linear (VLSI) integrate-and-fire neurons

We analyze in detail the statistical properties of the spike emission process of a canonical integrate-and-fire neuron, with a linear integrator and a lower bound for the depolarization, as often used in VLSI implementations (Mead, 1989). The spike statistics of such neurons appear to be qualitatively similar to conventional (exponential) integrate-and-fire neurons, which exhibit a wide variety of characteristics observed in cortical recordings. We also show that, contrary to current opinion, the dynamics of a network composed of such neurons has two stable fixed points, even in the purely excitatory network, corresponding to two different states of reverberating activity. The analytical results are compared with numerical simulations and are found to be in good agreement.

Specific expression of soluble adenylyl cyclase in male germ cells

The cAMP signaling pathway is an important mediator of extracellular signals in organisms from prokaryotes to higher eukaryotes. In mammals two types of adenylyl cyclase synthesize cAMP; a ubiquitous family of transmembrane isoforms regulated by G proteins in response to extracellular signals, and a recently isolated soluble enzyme insensitive to heterotrimeric G protein modulation. Using the very sensitive reverse transcription-polymerase chain reaction (RT-PCR), soluble adenylyl cyclase (sAC) expression is detectable in almost all tissues examined; however, Northern analysis and in situ hybridization indicate that high levels of sAC message are unique to male germ cells. Elevated levels of sAC mRNA are first observed in pachytene spermatocytes and expression increases through spermiogenesis. The accumulation of high levels of message in round spermatids suggests sAC protein plays an important role in the generation of cAMP in spermatozoa, implying possible roles in sperm maturation through the epididymis, capacitation, hypermotility, and/or the acrosome reaction.

Temperature Modulation of Slow and Fast Cortical Rhythms

In the local cortical network, spontaneous emergent activity self-organizes in rhythmic patterns. These rhythms include a slow one (<1 Hz), consisting in alternation of up and down states, and also faster rhythms (10–80 Hz) generated during up states. Varying the temperature in the bath between 26 and 41°C resulted in a strong modulation of the emergent network activity. Up states became shorter for warmer temperatures and longer with cooling, whereas down states were shortest at physiological (36–37°C) temperature. The firing rate during up states was robustly modulated by temperature, increasing with higher temperatures. The sparse firing rate during down states hardly varied with temperature, thus resulting in a progressive merging of up and down states for temperatures around 30°C. Below 30°C and down to 26°C the firing lost rhythmicity, becoming progressively continuous. The slope of the down-to-up transitions, which reflects the speed of recruitment of the local network, was progressively steeper for higher temperatures, whereas wave-propagation speed exhibited only a moderate increase. Fast rhythms were particularly sensitive to temperature. Broadband high-frequency fluctuations in the local field potential were maximal for recordings at 36–38°C. Overall, we found that maintaining cortical slices at physiological temperature is critical for the generated activity to be analogous to that in vivo. We also demonstrate that changes in activity with temperature were not secondary to oxygenation changes. Temperature variation sets the in vitro cortical network at different functional regimes, allowing the exploration of network activity generation and control mechanisms.

Stop-event-related potentials from intracranial electrodes reveal a key role of premotor and motor cortices in stopping ongoing movements

In humans, the ability to withhold manual motor responses seems to rely on a right-lateralized frontal–basal ganglia–thalamic network, including the pre-supplementary motor area and the inferior frontal gyrus (IFG). These areas should drive subthalamic nuclei to implement movement inhibition via the hyperdirect pathway. The output of this network is expected to influence those cortical areas underlying limb movement preparation and initiation, i.e., premotor (PMA) and primary motor (M1) cortices. Electroencephalographic (EEG) studies have shown an enhancement of the N200/P300 complex in the event-related potentials (ERPs) when a planned reaching movement is successfully stopped after the presentation of an infrequent stop-signal. PMA and M1 have been suggested as possible neural sources of this ERP complex but, due to the limited spatial resolution of scalp EEG, it is not yet clear which cortical areas contribute to its generation. To elucidate the role of motor cortices, we recorded epicortical ERPs from the lateral surface of the fronto-temporal lobes of five pharmacoresistant epileptic patients performing a reaching version of the countermanding task while undergoing presurgical monitoring. We consistently found a stereotyped ERP complex on a single-trial level when a movement was successfully cancelled. These ERPs were selectively expressed in M1, PMA, and Brodmann's area (BA) 9 and their onsets preceded the end of the stop process, suggesting a causal involvement in this executive function. Such ERPs also occurred in unsuccessful-stop (US) trials, that is, when subjects moved despite the occurrence of a stop-signal, mostly when they had long reaction times (RTs). These findings support the hypothesis that motor cortices are the final target of the inhibitory command elaborated by the frontal–basal ganglia–thalamic network.

Slow wave activity as the default mode of the cerebral cortex

The function of sleep remained one of largest enigmas of neuroscience for most of the 20th century. However in recent years different evidence has accumulated in support of a critical role of sleep on functions such as replay and memory consolidation. In particular slow wave sleep, and its underlying corticothalamocortical activity, slow oscillations, could be critical not only for memory but also for the maintenance of the brain's structural and func- tional connectivity. In this article we ask: why slow oscillations? To answer this question we put forward the idea that slow oscillations are the default activity of the cortical network based on both experimental and theoretical evidence. We go on to discuss why slow oscillations emerge from the cortical circuits and what are the dynamic advantages of this activity pattern, such as the resilience to perturbances and the facilitation of transitions between a disconnected (e.g. deep sleep) brain and a connected, awake brain.

Energy-dependent nucleolar localization of p53 in vitro requires two discrete regions within the p53 carboxyl terminus

The p53 tumor suppressor is a nucleocytoplasmic shuttling protein that is found predominantly in the nucleus of cells. In addition to mutation, abnormal p53 cellular localization is one of the mechanisms that inactivate p53 function. To further understand features of p53 that contribute to the regulation of its trafficking within the cell, we analysed the subnuclear localization of wild-type and mutant p53 in human cells that were either permeabilized with detergent or treated with the proteasome inhibitor MG132. We, here, show that either endogenously expressed or exogenously added p53 protein localizes to the nucleolus in detergent-permeabilized cells in a concentration- and ATP hydrolysis-dependent manner. Two discrete regions within the carboxyl terminus of p53 are essential for nucleolar localization in permeabilized cells. Similarly, localization of p53 to the nucleolus after proteasome inhibition in unpermeabilized cells requires sequences within the carboxyl terminus of p53. Interestingly, genotoxic stress markedly decreases the association of p53 with the nucleolus, and phosphorylation of p53 at S392, a site that is modified by such stress, partially impairs its nucleolar localization. The possible significance of these findings is discussed.

Adaptation shapes local cortical reactivity: from bifurcation diagram and simulations to human physiological and pathological responses

Human studies employing intracerebral and transcranial perturbations suggest that the input-output properties of cortical circuits are dramatically affected during sleep in healthy subjects as well as in awake patients with multifocal and focal brain injury. In all these conditions, cortical circuits react to direct stimulation with an initial activation followed by suppression of activity (Off-period) that disrupts the build-up of sustained causal interactions typically observed in healthy wakefulness. The transition to this stereotypical response has important clinical implications, being associated with loss of consciousness or loss of function. Here, we provide a mechanistic explanation of these findings by means of simulations of a cortical-like module endowed with activity-dependent adaptation and mean-field theory. First, we show that fundamental aspects of the local responses elicited in humans by direct cortical stimulation can be replicated by systematically varying the relationships between adaptation strength and excitation level in the network. Then, we reveal a region in the adaptation-excitation parameter space of crucial relevance for both physiological and pathological conditions, where spontaneous activity and responses to perturbation diverge in their ability to reveal Off-periods. Finally, we substantiate through simulations of connected cortical-like modules the role of adaptation mechanisms in preventing cortical neurons from engaging in reciprocal causal interactions, as suggested by empirical studies. These modeling results provide a general theoretical framework and a mechanistic interpretation for a body of neurophysiological measurements that bears critical relevance for physiological states as well as for the assessment and rehabilitation of brain-injured patients.Significance StatementSuppression of cortical activity following an initial activation is a defining feature of deep sleep in healthy subjects and wakefulness in patients affected by focal and multifocal brain injuries. Experimental findings suggest that these bimodal responses disrupt the emergence of complex interactions among cortical regions, leading to loss of consciousness or functional impairments. Given their practical implications, studying the mechanisms involved within a general theoretical framework is essential. Using a neuronal network model, we provide evidence for the key role of activity-dependent adaptation mechanisms in shaping the responses to perturbation and affecting the build-up of complex cortical interactions. Overall, this work provides a mechanistic interpretation relevant to the stratification, follow-up, and rehabilitation of brain-injured patients.

Some effects of tonal fatiguing on spontaneous and distortion-product otoacoustic emissions

Auditory fatiguing can be considered a suitable test to assess some cochlear mechanisms and diseases otherwise not easily detectable. Since spontaneous otoacoustic emissions (SOAEs) originate from active sources within the cochlea they show sensitive and early vulnerability to noise, displaying informative time-courses after overstimulation in the short (0-6 s) and in the long term (1-10 min) depending on the frequency of the fatiguing stimulus. Distortion product otoacoustic emissions too, in subjects with SOAEs, show interesting modifications after pure-tone exposure, detectable either on distortion product audiograms or in the growth functions. The modifications take place within a period of 5-7 minutes and strongly depend on the frequency of the fatiguing stimulus and on the closeness between SOAE and distortion product place. The data suggest that not only the interaction place between f1 and f2 has to be considered from a biomechanical and clinical point of view, but also the specific distortion product place on the cochlear partition.

[Pharmacotherapy of generalized anxiety disorder: state of the art]

A large percentage of patients in primary care suffer from Generalized Anxiety Disorder (GAD). A task force of the Swiss GAD Society has reviewed the scientific literature and has developed treatment recommendations. Basic treatment, adjunctive treatment and therapy of specific problems like insomnia and comorbidities are differentiated. Newer antidepressants are recommended as basic treatment, especially venlafaxine and paroxetine, which are licensed for that indication.

[The psychosocial problems in a population of emigrant Italian workers]

The psycho-social problems of a group of 365 Italian emigrants to Switzerland, admitted to psychiatric hospitals, were studied. It was found that non professional subjects (housewives, unemployed, retired) and manual workers (builders, waiters, factory workers) were more likely to suffer from psycho-social disorders. Only 11.1% of those studied had received a secondary education, therefore it could be assumed that subjects having a low technical and cultural level were less prepared, psychologically, for their new situation and were thus more susceptible to psycho-social disorders. The most frequent diagnoses were found to be: affective disorders, alcohol addiction, opiate addiction and schizophrenia. The study revealed that emigrants ran a greater risk of first time admittance to psychiatric wards whereas the local population had a greater number of relapses. In conclusion it is stressed that assistance with an aim to improve work, social and relationship conditions and to integrate the emigrants into their host society can reduce some forms of psycho-social disorders.

[Work psychopathology and migration]

The author analyses the correlation between work psychopathology, migration and the role of the occupational physician. Migration was studied and analysed by many clinical psychiatrists, occupational psychiatrists and sociologists, who tried to investigate the possible consequences produced by this event on the psychological equilibrium, on social life and on working activities. It is unquestionable, according to the actual knowledge, that a strict correlation exists between work and psyche, or among the uneasiness of the work, cultural belonging and a psychopathology of the work. Work psycophatology appears to be the measurement of the repercussion of a dangerous work on human mental health and, consequently, could have a role in better adapting the man to his work. Therefore, in every work situation, the risk of a somatic or psycological, or even toxicological, pathology, must not be underevaluated. In a perspective of transcultural evaluation of the "migrant" worker who has to be introduced in a work environment, the knowledge of the "unspecific" health risks of the migrant, and of the cultural differences specifying the cultural value of work, becomes fundamental. Among the symptoms that require particular attention in the evaluation of a worker, the concept of fatigue is certainly fundamental and crucial. Consequently, it is necessary to safeguard the health of "the whole worker", not only acting on the physical and chemical occupational hazards, but also considering the relational and cultural risk factors.

Correction: Energy-dependent nucleolar localization of p53 in vitro requires two discrete regions within the p53 carboxyl terminus

Following the publication of this article the authors noted that two images were duplicated in Figure 2B. The corrected figure 2B is below. The authors wish to apologize for any inconvenience caused.

"We Are the Sons of Our Own Deeds": Comparing Skeletal Health and Frailty Indices in Deceased Individuals Across 2000 Years of Milanese History

OBJECTIVES

In bioarchaeology, the concepts of resilience and frailty, and their quantification through indices, have gathered significant attention. This study is the first to apply, evaluate, and compare skeletal frailty indices and aims to trace frailty over time while identifying methodological challenges in their use on a sample representative of urban Milan's history.

MATERIALS AND METHODS

Two-hundred fifty individuals from five historical periods over 2000 years in urban Milan, equally represented by estimated males and females, were analyzed. Three skeletal frailty indices were applied-the "Health Index" GHHP, "Skeletal Frailty Index" (SFI), and "Biological Index of Frailty" (BIF)-and their diachronic variations interpreted. Index values were compared to each other through Spearman's correlations, and frailty values were assessed by periods (overall and by estimated sex) and by estimated sex through ANOVA and General Linear Models.

RESULTS

Diachronic analyses revealed a gradual increase in frailty from the Roman era to the Late Middle Ages, which then progressively decreased, corroborating historical sources. While all methods identified the Late Middle Ages sample as the frailest, discrepancies arose when defining the least frail group, especially when considering estimated biological sex and age variables.

DISCUSSION

Our study found practical and conceptual limitations in the GHHP. Most noticeably, criteria for GHHP and SFI limited sample size (and consequently) representation, while the more inclusive BIF proved overly permissive, allowing direct comparisons between skeletons with differential preservation. This study highlights common challenges and prospects, defines common criteria to standardize methodologies, and further investigates the relevance of stress markers in relation to frailty.

Implementation of neuromorphic systems: from discrete components to analog VLSI chips (testing and communication issues)

We review a series of implementations of electronic devices aiming at imitating to some extent structure and function of simple neural systems, with particular emphasis on communication issues. We first provide a short overview of general features of such "neuromorphic" devices and the implications of setting up "tests" for them. We then review the developments directly related to our work at the Istituto Superiore di Sanità (ISS): a pilot electronic neural network implementing a simple classifier, autonomously developing internal representations of incoming stimuli; an output network, collecting information from the previous classifier and extracting the relevant part to be forwarded to the observer; an analog, VLSI (very large scale integration) neural chip implementing a recurrent network of spiking neurons and plastic synapses, and the test setup for it; a board designed to interface the standard PCI (peripheral component interconnect) bus of a PC with a special purpose, asynchronous bus for communication among neuromorphic chips; a short and preliminary account of an application-oriented device, taking advantage of the above communication infrastructure.

Optimization of intensity modulated radiation therapy: assessing the complexity of the problem

Intensity modulated radiation therapy (IMRT) is one of the most innovative techniques in oncological radiotherapy, allowing to conform the dose delivery to the tumoral target, preserving the normal tissue. The high number of parameters involved in the IMRT treatment planning requires an automated approach to the beam modulation. Such optimization process consists in the search of the global minimum of a cost function representing a quality index for the treatment. The complexity of this task, has been analyzed with a statistical approach for three clinical cases of particular interest in IMRT. Our main result is that a cost function based on dose-volume constraints entails lower complexity of the optimization process, in terms of the choice of the parameters defining the cost function and in a smaller sensitivity to the initial conditions for the optimization algorithm.

[Judgment on work capacity of a health worker with schizophrenia]

We show the case of a worker affected with schizophrenia who was declared by the psychiatrist fit to work. The "Medico Competente" on the contrary, on the basis of his studies and in particular on his deep knowledge of the features of the office and the place where that worker works, decides that he is not fit to do that job. This judgement is confirmed by the "ASL-Collegio Medico". The case we described emphasizes in a very strong way what is written in the item 17, paragraph 1, letter a of the D.Lgs 626/94, which extends the judgement of the fitness to work given by the "Medico Competente" even to the psychic sphere of the worker.

[Acute accidental poisoning by mercury vapors in the home environment]

The authors report 2 cases of acute mercury intoxication due to accidental breakage of barometer on to a lit gas ring. Within 24-48 hours the subjects developed neurological, gastrointestinal and dermatological symptoms. A 24-hour urine sample contained 600 and 400 micrograms of mercury per liter respectively (reference value 0.1-6.9 micrograms/L); blood concentration of mercury was 130 and 100 micrograms per liter (reference value 1.7-9.9 micrograms/L). The patients were treated with penicillamine and daily excretion of mercury was monitored. The residual sources of pollution in the kitchen were identified and bonificated.