Real time unsupervised learning of visual stimuli in neuromorphic VLSI systems

Neuromorphic chips embody computational principles operating in the nervous system, into microelectronic devices. In this domain it is important to identify computational primitives that theory and experiments suggest as generic and reusable cognitive elements. One such element is provided by attractor dynamics in recurrent networks. Point attractors are equilibrium states of the dynamics (up to fluctuations), determined by the synaptic structure of the network; a `basin' of attraction comprises all initial states leading to a given attractor upon relaxation, hence making attractor dynamics suitable to implement robust associative memory. The initial network state is dictated by the stimulus, and relaxation to the attractor state implements the retrieval of the corresponding memorized prototypical pattern. In a previous work we demonstrated that a neuromorphic recurrent network of spiking neurons and suitably chosen, fixed synapses supports attractor dynamics. Here we focus on learning: activating on-chip synaptic plasticity and using a theory-driven strategy for choosing network parameters, we show that autonomous learning, following repeated presentation of simple visual stimuli, shapes a synaptic connectivity supporting stimulus-selective attractors. Associative memory develops on chip as the result of the coupled stimulus-driven neural activity and ensuing synaptic dynamics, with no artificial separation between learning and retrieval phases.Comment: submitted to Scientific Repor

Networks of spiking neurons and plastic synapses: implementation and control

The brain is an incredible system with a computational power that goes further beyond those of our standard computer. It consists of a network of 1011 neurons connected by about 1014 synapses: a massive parallel architecture that suggests that brain performs computation according to completely new strategies which we are far from understanding. To study the nervous system a reasonable starting point is to model its basic units, neurons and synapses, extract the key features, and try to put them together in simple controllable networks. The research group I have been working in focuses its attention on the network dynamics and chooses to model neurons and synapses at a functional level: in this work I consider network of integrate-and-fire neurons connected through synapses that are plastic and bistable. A synapses is said to be plastic when, according to some kind of internal dynamics, it is able to change the “strength”, the efficacy, of the connection between the pre- and post-synaptic neuron. The adjective bistable refers to the number of stable states of efficacy that a synapse can have; we consider synapses with two stable states: potentiated (high efficacy) or depressed (low efficacy). The considered synaptic model is also endowed with a new stop-learning mechanism particularly relevant when dealing with highly correlated patterns. The ability of this kind of systems of reproducing in simulation behaviors observed in biological networks, give sense to an attempt of implementing in hardware the studied network. This thesis situates at this point: the goal of this work is to design, control and test hybrid analog-digital, biologically inspired, hardware systems that behave in agreement with the theoretical and simulations predictions. This class of devices typically goes under the name of neuromorphic VLSI (Very-Large-Scale Integration). Neuromorphic engineering was born from the idea of designing bio-mimetic devices and represents a useful research strategy that contributes to inspire new models, stimulates the theoretical research and that proposes an effective way of implementing stand-alone power-efficient devices. In this work I present two chips, a prototype and a larger device, that are a step towards endowing VLSI, neuromorphic systems with autonomous learning capabilities adequate for not too simple statistics of the stimuli to be learnt. The main novel features of these chips are the implemented type of synaptic plasticity and the configurability of the synaptic connectivity. The reported experimental results demonstrate that the circuits behave in agreement with theoretical predictions and the advantages of the stop-learning synaptic plasticity when highly correlated patterns have to be learnt. The high degree of flexibility of these chips in the definition of the synaptic connectivity is relevant in the perspective of using such devices as building blocks of parallel, distributed multi-chip architectures that will allow to scale up the network dimensions to systems with interesting computational abilities capable to interact with real-world stimuli

a novel computational model of the wheat global market with an application to the 2010 russian federation case

In this paper, we build a computational model for the analysis of international wheat spot price formation, its dynamics and the dynamics of quantities traded internationally. The model has been calibrated using FAOSTAT data to evaluate its in-sample predictive power. The model is able to generate wheat prices in twelve international markets and traded wheat quantities in twenty-four world regions. The time span considered is from 1992 to 2013. In our study, particular attention was paid to the impact of the Russian Federation's 2010 grain export ban on wheat price and quantities traded internationally. Among other results, we found that the average weighted world wheat price in 2013 would have been 3.55% lower than the observed one if the Russian Federation had not imposed the export ban in 2010

Controversy and Consensus on Indications for Sperm DNA Fragmentation Testing in Male Infertility: A Global Survey, Current Guidelines, and Expert Recommendations

Purpose: Sperm DNA fragmentation (SDF) testing was recently added to the sixth edition of the World Health Organization laboratory manual for the examination and processing of human semen. Many conditions and risk factors have been associated with elevated SDF; therefore, it is important to identify the population of infertile men who might benefit from this test. The purpose of this study was to investigate global practices related to indications for SDF testing, compare the relevant professional society guideline recommendations, and provide expert recommendations. Materials and Methods: Clinicians managing male infertility were invited to take part in a global online survey on SDF clinical practices. This was conducted following the CHERRIES checklist criteria. The responses were compared to professional society guideline recommendations related to SDF and the appropriate available evidence. Expert recommendations on indications for SDF testing were then formulated, and the Delphi method was used to reach consensus. Results: The survey was completed by 436 experts from 55 countries. Almost 75% of respondents test for SDF in all or some men with unexplained or idiopathic infertility, 39% order it routinely in the work-up of recurrent pregnancy loss (RPL), and 62.2% investigate SDF in smokers. While 47% of reproductive urologists test SDF to support the decision for varicocele repair surgery when conventional semen parameters are normal, significantly fewer general urologists (23%; p=0.008) do the same. Nearly 70% would assess SDF before assisted reproductive technologies (ART), either always or for certain conditions. Recurrent ART failure is a common indication for SDF testing. Very few society recommendations were found regarding SDF testing. Conclusions: This article presents the largest global survey on the indications for SDF testing in infertile men, and demonstrates diverse practices. Furthermore, it highlights the paucity of professional society guideline recommendations. Expert recommendations are proposed to help guide clinicians

Controversy and consensus on the management of elevated sperm DNA fragmentation in male infertility: A global survey, current guidelines, and expert recommendations

Purpose Sperm DNA fragmentation (SDF) has been associated with male infertility and poor outcomes of assisted reproductive technology (ART). The purpose of this study was to investigate global practices related to the management of elevated SDF in infertile men, summarize the relevant professional society recommendations, and provide expert recommendations for managing this condition. Materials and Methods An online global survey on clinical practices related to SDF was disseminated to reproductive clinicians, according to the CHERRIES checklist criteria. Management protocols for various conditions associated with SDF were captured and compared to the relevant recommendations in professional society guidelines and the appropriate available evidence. Expert recommendations and consensus on the management of infertile men with elevated SDF were then formulated and adapted using the Delphi method. Results A total of 436 experts from 55 different countries submitted responses. As an initial approach, 79.1% of reproductive experts recommend lifestyle modifications for infertile men with elevated SDF, and 76.9% prescribe empiric antioxidants. Regarding antioxidant duration, 39.3% recommend 4–6 months and 38.1% recommend 3 months. For men with unexplained or idiopathic infertility, and couples experiencing recurrent miscarriages associated with elevated SDF, most respondents refer to ART 6 months after failure of conservative and empiric medical management. Infertile men with clinical varicocele, normal conventional semen parameters, and elevated SDF are offered varicocele repair immediately after diagnosis by 31.4%, and after failure of antioxidants and conservative measures by 40.9%. Sperm selection techniques and testicular sperm extraction are also management options for couples undergoing ART. For most questions, heterogenous practices were demonstrated. Conclusions This paper presents the results of a large global survey on the management of infertile men with elevated SDF and reveals a lack of consensus among clinicians. Furthermore, it demonstrates the scarcity of professional society guidelines in this regard and attempts to highlight the relevant evidence. Expert recommendations are proposed to help guide clinicians

A closed-loop stimulation system to modulate slow oscillations frequency of in vitro acute cortical slices

We present a closed-loop system able to set the frequency of slow oscillations dynamics observed in acute preparation of ferret cortical slices. The biological activity is recorded through a single electrode, is analyzed on-line and steered through the injection of a DC current of variable intensity. The system guarantees artifacts removal, minimal gaps in the data acquisition and proved to be robust to slice heterogeneities. To achieve such a control we designed a custom programmable stimulator ensuring low-noise and accurate tuning over low current levels. For data recording and analysis we rely on commercial acquisition and software tools. The result is a flexible and reliable system that ensures control over in vitro slow oscillation frequency. Our tool opens new possibilities for the investigation of slow-wave dynamics which have been associated with memory consolidation and whose alteration are observed in different pathologies such as neurodegenerative diseases