Stochastic Minority on Graphs

Cellular automata have been mainly studied on very regular graphs carrying the cells (like lines or grids) and under synchronous dynamics (all cells update simultaneously). In this paper we study how the asynchronism and the topology of cells act upon the dynamics of the classical Minority rule. Beyond its apparent simplicity, this rule yields complex behaviors which are clearly linked to the structure of the graph carrying the cells

Towards a Brain-inspired Information Processing System: Modelling and Analysis of Synaptic Dynamics: Towards a Brain-inspired InformationProcessing System: Modelling and Analysis ofSynaptic Dynamics

Biological neural systems (BNS) in general and the central nervous system (CNS) specifically exhibit a strikingly efficient computational power along with an extreme flexible and adaptive basis for acquiring and integrating new knowledge. Acquiring more insights into the actual mechanisms of information processing within the BNS and their computational capabilities is a core objective of modern computer science, computational sciences and neuroscience. Among the main reasons of this tendency to understand the brain is to help in improving the quality of life of people suffer from loss (either partial or complete) of brain or spinal cord functions. Brain-computer-interfaces (BCI), neural prostheses and other similar approaches are potential solutions either to help these patients through therapy or to push the progress in rehabilitation. There is however a significant lack of knowledge regarding the basic information processing within the CNS. Without a better understanding of the fundamental operations or sequences leading to cognitive abilities, applications like BCI or neural prostheses will keep struggling to find a proper and systematic way to help patients in this regard. In order to have more insights into these basic information processing methods, this thesis presents an approach that makes a formal distinction between the essence of being intelligent (as for the brain) and the classical class of artificial intelligence, e.g. with expert systems. This approach investigates the underlying mechanisms allowing the CNS to be capable of performing a massive amount of computational tasks with a sustainable efficiency and flexibility. This is the essence of being intelligent, i.e. being able to learn, adapt and to invent. The approach used in the thesis at hands is based on the hypothesis that the brain or specifically a biological neural circuitry in the CNS is a dynamic system (network) that features emergent capabilities. These capabilities can be imported into spiking neural networks (SNN) by emulating the dynamic neural system. Emulating the dynamic system requires simulating both the inner workings of the system and the framework of performing the information processing tasks. Thus, this work comprises two main parts. The first part is concerned with introducing a proper and a novel dynamic synaptic model as a vital constitute of the inner workings of the dynamic neural system. This model represents a balanced integration between the needed biophysical details and being computationally inexpensive. Being a biophysical model is important to allow for the abilities of the target dynamic system to be inherited, and being simple is needed to allow for further implementation in large scale simulations and for hardware implementation in the future. Besides, the energy related aspects of synaptic dynamics are studied and linked to the behaviour of the networks seeking for stable states of activities. The second part of the thesis is consequently concerned with importing the processing framework of the dynamic system into the environment of SNN. This part of the study investigates the well established concept of binding by synchrony to solve the information binding problem and to proposes the concept of synchrony states within SNN. The concepts of computing with states are extended to investigate a computational model that is based on the finite-state machines and reservoir computing. Biological plausible validations of the introduced model and frameworks are performed. Results and discussions of these validations indicate that this study presents a significant advance on the way of empowering the knowledge about the mechanisms underpinning the computational power of CNS. Furthermore it shows a roadmap on how to adopt the biological computational capabilities in computation science in general and in biologically-inspired spiking neural networks in specific. Large scale simulations and the development of neuromorphic hardware are work-in-progress and future work. Among the applications of the introduced work are neural prostheses and bionic automation systems

Hva kontrollerer kalving av breer? Fra observasjoner til prediksjoner

This thesis addresses the process of iceberg calving at the front of tidewater glaciers and tries to clarify what controls the calving of glaciers, from observations in the field to modeling and predictions. Iceberg calving is the detachment of ice from a parent glacier and it makes the glacier very sensitive to its local environment. In turn, calving at a glacier front has a strong impact on the glacier dynamics and can trigger and/or enhance glacier instabilities, acceleration and glacier retreat, making the calving process a crucial factor in glacier dynamics and hence in sea level rise. This thesis is based on field observations, collected throughout 4 years at the front of Kronebreen, Svalbard. A special emphasis has been given to trying various observation techniques: ground-based RADAR, direct observations, seismic monitoring, terrestrial photogrammetry and remote sensing. Using ground-based RADAR we were able to automatically detect 92% of the largest calving events. The percentage detected by seismic monitoring is lower (about 10%) but the technique allows for finer distinction between different calving types and glacier-related seismic events. Seismic equipment also requires less maintenance, less technical expertise and less funding, and can be left in the field for several months. Terrestrial photogrammetry is a very useful tool that can provide glacier dimensions and a continuous monitoring of the general conditions at the front. Finally, direct observations are recommended for the study of calving because it can provide, when used together with terrestrial photogrammetry, both qualitative and quantitative data. The qualitative aspect provides key information for understanding the calving process but is especially hard to obtain with technical methods. The question of seasonal calving variations is also addressed and we show that glacial seismic activity is highly variable throughout the year with recurrent increased activity in autumn, while velocity is low. However this thesis focuses on explaining very short-term variations: the individual calving events. Individual calving events have received so far very little attention in the field and no attention in modeling studies. This thesis was inspired by other studies of complex natural processes in which individual events are all equally considered, large and small, and which emphasize the value of understanding a process at the individual scale, for example the study of earthquakes or forest fires. We first show that general spatial patterns in calving activity can be explained by glacier characteristics like longitudinal stretching rate, which themselves are very linked to the glacier geometry. We then created a simple calving model with the object of understanding what controls the size and timing of calving events. Our simple model, focussing solely on the interplay between calving and its impact on the front stability, manages to reproduce the size and timing distribution of calving events as observed in the field. This result highlights the role of calving on front stability and on calving itself. Front stability is shown to be crucial in the control of calving. Implications of this new finding are that the size distribution of calving depends on the glacier stability: a glacier becoming unstable will produce a higher proportion of large calving events. Beyond a critical glacier stability, calving can become self-sustained and ongoing, leading to very rapid glacier retreat. We propose that the characteristics of the calving event sizes distribution indicate how close a glacier is to rapid retreat. One main point of this thesis is to show the importance of studying calving events at an individual scale to gain more understanding of the process.Denne avhandlingen omhandler kalvingsprosessen i fronten av en tidevannsbre og den forsøker å klargjøre hva som kontrollerer kalving av breer, ved hjelp av feltobservasjoner, modellering og prediksjon. Kalving av isfjell skjer når is brekker av fra en isbre, og kalving gjør breer svært sensitive til det lokale miljøet. Motsatt har også kalvingen ved brefronten en stor innflytelse på breens dynamikk, kalvingen kan initiere eller forsterke ustabilitet, akselerasjon eller tilbaketrekking av breen, hvilket gjør kalvingsprosessen til en sentral faktor for isdynamikken, og for havnivået. Denne avhandlingen er basert på feltobservasjoner som er samlet gjennom fire år ved fronten av Kronebreen på Svalbard. Det er blitt lagt spesielt vekt på å prøve ut forskjellige observasjonsteknikker, bakkebasert RADAR, direkte observasjoner, seismisk monitorering, terrestrisk fotogrammetri, og fjernanalyse. Ved hjelp av bakkebasert RADAR kunne vi detektere 92% av de storste kalvingsepisodene. Prosentandelen for seismisk monitorering er mye lavere, ca 10%, men denne monitoreringen tillater finere distinksjon av forskjellige kalvingsformer og brerelaterte seismiske episoder. Seismisk utstyr krever også mindre ettersyn, mindre teknisk ekspertise og lavere finansiering, og utstyret kan være utplassert i felt uten tilsyn i flere måneder. Terrestrisk fotogrammetri er et svært nyttig verktøy som kan fortelle om breens dimensjoner og som muliggjør en kontinuerlig monitorering av generelle forhold ved fronten. Tilsutt anbefales direkte observasjoner for å studere kalving, fordi disse i kombinasjon med terrestrisk fotogrammetri kan gi både kvalitative og kvantitative data. Det kvalitative aspektet gir essensiell informasjon for forståelsen av kalvingsprosessen, men er spesielt vanskelig å oppnå ved teknologiske metoder. Spørsmålet om sesongbaserte kalvingsvariasjoner er også undersøkt og vi viser at kalvingsaktiviteten er svært variabel gjennom året, med gjentagende økning i aktivitet på høsten når også hastigheten er på sitt laveste. Allikevel fokuserer denne avhandlingen på å forklare de svart raske variasjonene, nemlig individuelle kalvingshendelser. Så langt har det blitt viet svært lite oppmerksomhet mot individuelle kalvingshendelser i felt, og ingen oppmerksomhet innen modelleringsstudier. Denne avhandlingen er inspirert av studier av komplekse prosesser hvor individuelle hendelser er vurdert likeverdige, store som små, og som vektlegger verdien av å forstå prosessen på en skala på individuelt nivå, for eksempel for studier av jordskjelv. Vi viser først at generelle romlige monstre i kalvingsaktivitet kan forklares ved brekarakteristikker som longitudinell tøynings rate (stretching rate), som igjen er knyttet til breens geometri. Vi har laget en enkel kalvingsmodell hvor intensjonen er å forstå hva som kontrollerer størrelse og tidspunkt for kalvingshendelsen. Vår modell, som fokuserer kun på interaksjon mellom kalving og dennes innflytelse på frontstabiliteten, greier å reprodusere størrelses- og tidsfordeling av kalvingshendelser som observert i felt. Dette resultatet fremhever kalvingens rolle på frontstabiliteten og på kalvingen selv. Det viser seg at frontstabiliteten er en essensiell styringsmekanisme for kalvingen. Konsekvensene av dette nye funnet er at størrelsesfordelingen av kalvingshendelsene avhenger av breens stabilitet; en bre som blir ustabil produserer høyere proporsjon av større kalvingshendelser. Over en kritisk brestabilitet vil kalvingen bli selvopprettholdende og vedvarende, hvilket vil medføre en svart rask tilbaketrekning av brefronten. Vi fremsetter en påstand om at karakteristikken av fordelingen av størrelsene på kalvingshendelsene indikerer hvor nært forestående en rask tilbaketrekning er for breen. Et hovedpoeng ved denne avhandlingen er å vise hvor viktig det er å studere kalvingshendelser på en skala på individuelt nivå for å oppnå en bedre forståelse av prosessen.GLACIODY