Short-Term Memory Through Persistent Activity: Evolution of Self-Stopping and Self-Sustaining Activity in Spiking Neural Networks

Memories in the brain are separated in two categories: short-term and long-term memories. Long-term memories remain for a lifetime, while short-term ones exist from a few milliseconds to a few minutes. Within short-term memory studies, there is debate about what neural structure could implement it. Indeed, mechanisms responsible for long-term memories appear inadequate for the task. Instead, it has been proposed that short-term memories could be sustained by the persistent activity of a group of neurons. In this work, we explore what topology could sustain short-term memories, not by designing a model from specific hypotheses, but through Darwinian evolution in order to obtain new insights into its implementation. We evolved 10 networks capable of retaining information for a fixed duration between 2 and 11s. Our main finding has been that the evolution naturally created two functional modules in the network: one which sustains the information containing primarily excitatory neurons, while the other, which is responsible for forgetting, was composed mainly of inhibitory neurons. This demonstrates how the balance between inhibition and excitation plays an important role in cognition.Comment: 28 page

Evaluation de la qualité des situations spatiales des premières métropoles d'équilibre françaises : une application par morphologie mathématique

AucuneThis study aims to outline both spatial advantages and disadvantages of the first French major cities of 1963 by morphological techniques.Cette étude vise à diagnostiquer par la technique de la morphologie mathématique les avantages et désavantages spatiaux des huit premières métropoles d'équilibre françaises (1963)

Oxidation of nitrobenzene by ozone in the presence of faujasite zeolite in a continuous flow gas–liquid–solid reactor

This work investigates the oxidation of nitrobenzene (NB) by ozone in the presence of faujasite zeolite. Experiments were carried out in a gas–liquid–solid reactor were ozone transfer and NB oxidation took place at the same time. Three configurations of the reactor were compared: empty, filled with inert glass beads and filled with faujasite pellets. First, ozone transfer coefficient (kLa) and decomposition rate constant (kC) were determined for each configuration. In presence of solid, kLa was 2.0 to 2.6 times higher and kC was 5.0 to 6.4 times higher compared to the empty reactor. Then, the various configurations were evaluated in terms of NB removal and chemical oxygen demand (COD) decrease. The faujasite reactor showed higher removal of NB and decrease of COD compared to other configurations under the same conditions suggesting that the faujasite increases the oxidation rate of NB. Oxidation of NB in presence of faujasite also proved to be limited by the transfer of ozone from the gas to the liquid phase

Adsorption of Nitrobenzene from Water onto High Silica Zeolites and Regeneration by Ozone

This work investigates the removal of nitrobenzene (NB), a model pollutant from water, by combining adsorption onto zeolites and regeneration with ozone. The adsorption equilibrium isotherms of NB onto zeolites enabled the best adsorbent to be selected and zeolites with a high Si/Al ratio were the most efficient. The adsorption capacity depended on the Si/Al ratio and on the pore size. In a sequential process coupling adsorption and oxidation by ozone, NB was completely removed from water and the initial adsorption capacity of the zeolite was totally restored. Although no catalytic effect was noticed, the adsorption produced locally high concentrations, thus enhancing the oxidation rate for NB

The use of ozone and high silica zeolites to enhance refractory compounds removal

This work investigates the removal of organic pollutants from water through a three-phase process combining adsorption onto hydrophobic zeolites and in situ oxidation by ozone gas. Zeolites are aluminosilicates with various crystalline structures – and especially different pore sizes - which offer a great selectivity, profitable to organic pollutants separation in a complex matrix like natural water. They are highly resistant to chemical agents as acids, bases or oxidising agents such as ozone. Moreover, high silica zeolites have a high adsorption capacity for organics. Ozone is known to be a powerful oxidising agent able to react with various organic compounds. Its action – either direct or indirect - leads to the decomposition of organics into smaller molecules that are generally biodegradable. Previous studies have shown that organics adsorbed onto zeolites could be oxidised by ozonated water faster than in bulk water because of a micropore concentration effect (Fujita et al, 2004 and Sagehashi et al, 2005). In the gas phase, Monneyron et al (2003) showed that high silica zeolites could catalyse ozone decomposition into radical species and that adsorption properties were not significantly modified after exposition to ozone. Hence it is expected that zeolites saturated with organics could undergo regeneration by ozone without degradation of their structures or decrease of their adsorption capacities. The present work showed that high silica zeolites could efficiently adsorb nitrobenzene from water although their capacity stayed beyond that of activated carbon, except at low concentrations. The adsorption capacity depended on the zeolite structure and the Si/Al ratio. Adsorption isotherms could be well described by Langmuir or Freundlich models. As regards the three phase coupled process, the adsorbent could be efficiently regenerated during an ozonation step consisting in bubbling ozone through a suspension of saturated zeolite in a nitrobenzene solution at equilibrium. The initial adsorption capacity was quickly recovered and, continuing the treatment, the adsorption capacity of the zeolite was even increased. This may be due to the cleaning of zeolites pores by ozone (Pic et al, 2005). Yet traces of template molecules could probably remain from the zeolite synthesis process. Until now the catalytic effect of the zeolites has not been evidenced in the liquid phase. Therefore future work will focus on the conditions in which the oxidation kinetics can be enhanced in the presence of zeolites through a concentration effect, and to what extent

Loci in strata of meromorphic differentials with fully degenerate Lyapunov spectrum

International audienceWe construct explicit closed GL(2, R)-invariant loci in strata of meromorphic differentials of arbitrary large dimension with fully degenerate Lyapunov spectrum. This answers a question of Forni-Matheus-Zorich

Structural and functional studies of phosphoenolpyruvate carboxykinase

ATP-dependent phosphoenolpyruvate carboxykinase (E. C. 4.1.1.49; PCK) is an enzyme that catalyses the reversible conversion of oxaloacetate and ATP into phosphoenolpyruvate, ADP and CO2. PCK is made up of about 500 to 600 amino acid residues and is divided into two roughly equal domains. Upon binding of substrates, the two domains of PCK move towards each other. PCK is well known for its role in gluconeogenesis but in some species, it can have an anaplerotic role. In other species, PCK is important for metabolic steps involved in fermentation.Presented in this thesis are five solved crystal structures of the ATP-dependent form of PCK. Three of the PCK crystal structures determined were from Escherichia coli; one was a complex of ATP, Mg2+ and CO2, the second structure was an ATP, Mg2+, Mn2+, CO2 and oxaloacetate complex and, the third E. coli structure was a Lys213Ser mutant complexed with ATP, Mg2+and Mn2+. Two Anaerobiospirillum succiniciproducens PCK crystal structures were also solved; one was in the native form and the other was an ATP-Mg2+-Mn2+-oxalate complex. In the E. coli-PCK-ATP-Mg2+-CO2 crystal complex structure, the observed location of CO2 was in agreement with a previously determined E. coli PCK-CO2 crystal structure, which incorporated CO2 into the structure by a different technique. The findings from the E. coli PCK-ATP-Mg2+-CO2 crystal structure allowed the reaction mechanism presented in this work to be proposed.The PCK-ATP-Mg2+-Mn2+-CO2-oxaloacetate structure is the first structure where oxaloacetate is observed bound to PCK. Surprisingly, the observed location of oxaloacetate in this structure is 5 Angstroms away from its expected position near Mn2+. Oxaloacetate is weakly bound to a non-catalytic region of the enzyme. It is proposed that when the domains of PCK move towards each other upon binding nucleotide, oxaloacetate experiences steric crowding which results in it being ‘pushed’ towards the active site to react. Previous kinetic studies on the E. coli PCK mutant Lys213Ser have determined that Mn2+ is unexpectedly inhibitory. A crystal structure of K213S-PCK-ATP-Mg2+-Mn2+ demonstrates that Mn2+ is tetrahedrally coordinated in the active site, not octahedrally as occurred in other structures. By having Mn2+ in the tetrahedral coordination state, substrate binding in the active site of PCK is altered in a way that does not allow catalysis to occur.The two crystal structures of A. succiniciproducens PCK were useful in quantifying the substrate-induced domain movement. A surface active site lid made up of residues 385 to 405 that had never been observed in any of the previous PCK crystal structures was observed in the A. succiniciproducens PCK-ATP-Mg2+-Mn2+-oxalate crystal structure. Mutational studies of this lid have shown it to be essential for the function of PCK; however, its exact function is not certain. It has been proposed that the lid has multiple functions. One is to sequester the substrates from bulk solvent. Another function may be to assist in domain closure. The third function may be to assist in the proper positioning of substrates in the active site