Au cours des dernières années, l'élaboration de surfaces fonctionnelles par voie chimique et/ou topographique a connu un intérêt croissant. Des avancées significatives ont été réalisées pour l'optimisation des propriétés de surface, notamment tribologiques, via l'introduction de traitement visant à structurer à l'échelle nano- et micrométrique la surface de certains composants mécaniques. Différentes approches, chimiques, mécaniques ou énergétiques, peuvent être employées pour l'amélioration ou la maîtrise du frottement et de l'usure, néanmoins le traitement de surface par laser femtoseconde se distingue comme un des meilleurs procédés d'ablation, capable dans certaines conditions de générer des morphologies de surface auto-organisées périodiques, parfois multi-échelles, et appelées ripples. Cette technique appelée Laser Surface Texturing (LST), désormais couramment utilisée en tribologie et applicable à une grande variété de matériaux, présente de nombreux avantages parmi lesquels peuvent être mentionnés les vitesses de texturation atteintes ainsi que la qualité des structures de surface obtenues. De nombreux domaines ont actuellement recours à la texturation de surface par laser comme voie d'amélioration des propriétés tribologiques, comme l'automobile, et d'autres dont la filière du biomédical incarnent progressivement un nouveau champ d'expérimentations. Récemment, une nouvelle génération de prothèses tout en polymère poly(éther éther cétone) (PEEK) a vu le jour, suscitant un intérêt pour des études tribologiques. Ainsi, la texturation de surface par laser femtoseconde du PEEK comme matériau biomédical, en vue d'améliorer ses performances tribologiques, constitue le contexte général de ce travail de thèse. Cette étude traite d'une part, à l'échelle d’un impact laser, du phénomène d'interaction laser-PEEK en mode ultrabref et d'autre part, à l'échelle d’un contact mécanique, de l'optimisation d'une texturation de surface pour la maîtrise du frottement et la réduction de l'usure d'un couple PEEK/PEEK. / Over the past few years, the development of functional surfaces by chemical and/or topographical ways has shown an increasing interest. Significant advances have been made to optimize surface properties, including tribological properties, through the introduction of processes for the surface texturing at micro and nano scales. Chemical, mechanical and energetic approaches can be used to improve or control the friction and the wear of materials. The femtosecond laser stands out as one of the best methods for ablation, being able, under certain conditions, to generate periodic self-organized surface morphologies (sometimes multiscales) and called ripples. This technique known as laser surface texturing (LST), nowadays commonly used in tribology and applicable to a wide variety of materials, has many advantages including the texturing speeds and the quality of surface structures obtained. Currently, many fields such as the automobile industry and the biomedical sector use the laser surface texturing as a means of improving the tribological properties. Recently, a new generation of poly(ether ether ketone) (PEEK) polymer prostheses has emerged, arousing interest for tribological studies. The surface texturing of PEEK, as a biomedical material, by femtosecond laser, in order to improve its tribological performance, provides the general context of this thesis. This thesis first deals with the laser-PEEK interaction at the scale of a laser impact and then it focuses on the optimization of the surface texturing in order to control friction and reduce wear of a PEEK/PEEK tribosystem.

L’appauvrissement en lubrifiant d’un contact élasto-hydrodynamique peut menacer la pérennité du film interfacial séparant les surfaces, entraîner l’augmentation des forces de frottement et l’endommagement rapide du tribosystème. Industriellement, la volonté de réduire les quantités de lubrifiant embarquées et de limiter les opérations de maintenance favorisent la sous-alimentation. L’objectif de cette thèse est de contrôler l’alimentation d’un contact EHD sous-alimenté par une texturation de surface multi-échelle, afin d’assurer une lubrification minimale à l’interface. Une démarche scientifique s’appuyant sur la dissociation des échelles spatiales, temporelles ainsi que sur l’analyse des contributions des écoulements dans les différentes zones du contact a été mise en oeuvre. La compréhension de l’interaction laser/matière en irradiation ultra-brève a permis de générer des texturations nanométriques (ripples) et micrométriques (ondulations et réseaux de cavités) à l’aide d’un laser femtoseconde. La contribution de chaque échelle topographique sur la lubrification d’un contact a été étudiée. Lorsque les forces hydrodynamiques sont faibles (contact statique), l’alimentation d’un contact résulte d’une compétition entre une contribution capillaire et une contribution visqueuse. La macro géométrie des solides déformés ainsi que la viscosité pilotent au premier ordre la propagation du lubrifiant à l’extérieur de la zone haute pression. L’imbibition de la zone de Hertz n’est possible qu’avec l’introduction d’une surface nanotexturée et sa cinétique dépend de l’orientation et de l’amplitude des ripples. Pour des nombres capillaires élevés (contact dynamique), un critère de prédiction de l’apparition de la sous-alimentation a été établi. En présence de réseaux de cavités et sous l’action d’un cisaillement interfacial, les surfaces texturées possédant une capacité de rétention élevée réalimentent la zone de haute pression. Le volume piégé envahit alors le contact puis crée une surépaisseur de lubrifiant qui protège les solides de l’endommagement tout en limitant l’augmentation du frottement. Les texturations nanométriques (ripples) et micrométriques, anisotropes et périodiques, modifient l’équilibre entre les débits de réalimentation et les débits de fuite. Une orientation adaptée des motifs peut contenir le drainage du lubrifiant dans le contact et ainsi empêcher ou retarder l’apparition d’un régime de sous-alimentation totale. Au final, chaque échelle topographique contribue à la réalimentation d’un contact sousalimenté, soit en favorisant l’expansion des réservoirs latéraux, soit en apportant localement du lubrifiant, ou en maintenant un film fluide résiduel sur les surfaces. / The lubricant depletion of an elasto-hydrodynamic contact may threaten the sustainability of the interfacial film separating the surfaces, can result in the increase of frictional forces and a rapid damage of the tribosystem. Industrially, the trend to reduce the amount of initial lubricant and to limit maintenance promotes starvation of the lubricated contact. The aim of this work is to control the lubricant feeding of a starved EHL contact by a multi-scale surface texturing, in order to ensure minimal lubrication at the interface. A scientific approach based on the separation of spatial and temporal scales, as well as the analysis of the flow contributions in different zones of the contact has been implemented.The understanding of the laser/matter interaction in ultra-short irradation has helped us to generate both nanoscale texturing (ripples) and microscale texturing such as microwaves and networks of micrometric cavities, using a femtosecond laser. The contribution of each topographic scale on the contact lubrication has been analysed. When the hydrodynamic forces are low (static contact), the feeding of the contact results from a competition between capillary and viscous contributions. The macro geometry of the deformed solids and the lubricant viscosity mainly control the lubricant spread around the high-pressure zone. The imbibition of the Hertz contact area is only possible with the introduction of a nanotextured surface. The imbibition kinetics depends on the orientation and amplitude of the ripples. For high capillary numbers (dynamic contact), a criterion has been established in order to predict the occurrence of starvation. Using a network of micro cavities and the action of an interfacial shear, the high retention capacity textured surfaces brings lubricant to the high-pressure zone. The trapped volume propagates inside the contact and creates a film thickness that protects the solids from damages while limiting the increase in friction. Anisotropic and periodic nanoscale and microscale texturing, like ripples, influences the balance between re-feeding and leakage flow rates. A suitable orientation of the geometries may prevent the lubricant from drainage and thus delay the onset of a starved lubrication regime. In conclusion, each topographic scale contributes to re-feed a starved contact, by promoting lateral reservoir extension, by providing lubricant locally where it is needed, or maintaining a residual fluid film on surfaces.

In order to successfully incorporate the tremendous possibilities of nanoscale applications into devices and manufacturing, significant studies need to be conducted of the properties and mechanics of materials of this small scale. In this thesis, the effect of repeated scanning of KBr, aluminum, and gold was studied by using a nanoindenter and Atomic Force Microscope (AFM) in varying environments. Additional research was performed to study the environmental effects of gold film scratching using a Taber Shear/Scratch Tester. Scanning of KBr single crystal surface in air with a diamond tip in the Hysitron Triboindenter formed surface ripples 100 nm high, 1 micron apart. It has been observed that the nanoripple's initial height and period increase with the number of repeated scans. The surface ripples form perpendicular to the scanning direction, beginning at the bottom of sloped samples and working their way up the entire scan area. The addition of water to a wear experiment on gold film produced considerably deeper wear areas than its ambient air counterpart in both scanning machines. Scratch testing with a conical diamond tip of 77 µm radius with 125 g normal load also produced deeper wear tracks in water than in ambient air conditions. Several mechanisms may be responsible for the ripples formation, including dislocation dynamics, chatter, piezo hysteresis and others. Most likely there is a combination of effects, with a clear differentiation between nanoripple's origination and propagation. Mechanisms responsible for rippling, including system dynamic response and stick slip behavior are investigated. Topography modification appears to be the main result of ambient wear tests at the nanoscale, whereas much higher wear rate and nanoripples are observed in water. It is possible that this moisture is assisting grain fracture and pull off.

A volume-filtered Large-Eddy Simulation (LES) of oscillatory flow over a rippled mobile bed is conducted using an Euler-Lagrange approach. As in unsteady marine flows over sedimentary beds, the experimental data, referenced in this work for validation, shows quasi-steady state ripples in the sand bed under oscillatory flow. This work approximately reproduces this configuration with a sinusoidal pressure gradient driven flow and a sinusoidally rippled bed of particles. The LES equations, which are volume-filtered to account for the effect of the particles, are solved on an Eulerian grid, and the particles are tracked in a Lagrangian framework. In the Discrete Particle Method (DPM) used in this work, the particle collisions are handled by a soft-sphere model, and the liquid and solid phases are coupled through volume fraction and momentum exchange terms. Comparison of the numerical results to the experimental data show that the LES-DPM is capable of capturing the mesoscale features of the flow. The large scale shedding of vortices from the ripple peaks are observed in both datasets, which is reflected in the good quantitative agreement between the wall-normal flow statistics, and good qualitative agreement in ripple shape evolution. Additionally, the numerical data provides three insights into the complex interaction between the three-dimensional flow dynamics and bed morphology: (1) there is no observable distinction between reptating and saltating particle velocities, angular velocities or observed Shields parameters; (2) the potential motion of the mobile bed may create issues in the estimation of the bed shear stress used in classical models; and, (3) a helical pairing of vortices is observed, heretofore not known to have to have been identified in this type of flow configuration.

Ce travail vise à accroître la compréhension et la maîtrise des processus d'ablation laser en régime femtoseconde en s'appuyant sur l'étude, en particulier spectroscopique, de l'émission optique du panache de matière ablatée par des impulsions dont on contrôle la fluence laser et la forme temporelle. Le chapitre 1 consiste en une synthèse de la bibliographie sur l'ablation en régime femtoseconde en partant de l'absorption de l'énergie optique par le système électronique, jusqu'à la thermalisation des électrons et des ions et à l'évolution thermodynamique consécutive subie par le matériau irradié. Les conséquences induites sur les produits de l'ablation et sur la formation du plasma sont abordées. L'ensemble du dispositif expérimental mis en jeu dans cette étude est présenté au chapitre 2 avec en particulier la description de la mise en forme temporelle des impulsions laser par la modulation spectrale de leur phase à partir d'un modulateur spatial de lumière. Les chapitres 3 et 4 présentent les résultats de l'étude des panaches d'ablation respectivement induits à partir de cibles d'aluminium et de laiton. Une attention particulière est portée à l'influence de la mise en forme temporelle des impulsions et plusieurs optimisations de l'interaction sont décrites. Dans le cas de l'aluminium, les données expérimentales sont confrontées aux résultats de simulations numériques afin de proposer des schémas d'interprétation du phénomène d'ablation en régime femtoseconde et de sa réponse à la mise en forme temporelle des impulsions laser. Ces études ont permis de montrer la possibilité de modifier le couplage énergétique entre les impulsions laser et la cible irradiée, d'influencer les proportions de matière éjectée sous forme liquide et sous forme atomisée, de contrôler l'état d'excitation du panache d'ablation et enfin de maîtriser la génération de micro et nanoparticules. Le chapitre 5 clôture ce travail par une analyse de la structuration périodique induite à la surface de cibles métalliques en régime femtoseconde puis en fonction de la forme temporelle employée pour le dépôt énergétique. Ce dernier chapitre permet de compléter l'étude des comportements de la matière éjectée par une exploration des structures adoptées par la matière non-ablatée.

AC/DC Buck converters have been used widely in many applications from cell phones to vehicle battery chargers. Due to their importance many researchers have been studying their behavior to improve their efficiency and reduce their size and/or cost. One of the most common defects of these converters, whether they are used for high power or low power applications, is the unwanted ripples in the input voltage across the input stage. It is believed that these ripples are caused by the interaction between the converter itself or its controller with the rectifier required to change the AC input into DC followed by an input filter. Many strategies have been suggested to tackle this problem. A new strategy to improve the controller of that converter was suggested by M. Lenells [1] and it was based on a Hamiltonian model for the 3-phase AC/DC converter together with its rectifier. As a first step, we simulated this model for a single-phase DC/DC buck converter only using the so-called S-Functions in MATLAB/SIMULINK. Then we could find a control law that would reduce the ripples in the input voltage and keep the output voltage constant simultaneously. In this report, we present this model and its simulation to pave the way for the control and simulation of the 3-phase AC/DC converter.

In this project we studied the formation of nanoripples in calcite and InP single crystals by continuous scanning using the nanoindenter in the ambient environment and by Argon ion irradiation under ultra high vacuum conditions, respectively. Formation of tip induced nanowear ripples is studied on a freshly cleaved calcite single crystal as a function of scanning frequency and contact load of the diamond tip. At lower loads, initiation of the ripples takes place at the bottom of the surface slope at 3 Hz scanning frequency, which continue to propagate as scanning progresses. The orientation of these ripple structures is perpendicular to the scan direction. As the number of scans increases, ripples fully develop, and their height and periodicity increase with the number of scans by merging ripples together. At 6 mu N normal load, tip induced wear occurred as the tip started removing the ripple structures with increased number of scan cycles. As the contact load increased further, a ripple structure was not initiated and only tip induced wear occurred on the surface. At 1 Hz frequency material removal takes place as the tip moves back and forth and material slides towards the scan edges. Material removal rate increased with contact load and it is observed that the number of scans required to create a new surface is inversely proportional to the contact load. Possible mechanisms responsible for the formation of ripples at higher frequencies are attributed to the slope of the surface, piezo hysteresis,system dynamics or a combination of effects. Single crystal calcite hardness of 2.8 GPa and elastic modulus of 80 GPa were measured using nanoindentation. Evolution of nanostructures on the InP surface due to ion bombardment has been studied with scanning tunneling microscopy in UHV environment. InP crystal surfaces were irradiated by Argon ion incident beam with 3 KeV energy at an incident angle of 75 degrees. Self-organization of the surface was studied by varying the ion fluence from 7.7E13 to 4.6E17 ions per square centimeter. The observed nanoripple morphologies have been explained based on the concept of interplay between roughening and smoothing processes. Wavelength of the nanostructures linearly increases with the logarithm of the fluence. The rms roughness is approximately linear with the logarithm of the fluence. Nanoindentation experiments were performed on InP surface before and after ion bombardment to determine variation in hardness and elastic modulus. Surface of irradiated InP has higher H and E values as the surface become amorphized after Ar+ ion bombardment.

Η δραστηριότητα των Οξύαιχμων κυμάτων-Ριπιδισμών (Sharp waves-Ripples, SPWs-R) αποτελεί μια πληθυσμιακή δραστηριότητα του ιπποκάμπου που εμπλέκεται στην παγίωση της μνήμης. Η υψίσυχνη ταλάντωση των ριπιδισμών (100-200Hz) είναι αποτέλεσμα της πολύπλοκης αλληλεπίδρασης μεταξύ των πυραμιδικών κυττάρων και ορισμένων τύπων GABAεργικών ενδονευρώνων, μεταξύ των οποίων είναι τα καλαθοειδή κύτταρα που εκφράζουν παρβαλβουμίνη (PV-basket cells). Ο υποδοχέας για τα οπιοειδή τύπου «μ» εντοπίζεται στις απολήξεις των καλαθοειδών κυττάρων που εκφράζουν παρβαλβουμίνη, τα οποία αυξάνουν το βαθμό πυροδότησής τους ταυτόχρονα με την ταλάντωση των ριπιδισμών. Στην παρούσα μελέτη, χρησιμοποιώντας ένα in-vitro μοντέλο των Οξύαιχμων κυμάτων-Ριπιδισμών, μελετήσαμε την επίδραση δύο εκλεκτικών αγωνιστών του υποδοχέα των οπιοειδών τύπου «μ», της φαιντανύλης (1, 5, 10, 100, 500 και 1000 nM) και της μορφίνης (5, 10, 100, 1000 και 10000 nM). Και τα δύο φάρμακα αύξησαν σημαντικά το πλάτος και μείωσαν σημαντικά τη συχνότητα των SPWs με τρόπο δοσοεξαρτώμενο. Επιπλέον, η μείωση της συχνότητας συνέχισε και μετά την πάροδο της μίας ώρας εφαρμογής των φαρμάκων, ενώ η αύξηση του πλάτους είχε νωρίτερα παρουσίασει φαινόμενο plateau (40-50 λεπτά). Τα δύο φάρμακα δεν μετέβαλλαν τις παραμέτρους διάρκειας των απομονομένων επεισοδίων SPWs. Στις υψηλές συγκεντρώσεις των δύο φαρμάκων η ρυθμικότητα των SPWs μειώθηκε. Τα δύο φάρμακα μείωσαν το πλάτος της ισχύος της ταλάντωσης των ριπιδισμών. Η μείωση αυτή ήταν της τάξεως του 70% στις συγκεντρώσεις άνω των 500 nM. Παρόλα αυτά, η μορφίνη σε χαμηλές συγκεντρώσεις (5 και 10 nM) αύξησε σημαντικά το πλάτος της ισχύος της ταλάντωσης των ριπιδισμών κατά 25-30%. Είναι σημαντικό ότι οι επιδράσεις των δύο οπιοειδών παρατηρήθηκαν και στις συγκεντρώσεις οι οποίες εμπίπτουν στα κλινικά θεραπευτικά πλαίσια. Οι επιδράσεις των δύο οπιοειδών ήταν πλήρως αντιστρεπτές μετά την εφαρμογή των ανταγωνιστών του υποδοχέα των οπιοειδών τύπου «μ» ναλοξόνη (20 μΜ) και CTOP (1-5 μM). Οι επιδράσεις των οπιοειδών μετά από εφαρμογή διαδοχικά αυξανόμενων συγκεντρώσεων παρουσίαζαν φαινόμενο plateau, πράγμα το οποίο οφείλεται στην απευαισθητοποίηση του υποδοχέα. Τα παραπάνω ευρήματα αποδεικνύουν τη σημαντική επίδραση της ενεργοποίησης του υποδοχέα των οπιοειδών τύπου «μ» στη δραστηριότητα Οξύαιχμων κυμάτων-Ριπιδισμών, πιθανότατα μέσω τροποποίησης του βαθμού ενεργοποίησης των ενδονευρώνων που εμπλέκονται στη γένεση αυτής της δικτυακής ταλάντωσης. Υποθέτουμε ότι η μεταβολή που προκαλούν τα οπιοειδή στην ισορροπία μεταξύ διέγερσης και αναστολής παρεμβαίνει στη γένεση της δραστηριότητας Οξύαιχμων κυμάτων-Ριπιδισμών και αυτό μπορεί να εμπλέκεται στις αμνησιακές δράσεις των οπιοειδών φαρμάκων. / Sharp wave-ripple complexes (SPW-Rs) are population activity of the hippocamus implicated in memory consolidation. The high-frequency ripple oscillation (100-200 Hz) results from the complicated interaction between pyramidal cells and some types of GABAergic interneurons, including basket PV cells. The μ opioid receptor is located at the terminals of basket PV cells which increase their firing rate in coincidence with ripple oscillation. In this study, using an in vitro model of SPW-Rs we examined the effects of two selective μ opioid receptor agonists, fentanyl (1, 5, 10, 100, 500 and 1000 nM) and morphine (5, 10, 100, 1000 and 10000 nM. All drugs significantly increased the amplitude and slowed down the rate of occurrence of SPWs in a concentration-dependent manner. Furthermore, the drug-induced rate reduction continued up to one hour of application although the effect on the amplitude reached a plateau earlier (40-50 min). None of the dugs induced a change in the duration of individual events. At relatively high concentrations the rhythmicity of SPWs was suppressed. All drugs produced suppression of the amplitude and power of the ripple oscillation. This suppressive effect was as great as 70% at concentrations greater than 500 nM. However, morphine at low concentrations (5 nM) significantly increased the ripple power by 25-30%. Importantly all the drug-induced effects were observed at drug concentrations falling into the range of clinically used values. The effects of all opioids were fully reversed upon application of the μ receptor antagonist naloxone (20 μΜ) or CTOP (1-5 μM). The drug effects following increased drug concentrations in consecutive applications reached a plateau indicating receptor desensitization. These findings show that opioids acting on the μ receptor induce significant changes in SPW-R activity presumably affecting the level of activity of those interneurons implicated in the generation of the network oscillation. We hypothesize that the opioid-induced change of the balance between excitation and inhibition interfere with the generation of SPW-R activity and this effect might be involved in the amnesic actions of the opioid substances.

Τα οξύαιχμα κύματα-ριπιδισμοί αποτελούν μία σημαντική και ενδογενή νευρική δραστηριότητα του ιπποκάμπου που εμφανίζεται κυρίως κατά το στάδιο των βραδέων κυμάτων του ύπνου και σε φάσεις ακινησίας κατά την εγρήγορση και πιστεύεται ότι διαδραματίζει κρίσιμο ρόλο στην διαδικασία της παγίωσης της μνήμης, που εμπλέκεται στη μεταφορά της πληροφορίας από τον ιππόκαμπο στον νεοφλοιό. Στην παρούσα μελέτη χρησιμοποιώντας καταγραφές των δυναμικών πεδίου από την CA1 περιοχή τομών του κοιλιακού πόλου του ιπποκάμπου, εξετάσαμε την επίδραση της νιφεδιπίνης, ενός ανταγωνιστή των L-τύπου τασεοελεγχόμενων διαύλων ασβεστίου, στην δραστηριότητα των οξύαιχμων κυμάτων-ριπιδισμών. Παρατηρήσαμε ότι η νιφεδιπίνη αύξησε το πλάτος των ριπιδισμών χωρίς να επηρεάζει σημαντικά την διάρκεια και τη συχνότητα της ριπιδικής ταλάντωσης. Επίσης η νιφεδιπίνη αύξησε το πλάτος των οξύαιχμων κυμάτων και προκάλεσε μία μετρημένη αύξηση στην εμφάνιση των επεισοδίων των οξύαιχμων κυμάτων. Αυτά τα αποτελέσματα καταδεικνύουν ότι η ροή ασβεστίου διαμέσου των L-τύπου διαύλων συμμετέχει στην τροποποίηση των οξύαιχμων κυμάτων-ριπιδισμών. / Sharp wave-ripples (SWRs) are a prominent and endogenous network activity in the hippocampus occurring during slow-wave sleep, and awake immobility, and it is thought to play a critical role in process of memory consolidation implicated in the transfer of information from the hippocampal to neocortex. In the present study using recordings of field potentials from the CA1 field of ventral hippocampal slices we examined the effect of nifedipine, a blocker of voltage-dependent calcium channels of L-type, in the activity of SWRs. We observed that nifedipine increased the amplitude of ripples without significantly affecting the duration and frequency of the ripple oscillation. Also, nifedipine increased the amplitude of sharp waves and produced a moderate increase in the incidence of episodes of SWRs. These results indicate that calcium influx through L-type calcium channels participate in the modulation of SWRs.

New memories are labile, but over time some of them are stabilized. This thesis investigates the network mechanisms in the brain underlying the gradual consolidation of memory representations. Specifically, I performed a causal test of the long-standing hypothesis that the offline reactivation of new, memory-representing cell assemblies supports memory consolidation by stabilizing those assemblies and increasing the likelihood of their later reinstatement - and therefore presumably of memory recall. I performed multi-unit extracellular recordings in the dorsal CA1 region of behaving mice, from which I detected short-timescale (25 ms) co-activation patterns of principal neurons during exploration of open-field enclosures. These cell assembly patterns appeared to represent space as their expression was spatially tuned and environment specific; and these patterns were preferentially reactivated during sharp wave-ripples (SWRs) in subsequent sleep. Importantly, after exposure to a novel - but not a familiar - enclosure, the strength with which an assembly pattern was reactivated predicted its later reinstatement strength during context re-exposure. Moreover, optogenetic silencing of hippocampal pyramidal neurons during on-the-fly detected SWRs during the sleep following exposure to a novel - but again not a familiar - enclosure impaired subsequent assembly pattern reinstatement. These results are direct evidence for a causal role of SWR-associated reactivation in the stability of new hippocampal cell assemblies. Surprisingly, offline reactivation was only important for the stability of a subset of the assembly patterns expressed in a novel enclosure. Optogenetic SWR silencing only impaired the reinstatement of "gradually strengthened" patterns that had had a significant increasing trend in their expression strength throughout the initial exposure session. Consistent with this result, a positive correlation between reactivation and subsequent reinstatement was only found for these gradually strengthened patterns and not for the other, "early stabilized" patterns. An interesting interpretation is that the properties of the gradually strengthened patterns are all consistent with the Hebbian postulate of "fire together, wire together". To enable investigation of the relation between interneurons and principal cell assembly patterns from extracellular recordings, as a final contribution this thesis describes a statistical framework for the unsupervised classification of interneurons based on their firing properties alone.