Spatial Learning and Action Planning in a Prefrontal Cortical Network Model

The interplay between hippocampus and prefrontal cortex (PFC) is fundamental to spatial cognition. Complementing hippocampal place coding, prefrontal representations provide more abstract and hierarchically organized memories suitable for decision making. We model a prefrontal network mediating distributed information processing for spatial learning and action planning. Specific connectivity and synaptic adaptation principles shape the recurrent dynamics of the network arranged in cortical minicolumns. We show how the PFC columnar organization is suitable for learning sparse topological-metrical representations from redundant hippocampal inputs. The recurrent nature of the network supports multilevel spatial processing, allowing structural features of the environment to be encoded. An activation diffusion mechanism spreads the neural activity through the column population leading to trajectory planning. The model provides a functional framework for interpreting the activity of PFC neurons recorded during navigation tasks. We illustrate the link from single unit activity to behavioral responses. The results suggest plausible neural mechanisms subserving the cognitive “insight” capability originally attributed to rodents by Tolman & Honzik. Our time course analysis of neural responses shows how the interaction between hippocampus and PFC can yield the encoding of manifold information pertinent to spatial planning, including prospective coding and distance-to-goal correlates

Etude des signaux EXAFS des laitons au cours de la transformation martensitique : mise au point d'un appareillage EXAFS de laboratoire adapte aux films minces

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Bronzing effect: predicting the color shine of printed surfaces by estimating the refractive index of the inks

International audienceThis paper investigates the optical phenomenon responsible for the colored shine that sometimes appears at the surface of ink layers in the specular direction, often called "bronzing" or "gloss differential." The prediction of this shine effect relies on the Fresnel formulas of the air/ink interface. The complex refractive index of the ink must therefore be determined, which is made difficult because of the roughness of inked printing supports. We propose a generic method that can be applied to any ink, without any prior knowledge of its composition or the printing substrate. In order to reduce light scattering, a solid colored area is printed with the studied ink on a glossy paper previously printed with black ink. By ellipsometry, we determine the effective refractive index of the sample. The intrinsic complex refractive index of the ink can then be extracted by modeling the optical response of the inked surface with a set of Gaussian oscillators, among which one of them approaches residual scattering. With this data, we could proceed to a fine colorimetric analysis of the bronzing color of some cyan, magenta, and yellow inks. In particular, we show that this gloss color is slightly shifted from the complementary of the ink's usual color in diffuse reflection

Contribution of the fluorescence to conversion electron yield X-ray absorption fine-structure measurements

International audienceInfluence of an extra electron yield induced by the fluorescence process has been experimentally shown and analytically described. Analyse of the Conversion Electron Yield signal induced by pure bulk silver (Ag K-edge) and pure bulk nickel or copper recovered by a thin iron layer (Ni and Cu K-edges) has been performed. This fluorescence-electron yield leads to enhance the edge-height to modify the shape of the XANES and to decrease the EXAFS amplitude. Extrapolation of the model concerning unrecovered bulk samples was carried out in order to explain the EXAFS amplitude reduction often observed in CEY measurements referring to that obtained in the transmission mode. Such reduction / increases with the atomic number Z of the studied element. / can be considered as negligible for low-Z element (Ni and Cu). It becomes important for high-Z element. K-dependence of the amplitude reduction is rather limited and such reduction can be simply explained by a proportional factor. In order to correct this unwanted effect, theoretical calculation process can be performed or sample structure can be especially designed to minimise it. Thin films deposited on substrate composed of low-Z element do not generate significantly such extra electron yield

Croissance, nanostructure et réponse optique de films minces d'agrégats d'argent dans des matrices diélectriques

L'objectif de ce travail est d'élaborer par pulvérisation ionique des films minces d'agrégats d'argent nanométriques dispersés dans des matrices diélectriques à la structure contrôlée, afin d'ajuster la position spectrale de leur(s) bande(s) d'absorption plasmon. Dans cette optique, nous avons cherché à contrôler la morphologie des agrégats. Par des mesures de microscopie électronique en transmission et diffusion centrale des rayons X en incidence rasante, nous montrons que le rapport hauteur/diamètre des agrégats (ellipsoïdaux) peut être ajusté par un choix adéquat du mode de dépôt (simultané ou alterné) des espèces diélectrique et métallique, de la nature de la matrice (Si3N4 ou BN), de la quantité d'argent déposée, des conditions d'assistance et de post-traitement. La position du mode de résonance (1,1) des films élaborés varie sur tout le spectre du visible (de 400 à 750 nm) en fonction de la forme des agrégats en accord qualitatif avec les prévisions du modèle de Maxwell-Garnett.The aim of this work is to get a fine control of the structure of nanocomposite thin films consisting of silver clusters embedded within dielectric matrices elaborated by ion beam sputtering, in order to adjust the spectral position of their plasmon absorption band(s). For this purpose, it is desirable to control clusters morphology. It is showed by an approach combining transmission electron microscopy and Grazing Incidence Small Angle X-ray Scattering that the height-to-diameter ratio of the (ellipsoidal) clusters can be adjusted by a proper choice of the deposition mode (simultaneous or alternate sputtering of the metal and the dielectric species), nature of the matrix (Si3N4 or BN), amount of deposited silver, assistance and post-treatment conditions. The spectral position of the (1,1) resonance mode of the elaborated films shifts in the visible range from 400 nm to 750 nm depending on clusters shape in qualitative agreement with simulations using the Maxwell-Garnett model.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Couches de carbone amorphe hydrogéné sur verres ophtalmiques traités antireflets (contribution à l'étude des mécanismes de salissure)

L'objectif de ce travail était de résoudre le problème de salissure des verres ophtalmiques antireflet par le dépôt d'une couche ultra-mince de carbone amorphe hydrogéné a-C:H. Nous avons tout d'abord fait varier la tension d'autopolarisation afin de réduire l'absorption optique du film a-C:H obtenu par PECVD à décharge RF. Les couches réalisées avec un self-bias de 35 V et une épaisseur de 3 nm étaient transparentes. Nous avons ensuite déterminé les propriétés de nettoyage des films a-C:H et montré que celles-ci étaient dues à leur forte oléophilie : la salissure peut s'apparenter à un quasi-film mince, presque invisible en transmission mais difficile à enlever, compte tenu du fort travail d'adhésion. Ce comportement est opposé à celui observé sur les top-coats oléophobes. Dans ce cas, la salissure qui s'agence sous forme de micro-gouttelettes diffusantes (et donc très visibles en transmission) est facile à enlever de la surface compte tenu du faible travail d'adhésion.The goal of this work was to resolve the problem of the smudge on antireflective ophthalmic lenses by depositing an ultra-thin hydrogenated amorphous carbon coating a-C:H. First, we made the self-bias varying in order to reduce the optical absorption of the a-C:H film obtained by PECVD using a planar RF discharge. The coating obtained with a self-bias of 35 V and a thickness of 3 nm were transparent. Then, we determined the cleaning properties of a-C:H films and showed that the latter were due to the strong oleophilic nature of the material : the smudge can be considered as a quasi thin film almost invisible by transmission but very hard to remove, owing to the large adhesion work. This behaviour is opposite to the one observed on the olephobic top-coats. In this case, the smudge which was organised as scattering micro droplets (therefore very visible by transmission), is easy to remove from the surface, owing to the small adhesion work.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Morphologie et auto-organisation de nanoparticules d'argent dispersées dans les matrices diélectriques (influence sur les propriétés optiques)

L objectif de ce travail est d ajuster la position spectrale de la résonance plasmon de surface de nanoparticules de métaux nobles dispersées dans un matrice diélectrique, en contrôlant leur morphologie et leur organisation spatiale. Nous montrons que lorsque la croissance est réalisée sur des substrats plans, il est possible de jouer sur la nature de la matrice pour modifier le rapport d aspect hauteur sur diamètre H/D des particules, et donc leur réponse optique. Des analyses structurales quantitatives menées par microscopie électronique en transmission à balayage (HAADF-STEM) font apparaître que H/D est une fonction décroissante du diamètre D, indépendamment de la quantité de métal déposé. Afin de comprendre ces effets de la matrice sur les propriétés structurales et optiques des particules, différentes études (influence du métal et de la quantité déposés, présence d une couche tampon, influence des conditions d élaboration, vitesse de recouvrement des particules, . . . ) sont menées et des simulations numériques des spectres de transmission sont réalisées en intégrant dans un modèle de Yamaguchi les paramètres structuraux issus de l analyse HAADF. Une autre approche consiste à utiliser des surfaces d alumine nanostructurées afin d induire une organisation surfacique des particules et ainsi entraîner une anisotropie de leurs propriétés optiques. Nous montrons que selon la géométrie de dépôt utilisée (incidence normale, incidence rasante, orientation et angle d incidence du flux atomique), il est possible par effet d ombrage de sélectionner le type de facettes sur lesquelles la croissance a lieu, et par conséquent d organiser les nanoparticules d argent sur la surface sous forme de bandes ou de chaînes linéaires, dont les propriétés optiques présentent une dépendance à la polarisation de la lumière incidente.The aim of this work is to adjust the spectral position of the surface plasmon resonance of noble-metal nanoparticles embedded in a dielectric matrix by tailoring their morphology and their spatial organization. We show that when the growth is performed on plane substrates, it is possible to modify the aspect ratio of the particles H/D, and thus their optical response by changing the nature of the matrix. Quantitative structural analyzes carried out by using high angle annular dark field scanning transmission electronic microscopy (HAADF-STEM) imaging reveal that H/D is a decreasing function of the diameter D, independently of the deposited metal amount. In order to understand these effects of the matrix on the structural and optical properties of the particles, various studies (influence of the metal and deposited amount, presence of a buffer layer, influence of the elaboration conditions, covering rate of the particles,. . . ) are presented and numerical simulations of the optical spectra are proposed by integrating the structural parameters from the HAADF analysis in a model of Yamaguchi. A second step consists in using nanostructured sapphire surfaces in order to induce a surface organization of the particles, and thus to involve an anisotropy of their optical properties. We show that according to the geometry used during the deposition (normal incidence, grazing incidence, orientation and angle of incidence of atomic flux), it is possible to select the type of facets on which the growth takes place (shadowing effect), and consequently to elaborate self-organized systems of metal nanoislands constituted of stripes or linear chains of particles, whose optical properties display a light-polarization dependence.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

On the importance of light scattering for high performances nanostructured antireflective surfaces

An antireflective coating presenting a continuous refractive index gradient is theoretically better than its discrete counterpart because it can give rise to a perfect broadband transparency. This kind of surface treatment can be obtained with nanostructures like moth-eye. Despite the light scattering behavior must be accounted as it can lead to a significant transmittance drop, no methods are actually available to anticipate scattering losses in such nanostructured antireflective coatings. To overcome this current limitation, we present here an original way to simulate the scattering losses in these systems and routes to optimize the transparency. This method, which was validated by a comparative study of coatings presenting refractive indices with either discrete or continuous gradient, shows that a discrete antireflective coating bilayer made by oblique angle deposition allows reaching ultra-high mean transmittance of 98.97% over the broadband [400-1800] nm. Such simple surface treatment outperforms moth-eye architectures thanks to both interference effect and small dimensions nanostructures that minimize the scattering losses as confirmed by finite-difference time domain simulations performed on reconstructed volumes obtained from electron tomography experiments