La dyslexie développementale chez l'enfant : comparaison de rééducations en imagerie par résonnance magnétique fonctionnelle et étude du déficit auditif pré-attentif à l'aide de l'électro-encéphalographie

La dyslexie développementale peut se définir comme étant un trouble de l'apprentissage de la lecture survenant en dépit d'une intelligence normale, de l'absence de troubles sensoriels ou neurologiques, d'une instruction scolaire adéquate, d'opportunités socio-culturelles suffisantes; en outre, elle dépend d'une perturbation d'aptitudes cognitives fondamentales souvent d'origines constitutionnelles. Environ 4% à 8% des enfants souffriraient d'un tel trouble. Au sein du présent travail de thèse, nous avons dans un premier temps comparé les effets de trois rééducations différentes (Sémiophonie, Parole ralentie et classique) en comportemental et en imagerie cérébrale. Il fut montré une absence de différences d'effets entre ces trois rééducation, malgré un bénéfice commun, ainsi que des modifications d'activités cérébrales communes. La deuxième partie est consacrée à l'étude du déficit auditif pré-attentif, cette étude s'inscrivant dans le vaste projet européen Neurodys. La Mismatch Negativity obtenue avec des voyelles de différentes nationalités (Français, Finnois, Hongrois et Allemand), fut comparée entre enfants dyslexiques et enfants normo-lecteurs. Il fut montré que des corrélations entre le niveau de lecture et le traitement des voyelles corroborent l'hypothèse d'un traitement allophonique du langage chez les enfants dyslexiques.Developmental dyslexia or reading difficulty is one of the most common problems that severely affect academic performance. It is displayed as a difficulty in learning to read despite conventional teaching, without being the direct result of any intellectual disadvantage or unfavourable environmental influence. It affects 4% to 8% of children. In this thesis work, we first compared effects of three remediation programs (Semiophony, modified speech and standard) on behavioral performances and on brain activations using functional MRI. The three different types of remediation elicited the same behavioral improvement. Moreover, they had a common effect upon brain activation changes. In the second part of this manuscript we studied pre-attentive auditory deficit in dyslexic children compared to control children. This research project is included in a European project called Neurodys. Mismatch Negativity elicited by different European vowels (French, Finnish, Hungarian and German) was compared in the two groups of subjects. Correlations between reading level and vowel treatment showed a strong support for allophonic perception mode in dyslexic children

Magnetic and crystal structures of the one-dimensional ferromagnetic chain pyroxene NaCrGe2O6

International audienceWe investigated the magnetic and structural properties of the ferromagnetic pyroxene NaCrGe2O6 by superconducting quantum interference device magnetometry and powder neutron diffraction. This material is the only ferromagnetic member of the recently reported multiferroic pyroxene family AMX2O6 A=Li,Na; M =Fe, Cr; X=Si,Ge . Below TC=6 K, the magnetic structure is characterized by one-dimensional magnetic chains with spins aligned along the c axis of the monoclinic cell. The magnetic moment of Cr3+ is significantly reduced by about 25%. We show that this is likely the result of the low dimensionality of the system. The associated magnetic space group is C2 /c . This symmetry does not allow a linear magnetoelectric effect. No structural phase transition was observed down to 1.8 K

Modulations of ongoing alpha oscillations predict successful short-term visual memory encoding

Alpha-frequency band oscillations have been shown to be one of the most prominent aspects of neuronal ongoing oscillatory activity, as reflected by electroencephalography (EEG) recordings. First thought to reflect an idling state, a recent framework indicates that alpha power reflects cortical inhibition. In the present study, the role of oscillations in the upper alpha-band (12 Hz) was investigated during a change-detection test of short-term visual memory. If alpha oscillations arise from a purely inhibitory process, higher alpha power before sample stimulus presentation would be expected to correlate with poorer performance. Instead, participants with faster reaction-times showed stronger alpha power before the sample stimulus in frontal and posterior regions. Additionally, faster participants showed stronger alpha desynchronization after the stimulus in a group of right frontal and left posterior electrodes. The same pattern of electrodes showed stronger alpha with higher working-memory load, so that when more items were processed, alpha power desynchronized faster after the stimulus. During memory maintenance, alpha power was greater when more items were held in memory, likely due to a faster resynchronization. These data are consistent with the hypothesis that the level of suppression of alpha power by stimulus presentation is an important factor for successfully encoding visual stimuli. The data are also consistent with a role for alpha as actively participating in attentional processes

Magnetic and crystal structures of the magnetoelectric pyroxene LiCrSi2O6

International audienceWe investigated the magnetic and crystal structures of the recent reported magnetoelectric system LiCrSi2O6 by powder neutron diffraction. Below TN=11.5 K, an antiferromagnetic order appears. It is characterized by an antiferromagnetic coupling within the CrO6 octahedra chains and a ferromagnetic coupling between the chains. The magnetic order is commensurate with the lattice with k=0. The associated magnetic space group is P21 /c. This symmetry is in agreement with the reported magnetoelectric effect. We show that the magnetic frustration in this system is small. Finally, we discuss our results using a Landau phenomenological model and in the light of the literature

Magnetodielectric coupling of a polar organic-inorganic hybrid Cr(II) phosphonate

Cr[(H(3)N-(CH(2))(2)-PO(3))(Cl)(H(2)O)] represents a rare example of a polar organic-inorganic hybrid material that exhibits a canted antiferromagnetic order below T(N)=5.5 K. The unusual coexistence of a polar crystal structure and magnetic order triggered our investigation of the magnetodielectric coupling. The coupling is evidenced by an anomaly in the temperature dependence of the dielectric constant epsilon below the Neel temperature. The magnetocapacitance is enhanced by one order of magnitude below T(N). The main characteristics of the magnetodielectic response are interpreted by Landau theoretical coupling terms

Coupled Negative magnetocapacitance and magnetic susceptibility in a Kagome staircase-like compound Co3V2O8

The dielectric constant of the Kagome staircase-like Co3V2O8 polycrystalline compound has been measured as function of temperature and magnetic field up to 14T. It is found that the application of an external magnetic field suppresses the anomaly for the dielectric constant beyond 6.1K. Furthermore, its magnetic field dependence reveals a negative magnetocapacitance which is proportional to the magnetic susceptibility, suggesting a common magnetostrictive origin for the magnetic field dependence of the two quantities. This result is very different from that obtained from the isostructural compound Ni3V2O8 that presents a peak in the dielectric constant at the incommensurate magnetic phase transition coupled to a sign change of the magnetocapacitance

High-pressure Raman study of Fe(IO3)3: Soft-mode behavior driven by coordination changes of iodine atoms

This document is the Accepted Manuscript version of a Published Work that appeared in final form in The Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.jpcc.0c06541.[EN] We report high-pressure Raman spectroscopy studies of Fe(IO3)(3) up to nearly 21 GPa that have been interpreted with the help of density functional theory calculations, which include the calculation of phonon dispersion curves and elastic constants at different pressures. Zero-pressure Raman-active mode frequencies and their pressure dependences have been determined. Modes have been assigned and correlated to atomic movements with the help of calculations. Interestingly, in the high-frequency region, there are several modes that soften under compression. These modes have been identified as internal vibrations of the IO3 coordination polyhedron. Their unusual behavior is a consequence of the changes induced by pressure in the coordination sphere of iodine, which gradually change from a threefold coordination to an almost sixfold coordination under compression. The coordination change is favored by the decrease of the stereoactivity of the iodine lone electron pair so that likely a real sixfold coordination is attained after a first-order phase transition previously reported to occur above 21 GPa. The strong nonlinear behavior found in Raman-active modes as well as in theoretically calculated elastic constants has been discovered to be related to the occurrence of two previously unreported isostructural phase transitions at 1.5-2.0 and 5.7-6.0 GPa as shown by dynamic instabilities close to the Brillouin zone center.This work was supported by the Spanish Ministry of Science, Innovation and Universities, the Spanish Research Agency (AEI), the European Fund for Regional Development (ERDF, FEDER) under grants MAT2016-75586-C4-1/2/3-P, PID2019-106383GB-C41/42/43, and RED2018-102612-T (MALTA Consolider-Team Network), and the Generalitat Valenciana under grant Prometeo/2018/123 (EFIMAT). A.L. and D.E. would like to thank the Generalitat Valenciana for the Ph.D. fellowship GRISOLIAP/2019/025).Liang, A.; Rahman, S.; Rodriguez-Hernandez, P.; Muñoz, A.; Manjón, F.; Nenert, G.; Errandonea, D. (2020). High-pressure Raman study of Fe(IO3)3: Soft-mode behavior driven by coordination changes of iodine atoms. The Journal of Physical Chemistry C. 124(39):21329-21337. https://doi.org/10.1021/acs.jpcc.0c06541S21329213371243