Left medial temporal lobe epilepsy (MTLE) is associated with verbal memory impairment usually related to hippocampal damage. We used functional MRI (fMRI) to investigate the patterns of functional activity in healthy volunteers and MTLE patients engaged in verbal episodic memory tasks to look for evidence of a reallocation of verbal memory in epileptic patients. fMRI data were collected from seven MTLE patients with left-sided hippocampal sclerosis and 10 healthy right-handed control subjects on a 3T scanner. Subjects were instructed to learn a list of 17 words (encoding) and then to recall them (retrieval) on successive trials. Healthy volunteers and patients both exhibited bilateral activation (right higher than left) of the parahippocampal gyrus during the retrieval. This effect was more marked in the control subjects. In contrast to the control subjects, patients exhibited consistent and extensive left prefrontal activations in all the memory tasks. These findings show that verbal memory tasks did not involve the same functional patterns in patients and healthy volunteers. This may be interpreted as a dysfunctional response due to the epilepsy and left hippocampal sclerosis, and could reflect the early onset and progressive course of the disease.

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There is growing interest in studying the role of connectivity patterns in brain functions. In recent years, functional brain networks were found to exhibit small-world properties during different brain states. In previous studies, time-independent networks were recovered from long time periods of brain activity. In this paper, we propose an approach, the event-related networks, that allows one to characterize the dynamical evolution of functional brain networks in time-frequency space. We illustrate this approach by characterizing connectivity patterns in magnetoencephalographic signals recorded during a visual stimulus paradigm. When compared with equivalent random and regular networks, the results reveal that functional connectivity varies with time and frequency during the processing of the stimulus, while maintaining a small-world structure. This approach may provide insights into the connectivity of other complex and spatially extended nonstationary systems.

Semiarid landscapes are characterized by vegetated surfaces. Understanding the impact of vegetation on aeolian soil erosion is important for reducing soil erosion or limiting crop damage through abrasion or burial. In the present study, a saltation model fully coupled with a large-eddy simulation airflow model is extended to vegetated landscapes. From this model, the sensitivity of sand erosion to different arrangements and type of plants (shrub versus tree) representative of semiarid landscapes is investigated and the wind erosion reduction induced by plants is quantified. We show that saltation processes over vegetated surfaces have a limited impact on the mean wind statistics, the momentum extracted from the flow by saltating particles being negligible compared to that extracted by plants. Simulated sand erosion patterns resulting from plant distribution, i.e., accumulation and erosion areas, appear qualitatively consistent with previous observations. It is shown that sand erosion reduction depends not only on vegetation cover but also on plant morphology and plant distribution relative to the mean wind direction. A simple shear stress partitioning approach applied in shrub cases gives similar trends of sand erosion reduction as the present model following wind direction and vegetation cover. However, the magnitude of the reduction appears significantly different from one approach to another. Although shrubs trap saltating particles, trees appear more efficient than shrubs to reduce sand erosion. This is explained by the large-scale sheltering effect of trees compared to the local shrub one.

We investigate theoretically the optomechanical interactions inside cavities created in two-dimensional infinite phoXonic crystals constituted by a square array of holes in silicon (Si) and gallium arsenide (GaAs) matrices. The cavity is simply obtained by removing one hole in the perfect crystal. Our calculations take into account two mechanisms that contribute to the optomechanical coupling, namely the bulk photoelastic effect and the deformations of the interfaces due to the acoustic strain. The coupling strength is estimated by two different methods, modulation of the photonic mode frequency during one period of the acoustic oscillations and calculation of the optomechanical coupling rate. We discuss the important roles of the symmetry and degeneracy of the modes to discriminate which ones are not able to interact efficiently. Calculations in Si and GaAs crystals at different optical wavelengths emphasize the dependence of the photoelastic contribution to the optomechanical interaction as a function of material and wavelength, especially owing to the significant variation of the photoelastic coefficients near the semiconductor band gap.

International audienc

This study assesses the role of the human medial temporal lobe (MTL) structures in the coordination of spatial information across perspective change and, in particular, in visual perspective taking--namely the capacity to know what another individual is seeing on the visual scene. Fourteen patients with unilateral temporal lobe resection and 21 control subjects performed two tasks, called 'object location memory' and 'viewpoint recognition', respectively. In the object location memory task, subjects had to memorize the position of a target object in the environment from an initial viewpoint. They were then shown the same environment from a new viewpoint and had to indicate whether or not the target object had moved. In the viewpoint recognition task, subjects had to imagine the perspective of an avatar from the initial viewpoint and then decide whether or not the new viewpoint was that of the avatar. The results showed a double dissociation, with left MTL patients being impaired in the object location memory task but not in the viewpoint recognition task and right MTL patients being impaired in the viewpoint recognition task but not in the object location memory task. Furthermore, based on multiple regression analyses between performance and the volumes of the different MTL structures, we discuss the specific involvement of the left temporopolar cortex and of the right hippocampus in different kinds of visual perspective taking.

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