MRI Techniques and New Animal Models for Imaging the Brain

Chapitre 10absen

Early visual evoked potentials are modulated by eye position in humans induced by whole body rotations

BACKGROUND: To reach and grasp an object in space on the basis of its image cast on the retina requires different coordinate transformations that take into account gaze and limb positioning. Eye position in the orbit influences the image's conversion from retinotopic (eye-centered) coordinates to an egocentric frame necessary for guiding action. Neuroimaging studies have revealed eye position-dependent activity in extrastriate visual, parietal and frontal areas that is along the visuo-motor pathway. At the earliest vision stage, the role of the primary visual area (V1) in this process remains unclear. We used an experimental design based on pattern-onset visual evoked potentials (VEP) recordings to study the effect of eye position on V1 activity in humans. RESULTS: We showed that the amplitude of the initial C1 component of VEP, acknowledged to originate in V1, was modulated by the eye position. We also established that putative spontaneous small saccades related to eccentric fixation, as well as retinal disparity cannot explain the effects of changing C1 amplitude of VEP in the present study. CONCLUSIONS: The present modulation of the early component of VEP suggests an eye position-dependent activity of the human primary visual area. Our findings also evidence that cortical processes combine information about the position of the stimulus on the retinae with information about the location of the eyes in their orbit as early as the stage of primary visual area

Top-Down Activation of Fusiform Cortex without Seeing Faces in Prosopagnosia

Face processing can be modified by bottom-up and top-down influences, but it is unknown how these processes interact in patients with face-recognition impairments (prosopagnosia). We investigated a prosopagnosic with lesions in right occipital and left fusiform cortex but whose right fusiform gyrus is intact and still activated during face-processing tasks. P.S., a patient with a well-established and selective agnosia for faces, was instructed to detect the presence of either faces or houses in pictures with different amounts of noise. The right fusiform face area (FFA) showed reduced responses to face information when visual images were degraded with noise. However, her right FFA still activated to noise-only images when she was instructed to detect faces. These results reveal that fusiform activation is still selectively modulated by task demands related to the anticipation of a face, despite severe face-recognition deficits and the fact that no reliable stimulus-driven response is evoked by actual facial information. Healthy controls showed stimulus-driven responses to faces in fusiform, and in right but not left occipital cortex, suggesting that the latter area alone might provide insufficient facial information in P.S. These results provide a novel account for residual activation of the FFA and underscore the importance of controlling task demands during functional magnetic resonance imagin

2.5D Representations Combining in vivo 3D MRI and ex vivo 2D MSI Approaches to Study the Lipid Distribution in the Whole Sheep Brain

National audienceMass Spectrometry Imaging (MSI) provides easily high spatially resolved masses allowing characterization of endogenous lipids. These latter constitute about 70% of the composition of the white matter of the brain which can be implicated in developmental and/or cognitive troubles. In order to examine the molecular distribution of lipids in whole sheep brain, and especially in white/grey matter, we combined in vivo and ex vivo images, obtained in the same animals, using Magnetic Resonance Imaging (MRI) and MSI, respectively. In order to view the topology of the molecular species within the organ, we propose the construction of a 2.5 D representation where a single section imaged with 2D MSI is localized within the tissue volume obtained by 3D MRI. 3D T1-weighted MPRAGE images were acquired on two anesthetized sheep with a 3 Tesla MRI (Siemens, Verio ®). The parameters of acquisition for the MPRAGE were: TR 2500ms, TE 3.2ms, FA 12, NEX 1, matrix 384×384, FOV 192mm, 288 slices with a thickness of 0.5mm. In order to improve data quality, the 3D MRI volumes have been pre-processed using in-house algorithms using volume fitting and Markov random field methods. T1 3D planes corresponding to MSI planes were reconstructed using Osirix imaging software.Brains were collected after sacrifice and frozen at -80°C. Frontal and sagittal 14 µm brain sections were performed with a cryostat adapted to large sections (CM3050 S, Leica) and mounted onto conductive ITO-coated slides. The spray of α-cyano-4-hydroxycinnamic acid matrix was performed using an Image Prep device (Bruker). Spectra were acquired using an UltrafleXtrem MALDI-TOF instrument (Bruker) in the 200–1200 m/z range with a spatial resolution set at 125 µm. Raw spectra were analyzed with SCiLS Lab software to generate 2D ion density maps and segmentation maps (data partitioning). The tissue sections analyzed by MSI were stained with cresyl violet to manually delimitate neuronal nuclei and areas. This histological map was used to delineate the MRI and MSI 2D views and overlay them regardless the same brain areas used as fiducials. After, a 2.5 D representation was proposed to visualize the lipid distribution within the entire organ.In conclusion, in this study, frontal and sagittal whole sheep brain sections analyzed by MSI showed a clear difference in lipid distribution between different compartments of brain tissues, especially between grey and white matter, until the cerebral envelopment presenting circumvolution. Furthermore, the alignment of 2D MALDI-imaging with T1-weighted images showed that MSI can provide finer details on the structural connectivity of myelinated fiber tracts. Here, the 2.5 D representation combining MRI and MSI was presented as an alternative approach to 3D anatomical and molecular atlas providing a perfect topology of the molecular species within an organ. For the moment, 3D MSI of whole sheep brain is a challenge, while the 2.5 D construction demonstrated to be a capable tool for exploring molecular distributions throughout sample volumes.Nowadays, the reported results may serve as a starting point for further experiments associating MSI and dynamic and functional MRI, especially for the characterization of brain

Evolution of predator dispersal in relation to spatio-temporal prey dynamics : how not to get stuck in the wrong place!

Peer reviewedPublisher PD

A computer simulation study of the effect of pressure on Mg diffusion in forsterite

Computer simulation techniques were used to investigate the effect of pressure on magnesium diffusionin forsterite between 0 and 10 GPa. We studied the diffusion path along the c crystallographic axis (we always refer to the Pbnm system) via a vacancy mechanism. Using a Mott-Littleton approach withinthe code GULP, we were able to precisely map the diffusion path of a Mg vacancy and we found theactivation energy, E = 3.97 eV at 0 GPa (with Ef = 3.35 eV for the formation energy and Em = 0.62 eV for the migration) and E = 4.46 eV at 10 GPa (Ef = 3.81 eV and Em = 0.65 eV). Preliminary results using the supercell technique gave the same saddle point coordinates and energies. This saddle point of the Mg vacancy diffusion found with GULP was then introduced in an ab initio code, confirming the values of the migration energy both at 0 and 10 GPa. We were therefore able to estimate the activation volume (V) to be around 5 cm3/mol and d(V)/dP = 0. The effect of pressure applies mostly on defect formation and little on migration

Immune-Mediated Change in the Expression of a Sexual Trait Predicts Offspring Survival in the Wild

BACKGROUND: The "good genes" theory of sexual selection postulates that females choose mates that will improve their offspring's fitness through the inheritance of paternal genes. In spite of the attention that this hypothesis has given rise to, the empirical evidence remains sparse, mostly because of the difficulties of controlling for the many environmental factors that may covary with both the paternal phenotype and offspring fitness. Here, we tested the hypothesis that offspring sired by males of a preferred phenotype should have better survival in an endangered bird, the houbara bustard (Chlamydotis undulata undulata). METHODOLOGY/PRINCIPAL FINDINGS: We tested if natural and experimentally-induced variation in courtship display (following an inflammatory challenge) predicts the survival of offspring. Chicks were produced by artificial insemination of females, ensuring that any effect on survival could only arise from the transfer of paternal genes. One hundred and twenty offspring were equipped with radio transmitters, and their survival monitored in the wild for a year. This allowed assessment of the potential benefits of paternal genes in a natural setting, where birds experience the whole range of environmental hazards. Although natural variation in sire courtship display did not predict offspring survival, sires that withstood the inflammatory insult and maintained their courtship activity sired offspring with the best survival upon release. CONCLUSIONS: This finding is relevant both to enlighten the debate on "good genes" sexual selection and the management of supportive breeding programs

Comparative Genomics Suggests that the Fungal Pathogen Pneumocystis Is an Obligate Parasite Scavenging Amino Acids from Its Host's Lungs

Pneumocystis jirovecii is a fungus causing severe pneumonia in immuno-compromised patients. Progress in understanding its pathogenicity and epidemiology has been hampered by the lack of a long-term in vitro culture method. Obligate parasitism of this pathogen has been suggested on the basis of various features but remains controversial. We analysed the 7.0 Mb draft genome sequence of the closely related species Pneumocystis carinii infecting rats, which is a well established experimental model of the disease. We predicted 8’085 (redundant) peptides and 14.9% of them were mapped onto the KEGG biochemical pathways. The proteome of the closely related yeast Schizosaccharomyces pombe was used as a control for the annotation procedure (4’974 genes, 14.1% mapped). About two thirds of the mapped peptides of each organism (65.7% and 73.2%, respectively) corresponded to crucial enzymes for the basal metabolism and standard cellular processes. However, the proportion of P. carinii genes relative to those of S. pombe was significantly smaller for the “amino acid metabolism” category of pathways than for all other categories taken together (40 versus 114 against 278 versus 427, P<0.002). Importantly, we identified in P. carinii only 2 enzymes specifically dedicated to the synthesis of the 20 standard amino acids. By contrast all the 54 enzymes dedicated to this synthesis reported in the KEGG atlas for S. pombe were detected upon reannotation of S. pombe proteome (2 versus 54 against 278 versus 427, P<0.0001). This finding strongly suggests that species of the genus Pneumocystis are scavenging amino acids from their host's lung environment. Consequently, they would have no form able to live independently from another organism, and these parasites would be obligate in addition to being opportunistic. These findings have implications for the management of patients susceptible to P. jirovecii infection given that the only source of infection would be other humans

Individual Attachment Style Modulates Human Amygdala and Striatum Activation during Social Appraisal

Adult attachment style refers to individual personality traits that strongly influence emotional bonds and reactions to social partners. Behavioral research has shown that adult attachment style reflects profound differences in sensitivity to social signals of support or conflict, but the neural substrates underlying such differences remain unsettled. Using functional magnetic resonance imaging (fMRI), we examined how the three classic prototypes of attachment style (secure, avoidant, anxious) modulate brain responses to facial expressions conveying either positive or negative feedback about task performance (either supportive or hostile) in a social game context. Activation of striatum and ventral tegmental area was enhanced to positive feedback signaled by a smiling face, but this was reduced in participants with avoidant attachment, indicating relative impassiveness to social reward. Conversely, a left amygdala response was evoked by angry faces associated with negative feedback, and correlated positively with anxious attachment, suggesting an increased sensitivity to social punishment. Secure attachment showed mirror effects in striatum and amygdala, but no other specific correlate. These results reveal a critical role for brain systems implicated in reward and threat processing in the biological underpinnings of adult attachment style, and provide new support to psychological models that have postulated two separate affective dimensions to explain these individual differences, centered on the ventral striatum and amygdala circuits, respectively. These findings also demonstrate that brain responses to face expressions are not driven by facial features alone but determined by the personal significance of expressions in current social context. By linking fundamental psychosocial dimensions of adult attachment with brain function, our results do not only corroborate their biological bases but also help understand their impact on behavior