Jean-Christophe Comte

Multimodal Convergence within the Intraparietal Sulcus of the Macaque Monkey

The parietal cortex is highly multimodal and plays a key role in the processing of objects and actions in space, both in human and nonhuman primates. Despite the accumulated knowledge in both species, we lack the following: (1) a general description of the multisensory convergence in this cortical region to situate sparser lesion and electrophysiological recording studies; and (2) a way to compare and extrapolate monkey data to human results. Here, we use functional magnetic resonance imaging (fMRI) in the monkey to provide a bridge between human and monkey studies. We focus on the intraparietal sulcus (IPS) and specifically probe its involvement in the processing of visual, tactile, and auditory moving stimuli around and toward the face. We describe three major findings: (1) the visual and tactile modalities are strongly represented and activate mostly nonoverlapping sectors within the IPS. The visual domain occupies its posterior two-thirds and the tactile modality its anterior one-third. The auditory modality is much less represented, mostly on the medial IPS bank. (2) Processing of the movement component of sensory stimuli is specific to the fundus of the IPS and coincides with the anatomical definition of monkey ventral intraparietal area (VIP). (3) A cortical sector within VIP processes movement around and toward the face independently of the sensory modality. This amodal representation of movement may be a key component in the construction of peripersonal space. Overall, our observations highlight strong homologies between macaque and human VIP organization.

Paradoxical Sleep in Mice Lacking M3 and M2/M4 Muscarinic Receptors

Acetylcholine is crucial for the regulation of paradoxical sleep (PS) and EEG theta activity. To determine the contribution of individual muscarinic receptors to these events, we analyzed the sleep-waking cycle and EEG activities of mice lacking functional M3 or M2/M4 receptors. Daily PS amounts were significantly decreased in M3–/– (–22%) but not in M2/M4–/– mice. Further, the theta peak frequency for PS was significantly increased in both M2/M4–/– and M3–/– mice. This study supports the potential role of M3 rather than M2 and M4 muscarinic receptors in the modulation of PS in mice and strengthens the idea that multiple muscarinic receptors contribute to the regulation of the EEG theta activity during PS.

Stochastic Resonance Another Way to Retrieve Subthreshold Digital Data

A new type of electronic receiver based on stochastic resonance properties is proposed to retrieve subthreshold modulated (or not) digital signals. In order to check the receiver performances, we investigated different kinds of modulations. Considering both uniform and Gaussian white noise, a total restoration of the emitted data is achieved for an unmodulated or amplitude modulated (AM) signals, while an almost total restoration is reached using a frequency shift keying (FSK) modulation. These experimental results confirm the great interest of stochastic resonance in the signal transmission field.

Implementation of an Absolute Brain

Magnetic resonance spectroscopy has emerged as a sensitive modality to detect early and diffuse alterations in multiple sclerosis. Recently, the hypothesis of neurodegenerative pathogenesis has highlightened the interest for measurement of metabolites concentrations, to gain specificity, in a large brain volume encompassing different tissue alterations. Therefore, we proposed in this paper the implementation of an absolute quantification method based on localized spectroscopy at short (30 ms) and long (135 ms) echo time of a volume including normal appearing white matter, cortical gray matter, and lesions. First, methodological developments were implemented including external calibration, and corrections of phased-array coil sensitivity and cerebrospinal fluid volume contribution. Second, these improvements were validated and optimized using an original methodology based on simulations of brain images with lesions. Finally, metabolic alterations were assessed in 65 patients including 26 relapsing-remitting, 17 primary-progressive (PP), 22 secondary-progressive (SP) patients, and in 23 normal subjects. Results showed increases of choline, creatine, and myo-inositol concentrations in PP and SP patients compared to controls, whereas the concentration of N-acetyl compounds remained constant. The major finding of this study was the identification of Cho concentration and Cho/tNA ratio as putative markers of progressive onset, suggesting interesting perspectives in detection and followup of neurodegenerative processes.

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A contrast enhancement and image inverting tool using a lattice of uncoupled nonlinear oscillators is proposed. We show theoretically and numerically that the gray scale picture contrast is strongly enhanced even if this one is initially very small. An image inversion can be also obtained in real time with the same Cellular Nonlinear Network (CNN) without reconfiguration of the network. A possible electronic implementation of this CNN is finally discussed.

H-MRS Quantification Method to Assess Different Tissue Alterations in Multiple Sclerosis

A general method allowing to construct nonlinear resistors with arbitrary current-voltage (I-V) characteristics is proposed. The example of a cubic I-V characteristic is presented showing a perfect agreement between the theoretical desired resistor and its electronic realization based on analog multipliers.

A NONLINEAR OSCILLATORS NETWORK DEVOTED TO IMAGE PROCESSING

An electronic receiver based on stochastic resonance is presented to rescue subthreshold modulated digital data. In real experiment, it is shown that a complete data restoration is achieved for both uniform and Gaussian white noise.

GENERATION OF NONLINEAR CURRENT–VOLTAGE CHARACTERISTICS: A GENERAL METHOD

We study the influence of spatiotemporal noise on the propagation of square waves in an electrical dissipative chain of triggers. By numerical simulation, we show that noise plays an active role in improving signal transmission. Using the Signal to Noise Ratio at each cell, we estimate the propagation length. It appears that there is an optimum amount of noise that maximizes this length. This specific case of stochastic resonance shows that noise enhances propagation.

DIGITAL INFORMATION RECEIVER BASED ON STOCHASTIC RESONANCE

Kink propagation failure induced by coupling inhomogeneities in a Nagumo lattice is investigated. Considering the case of weak couplings, we define analytically and numerically the coupling conditions leading to the pinning of the kink.

NOISE-ENHANCED PROPAGATION IN A DISSIPATIVE CHAIN OF TRIGGERS

A contour detection based on a diffusive cellular nonlinear network is proposed. It is shown that there exists a particular nonlinear function for which, numerically, the obtained contour is satisfactory. Furthermore, this nonlinear function can be achieved using analog components.