Laminar signal extraction over extended cortical areas by means of a spatial GLM.

There is converging evidence that distinct neuronal processes leave distinguishable footprints in the laminar BOLD response. However, even though the achievable spatial resolution in functional MRI has much improved over the years, it is still challenging to separate signals arising from different cortical layers. In this work, we propose a new method to extract laminar signals. We use a spatial General Linear Model in combination with the equivolume principle of cortical layers to unmix laminar signals instead of interpolating through and integrating over a cortical area: thus reducing partial volume effects. Not only do we provide a mathematical framework for extracting laminar signals with a spatial GLM, we also illustrate that the best case scenarios of existing methods can be seen as special cases within the same framework. By means of simulation, we show that this approach has a sharper point spread function, providing better signal localisation. We further assess the partial volume contamination in cortical profiles from high resolution human ex vivo and in vivo structural data, and provide a full account of the benefits and potential caveats. We eschew here any attempt to validate the spatial GLM on the basis of fMRI data as a generally accepted ground-truth pattern of laminar activation does not currently exist. This approach is flexible in terms of the number of layers and their respective thickness, and naturally integrates spatial regularisation along the cortex, while preserving laminar specificity. Care must be taken, however, as this procedure of unmixing is susceptible to sources of noise in the data or inaccuracies in the laminar segmentation.

A biologically plausible mechanism for neuronal coding organized by the phase of alpha oscillations.

The visual system receives a wealth of sensory information of which only little is relevant for behaviour. We present a mechanism in which alpha oscillations serve to prioritize different components of visual information. By way of simulated neuronal networks, we show that inhibitory modulation in the alpha range (~ 10 Hz) can serve to temporally segment the visual information to prevent information overload. Coupled excitatory and inhibitory neurons generate a gamma rhythm in which information is segmented and sorted according to excitability in each alpha cycle. Further details are coded by distributed neuronal firing patterns within each gamma cycle. The network model produces coupling between alpha phase and gamma (40-100 Hz) amplitude in the simulated local field potential similar to that observed experimentally in human and animal recordings.

Adhesion forces in the self-recognition of oligosaccharide epitopes of the proteoglycan aggregation factor of the marine sponge Microciona prolifera.

Cell aggregation in the marine sponge Microciona prolifera is mediated by a multimillion molecular-mass aggregation factor, termed MAF. Earlier investigations revealed that the cell aggregation activity of MAF depends on two functional domains: (i) a Ca(2+)-independent cell-binding domain and (ii) a Ca(2+)-dependent proteoglycan self-interaction domain. Structural analysis of involved carbohydrate fragments of the proteoglycan in the self-association established a sulfated disaccharide beta-D: -GlcpNAc3S-(1-->3)-alpha-L: -Fucp and a pyruvated trisaccharide beta-D: -Galp4,6(R)Pyr-(1-->4)-beta-D: -GlcpNAc-(1-->3)-alpha-L: -Fucp. Recent UV, SPR, and TEM studies, using BSA conjugates and gold nanoparticles of the synthetic sulfated disaccharide, clearly demonstrated self-recognition on the disaccharide level in the presence of Ca(2+)-ions. To determine binding forces of the carbohydrate-carbohydrate interactions for both synthetic MAF oligosaccharides, atomic force microscopy (AFM) studies were carried out. It turned out that, in the presence of Ca(2+)-ions, the force required to separate the tip and sample coated with a self-assembling monolayer of thiol-spacer-containing beta-D: -GlcpNAc-(1-->3)-alpha-L: -Fucp-(1-->O)(CH(2))(3)S(CH(2))(6)S- was found to be quantized in integer multiples of 30 +/- 6 pN. No binding was observed between the two monolayers in the absence of Ca(2+)-ions. Cd(2+)-ions could partially induce the self-interaction. In contrast, similar AFM experiments with thiol-spacer-containing beta-D: -Galp4,6(R)Pyr-(1-->4)-beta-D: -GlcpNAc-(1-->3)-alpha-L: -Fucp-(1-->O)(CH(2))(3)S(CH(2))(6)S- did not show a binding in the presence of Ca(2+)-ions. Also TEM experiments of gold nanoparticles coated with the pyruvated trisaccharide could not make visible aggregation in the presence of Ca(2+)-ions. It is suggested that the self-interaction between the sulfated disaccharide fragments is stronger than that between the pyruvated trisaccharide.

Domain formation in phosphatidylcholine bilayers containing transmembrane peptides: specific effects of flanking residues.

Lateral segregation in biological membranes leads to the formation of domains. We have studied the lateral segregation in gel-state model membranes consisting of supported dipalmitoylphosphatidylcholine (DPPC) bilayers with various model peptides, using atomic force microscopy (AFM). The model peptides are derivatives of the Ac-GWWL(AL)(n)WWA-Etn peptides (the so-called WALP peptides) and have instead of tryptophans, other flanking residues. In a previous study, we found that WALP peptides induce the formation of extremely ordered, striated domains in supported DPPC bilayers. In this study, we show that WALP analogues with other uncharged residues (tyrosine, phenylalanine, or histidine at pH 9) can also induce the formation of striated domains, albeit in some cases with a slightly different pattern. The WALP analogues with positively charged residues (lysine or histidine at low pH) cannot induce striated domains and give rise to a completely different morphology: they induce irregularly shaped depressions in DPPC bilayers. The latter morphology is explained by the fact that the positively charged peptides repel each other and hence are not able to form striated domains in which they would have to be in close vicinity. They would reside in disordered, fluidlike lipid areas, appearing below the level of the ordered gel-state lipid domains, which would account for the irregularly shaped depressions.