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Thomas Schmidt

Publications and source records attributed to Thomas Schmidt.

47 records · Page 3Linked to original sources
arXiv2018

Cytoskeletal Anisotropy Controls Geometry and Forces of Adherent Cells

We investigate the geometrical and mechanical properties of adherent cells characterized by a highly anisotropic actin cytoskeleton. Using a combination of theoretical work and experiments on micropillar arrays, we demonstrate that the shape of the cell edge is accurately described by elliptical arcs, whose eccentricity expresses the degree of anisotropy of the internal cell stresses. This results in a spatially varying tension along the cell edge, that significantly affects the traction forces exerted by the cell on the substrate. Our work highlights the strong interplay between cell mechanics and geometry and paves the way towards the reconstruction of cellular forces from geometrical data.

physics.bio-ph↗
arXiv2018

VMC proper motions of the Magellanic Bridge

Dwarf galaxies enable us to study the early phases of galaxy evolution and are key to many open questions about the hierarchical structure of the Universe. The Large and Small Magellanic Cloud (LMC and SMC) are the most luminous dwarf galaxy satellites of the Milky Way (MW). They are most likely gravitationally bound to each other, and their last interaction occurred about 200 Myr ago. Also, they are in an early phase of minor merging with the MW and will impact the Galactic structure in the future because of their relatively large mass. However, there are still major uncertainties regarding their origin and their interactions with one another and with the Milky Way. We cross-correlated the VMC and Gaia DR2 data to select a sample of stars that likely belong to the Magellanic Bridge, a feature formed of gas and stars which is connecting the LMC and the SMC. We removed potential MW foreground stars using a combination of parallax and colour-magnitude criteria and calculated the proper motions of the Bridge member stars. Our analysis supports a motion of star towards the LMC, which was found to be in good agreement with a dynamical simulation, of the SMC being stripped by the LMC.

astro-ph.GA↗
arXiv2018

An accumulator model for primes and targets with independent response activation rates: Basic equations for average response times

In response priming tasks, speeded responses are performed toward target stimuli preceded by prime stimuli. Responses are slower and error rates are higher when prime and target are assigned to different responses, compared to assignment to the same response, and those priming effects increase with prime-target SOA. Here, we generalize Vorberg et al.'s (2003) accumulator model of response priming, where response activation is first controlled exclusively by the prime and then taken over by the actual target. Priming thus occurs by motor conflict because a response-inconsistent prime can temporarily drive the process towards the incorrect response. While the original model assumed prime and target signals to be identical in strength, we allow different rates of response activation (cf. Mattler & Palmer, 2012; Schubert et al., 2012). Our model predicts that stronger primes mainly increase priming effects in response times and error rates, whereas stronger targets mainly diminish response times and priming effects.

q-bio.NC↗
arXiv2017

Sources of false positives and false negatives in the STATCHECK algorithm: Reply to Nuijten et al. (2016)

STATCHECK is an R algorithm designed to scan papers automatically for inconsistencies between test statistics and their associated p values (Nuijten et al., 2016). The goal of this comment is to point out an important and well-documented flaw in this busily applied algorithm: It cannot handle corrected p values. As a result, statistical tests applying appropriate corrections to the p value (e.g., for multiple tests, post-hoc tests, violations of assumptions, etc.) are likely to be flagged as reporting inconsistent statistics, whereas papers omitting necessary corrections are certified as correct. The STATCHECK algorithm is thus valid for only a subset of scientific papers, and conclusions about the quality or integrity of statistical reports should never be based solely on this program.

q-bio.QM↗
arXiv2015

Spherical Fused Silica Cells Filled with Pure Helium for NMR-Magnetometry

High magnetic fields (> 1 T) are measured by NMR magnetometers with un-rivaled precision if the precessing spin sample provides long coherence times. The longest coherence times are found in diluted ${}^{3}$He samples, which can be hyperpolarized for sufficient signal strength. In order to have minimal influence on the homogeneity and value of the measured magnetic field the optimal container for the ${}^{3}$He should be a perfect sphere. A fused silica sphere with an inner diameter of 8 mm and an outer diameter of 12 mm was made from two hemispheres by diffusion bonding leaving only a small hole for cleaning and evacuation. This hole was closed in vacuum by a CO${}_{2}$ laser and the inner volume was filled with a few mbars of ${}^3$He via wall permeation. NMR-measurements on such a sample had coherence times of 5 min. While the hemispheres were produced with < 1 $μ$m deviation from sphericity, the bonding left a step of ca. 50 $μ$m at maximum. The influence of such a mismatch, its orientation and materials in the direct vicinity of the sample are analyzed by FEM-simulations and discussed in view of coherence times and absolute fields.

physics.ins-det↗
arXiv2014

Behavioral criteria of feedforward processing in rapid-chase theory: Some formal considerations

The rapid-chase theory of response priming defines a set of behavioral criteria that indicate feedforward processing of visual stimulus features rather than recurrent processing. These feedforward criteria are strong predictions from a feedforward model that argues that sequentially presented prime and target stimuli lead to strictly sequential waves of visuomotor processing that can still be traced in continuous motor output, for instance, in pointing movements, muscle forces, or EEG readiness potentials. The feedforward criteria make it possible to evaluate whether some continuous motor output is consistent with feedforward processing, even though the neuronal processes themselves are not readily observable. This paper is intended as an auxiliary resource that states the criteria with some degree of formal precision.

q-bio.NC↗
arXiv2014

Invisible stimuli, implicit thresholds: Why invisibility judgments cannot be interpreted in isolation

Some studies of unconscious cognition rely on judgments of participants stating that they have "not seen" the critical stimulus (e.g., in a masked-priming experiment). Trials in which participants gave "not-seen" judgments are then treated as those where the critical stimulus was "subliminal" or "unconscious", as opposed to trials with higher visibility ratings. Sometimes, only these trials are further analyzed, for instance, for unconscious priming effects. Here I argue that this practice requires implicit assumptions about subjective measures of awareness incompatible with basic models of categorization under uncertainty (e.g., modern signal-detection and threshold theories). Most importantly, it ignores the potential effects of response bias. Instead of taking "not-seen" judgments literally, they would better be employed in parametric experiments where stimulus visibility is manipulated systematically, not accidentally. This would allow studying qualitative and double dissociations between measures of awareness and of stimulus processing per se.

q-bio.NC↗
arXiv2011

Continued fractions for a class of triangle groups

We give continued fraction algorithms for a particular class of Fuchsian triangle groups. In particular, we give an explicit form of each such group that is a subgroup of the Hilbert modular group of its trace field and provide an interval map that is piecewise linear fractional, given in terms of group elements. Using natural extensions, we find an ergodic invariant measure for the interval map. We also study diophantine properties of approximation in terms of the continued fractions; and furthermore show that these continued fractions are appropriate to obtain transcendence results.

math.NT↗
arXiv2007

Accurate determination of elastic parameters for multi-component membranes

Heterogeneities in the cell membrane due to coexisting lipid phases have been conjectured to play a major functional role in cell signaling and membrane trafficking. Thereby the material properties of multiphase systems, such as the line tension and the bending moduli, are crucially involved in the kinetics and the asymptotic behavior of phase separation. In this Letter we present a combined analytical and experimental approach to determine the properties of phase-separated vesicle systems. First we develop an analytical model for the vesicle shape of weakly budded biphasic vesicles. Subsequently experimental data on vesicle shape and membrane fluctuations are taken and compared to the model. The combined approach allows for a reproducible and reliable determination of the physical parameters of complex vesicle systems. The parameters obtained set limits for the size and stability of nanodomains in the plasma membrane of living cells.

cond-mat.soft↗
arXiv2007

Single-molecule imaging of the H-ras membrane-anchor reveals domains in the cytoplasmic leaflet of the cell membrane

In the last decade evidence has accumulated that small domains of 50-700 nm in diameter are located in the exoplasmic leaflet of the plasma membrane. Most of these domains supposedly consist of specific sets of lipids and proteins, and are believed to coordinate signal transduction cascades. Whether similar domains are also present in the cytoplasmic leaflet of the plasma membrane is unclear so far. To investigate the presence of cytoplasmic leaflet domains, the H-Ras membrane-targeting sequence was fused to the C-terminus of the enhanced yellow fluorescent protein. Using single-molecule fluorescence microscopy, trajectories of individual molecules diffusing in the cytoplasmic leaflet of the plasma membrane were recorded. From these trajectories, the diffusion of individual membrane-anchored enhanced yellow fluorescent protein molecules was studied in live cells on timescales from 5 to 200 ms. The results show that the diffusion of 30-40% of the molecules is constrained in domains with a typical size of 200 nm. Neither breakdown of actin nor cholesterol extraction changed the domain characteristics significantly, indicating that the observed domains may not be related to the membrane domains identified so far.

physics.optics↗