355 research outputs found
Reading Imagined Letter Shapes from the Mind's Eye Using Real-time 7 Tesla fMRI
We present a 7 Tesla fMRI proof-of-concept study of the first letter speller BCI that decodes imagined letter shapes from activity patterns in early visual cortical areas. New tools are developed to enable real-time population receptive field retinotopic mapping for encoding and decoding. Using two different letter shapes (H and T), classification performance of generated activity patterns during imagery reaches 80% accuracy in each individual. Using a denoising autoencoder, recognizable letter shapes could be reconstructed and displayed as feedback to participants in the scanner
Effects of Synaptic and Myelin Plasticity on Learning in a Network of Kuramoto Phase Oscillators
Models of learning typically focus on synaptic plasticity. However, learning
is the result of both synaptic and myelin plasticity. Specifically, synaptic
changes often co-occur and interact with myelin changes, leading to complex
dynamic interactions between these processes. Here, we investigate the
implications of these interactions for the coupling behavior of a system of
Kuramoto oscillators. To that end, we construct a fully connected,
one-dimensional ring network of phase oscillators whose coupling strength
(reflecting synaptic strength) as well as conduction velocity (reflecting
myelination) are each regulated by a Hebbian learning rule. We evaluate the
behavior of the system in terms of structural (pairwise connection strength and
conduction velocity) and functional connectivity (local and global
synchronization behavior). We find that for conditions in which a system
limited to synaptic plasticity develops two distinct clusters both structurally
and functionally, additional adaptive myelination allows for functional
communication across these structural clusters. Hence, dynamic conduction
velocity permits the functional integration of structurally segregated
clusters. Our results confirm that network states following learning may be
different when myelin plasticity is considered in addition to synaptic
plasticity, pointing towards the relevance of integrating both factors in
computational models of learning.Comment: 39 pages, 15 figures This work is submitted in Chaos: An
Interdisciplinary Journal of Nonlinear Scienc
An invariant distribution in static granular media
We have discovered an invariant distribution for local packing configurations
in static granular media. This distribution holds in experiments for packing
fractions covering most of the range from random loose packed to random close
packed, for beads packed both in air and in water. Assuming only that there
exist elementary cells in which the system volume is subdivided, we derive from
statistical mechanics a distribution that is in accord with the observations.
This universal distribution function for granular media is analogous to the
Maxwell-Boltzmann distribution for molecular gasses.Comment: 4 pages 3 figure
Minkowski tensors and local structure metrics: Amorphous and crystalline sphere packings
Robust and sensitive tools to characterise local structure are essential for investigations of granular or particulate matter. Often local structure metrics derived from the bond network are used for this purpose, in particular Steinhardt's bond-orientational order parameters ql . Here we discuss an alternative method, based on the robust characterisation of the shape of the particles' Voronoi cells, by Minkowski tensors and derived anisotropy measures. We have successfully applied these metrics to quantify structural changes and the onset of crystallisation in random sphere packs. Here we specifically discuss the expectation values of these metrics for simple crystalline unimodal packings of spheres, consisting of single spheres on the points of a Bravais lattice. These data provide an important reference for the discussion of anisotropy values of disordered structures that are typically of relevance in granular systems. This analysis demonstrates that, at least for sufficiently high packing fractions above φ > 0.61, crystalline sphere packs exist whose Voronoi cells are more anisotropic with respect to a volumetric moment tensor than the average value of Voronoi cell anisotropy in random sphere packs
L-arabinose co-ingestion delays glucose absorption derived from sucrose in healthy men and women : A double-blind, randomized crossover trial
Dietary interventions to delay carbohydrate digestion or absorption can effectively prevent hyperglycaemia in the early postprandial phase. L-arabinose can specifically inhibit sucrase. It remains to be assessed whether co-ingestion of L-arabinose with sucrose delays sucrose digestion, attenuates subsequent glucose absorption and impacts hepatic glucose output. In this double-blind, randomised crossover study, we assessed blood glucose kinetics following ingestion of a 200-ml drink containing 50 g of sucrose with 7·5 g of L-arabinose (L-ARA) or without L-arabinose (CONT) in twelve young, healthy participants (24 ± 1 years; BMI: 22·2 ± 0·5 kg/m2). Plasma glucose kinetics were determined by a dual stable isotope methodology involving ingestion of (U-13C6)-glucose-enriched sucrose, and continuous intravenous infusion of (6,6–2H2)-glucose. Peak glucose concentrations reached 8·18 ± 0·29 mmol/l for CONT 30 min after ingestion. In contrast, the postprandial rise in plasma glucose was attenuated for L-ARA, because peak glucose concentrations reached 6·62 ± 0·18 mmol/l only 60 min after ingestion. The rate of exogenous glucose appearance for L-ARA was 67 and 57 % lower compared with CONT at t = 15 min and 30 min, respectively, whereas it was 214 % higher at t = 150 min, indicating a more stable absorption of exogenous glucose for L-ARA compared with CONT. Total glucose disappearance during the first hour was lower for L-ARA compared with CONT (11 ± 1 v. 17 ± 1 g, P < 0·0001). Endogenous glucose production was not differentially affected at any time point (P = 0·27). Co-ingestion of L-arabinose with sucrose delays sucrose digestion, resulting in a slower absorption of sucrose-derived glucose without causing adverse effects in young, healthy adults
Minkowski tensors and local structure metrics: Amorphous and crystalline sphere packings
Robust and sensitive tools to characterise local structure are essential for investigations of granular or particulate matter. Often local structure metrics derived from the bond network are used for this purpose, in particular Steinhardt's bond-orientational order parameters ql . Here we discuss an alternative method, based on the robust characterisation of the shape of the particles' Voronoi cells, by Minkowski tensors and derived anisotropy measures. We have successfully applied these metrics to quantify structural changes and the onset of crystallisation in random sphere packs. Here we specifically discuss the expectation values of these metrics for simple crystalline unimodal packings of spheres, consisting of single spheres on the points of a Bravais lattice. These data provide an important reference for the discussion of anisotropy values of disordered structures that are typically of relevance in granular systems. This analysis demonstrates that, at least for sufficiently high packing fractions above φ > 0.61, crystalline sphere packs exist whose Voronoi cells are more anisotropic with respect to a volumetric moment tensor than the average value of Voronoi cell anisotropy in random sphere packs
Channel-Independent and Sensor-Independent Stimulus Representations
This paper shows how a machine, which observes stimuli through an
uncharacterized, uncalibrated channel and sensor, can glean machine-independent
information (i.e., channel- and sensor-independent information) about the
stimuli. First, we demonstrate that a machine defines a specific coordinate
system on the stimulus state space, with the nature of that coordinate system
depending on the device's channel and sensor. Thus, machines with different
channels and sensors "see" the same stimulus trajectory through state space,
but in different machine-specific coordinate systems. For a large variety of
physical stimuli, statistical properties of that trajectory endow the stimulus
configuration space with differential geometric structure (a metric and
parallel transfer procedure), which can then be used to represent relative
stimulus configurations in a coordinate-system-independent manner (and,
therefore, in a channel- and sensor-independent manner). The resulting
description is an "inner" property of the stimulus time series in the sense
that it does not depend on extrinsic factors like the observer's choice of a
coordinate system in which the stimulus is viewed (i.e., the observer's choice
of channel and sensor). This methodology is illustrated with analytic examples
and with a numerically simulated experiment. In an intelligent sensory device,
this kind of representation "engine" could function as a "front-end" that
passes channel/sensor-independent stimulus representations to a pattern
recognition module. After a pattern recognizer has been trained in one of these
devices, it could be used without change in other devices having different
channels and sensors.Comment: The results of a numerically simulated experiment, which illustrates
the proposed method, have been added to the version submitted on October 27,
2004. This paper has been accepted for publication in the Journal of Applied
Physics. For related papers, see http://www.geocities.com/dlevin2001
Post-exercise protein synthesis rates are only marginally higher in type I compared with type II muscle fibres following resistance-type exercise.
We examined the effect of an acute bout of resistance exercise on fractional muscle protein synthesis rates in human type I and type II muscle fibres. After a standardised breakfast (31 ± 1 kJ kg(−1) body weight, consisting of 52 Energy% (En%) carbohydrate, 34 En% protein and 14 En% fat), 9 untrained men completed a lower-limb resistance exercise bout (8 sets of 10 repetitions leg press and leg extension at 70% 1RM). A primed, continuous infusion of l-[ring-(13)C(6)]phenylalanine was combined with muscle biopsies collected from both legs immediately after exercise and after 6 h of post-exercise recovery. Single muscle fibres were dissected from freeze-dried biopsies and stained for ATPase activity with pre-incubation at a pH of 4.3. Type I and II fibres were separated under a light microscope and analysed for protein-bound l-[ring-(13)C(6)]phenylalanine labelling. Baseline (post-exercise) l-[ring-(13)C(6)]phenylalanine muscle tissue labelling, expressed as (∂(13)C/(12)C), averaged −32.09 ± 0.28, −32.53 ± 0.10 and −32.02 ± 0.16 in the type I and II muscle fibres and mixed muscle, respectively (P = 0.14). During post-exercise recovery, muscle protein synthesis rates were marginally (8 ± 2%) higher in the type I than type II muscle fibres, at 0.100 ± 0.005 versus 0.094 ± 0.005%/h, respectively (P < 0.05), whereby rates of mixed muscle protein were 0.091 ± 0.005%/h. Muscle protein synthesis rates following resistance-type exercise are only marginally higher in type I compared with type II muscle fibres
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