3,227 research outputs found
Median topographic maps for biomedical data sets
Median clustering extends popular neural data analysis methods such as the
self-organizing map or neural gas to general data structures given by a
dissimilarity matrix only. This offers flexible and robust global data
inspection methods which are particularly suited for a variety of data as
occurs in biomedical domains. In this chapter, we give an overview about median
clustering and its properties and extensions, with a particular focus on
efficient implementations adapted to large scale data analysis
Nanomechanical and topographical imaging of living cells by Atomic Force Microscopy with colloidal probes
Atomic Force Microscopy (AFM) has a great potential as a tool to characterize
mechanical and morphological properties of living cells; these properties have
been shown to correlate with cells' fate and patho-physiological state in view
of the development of novel early-diagnostic strategies. Although several
reports have described experimental and technical approaches for the
characterization of cell elasticity by means of AFM, a robust and commonly
accepted methodology is still lacking. Here we show that micrometric spherical
probes (also known as colloidal probes) are well suited for performing a
combined topographic and mechanical analysis of living cells, with spatial
resolution suitable for a complete and accurate mapping of cell morphological
and elastic properties, and superior reliability and accuracy in the mechanical
measurements with respect to conventional and widely used sharp AFM tips. We
address a number of issues concerning the nanomechanical analysis, including
the applicability of contact mechanical models and the impact of a constrained
contact geometry on the measured elastic modulus (the finite-thickness effect).
We have tested our protocol by imaging living PC12 and MDA-MB-231 cells, in
order to demonstrate the importance of the correction of the finite-thickness
effect and the change in cell elasticity induced by the action of a
cytoskeleton-targeting drug.Comment: 51 pages, 12 figures, 3 table
Mapping Topographic Structure in White Matter Pathways with Level Set Trees
Fiber tractography on diffusion imaging data offers rich potential for
describing white matter pathways in the human brain, but characterizing the
spatial organization in these large and complex data sets remains a challenge.
We show that level set trees---which provide a concise representation of the
hierarchical mode structure of probability density functions---offer a
statistically-principled framework for visualizing and analyzing topography in
fiber streamlines. Using diffusion spectrum imaging data collected on
neurologically healthy controls (N=30), we mapped white matter pathways from
the cortex into the striatum using a deterministic tractography algorithm that
estimates fiber bundles as dimensionless streamlines. Level set trees were used
for interactive exploration of patterns in the endpoint distributions of the
mapped fiber tracks and an efficient segmentation of the tracks that has
empirical accuracy comparable to standard nonparametric clustering methods. We
show that level set trees can also be generalized to model pseudo-density
functions in order to analyze a broader array of data types, including entire
fiber streamlines. Finally, resampling methods show the reliability of the
level set tree as a descriptive measure of topographic structure, illustrating
its potential as a statistical descriptor in brain imaging analysis. These
results highlight the broad applicability of level set trees for visualizing
and analyzing high-dimensional data like fiber tractography output
Spatial land-use inventory, modeling, and projection/Denver metropolitan area, with inputs from existing maps, airphotos, and LANDSAT imagery
A landscape model was constructed with 34 land-use, physiographic, socioeconomic, and transportation maps. A simple Markov land-use trend model was constructed from observed rates of change and nonchange from photointerpreted 1963 and 1970 airphotos. Seven multivariate land-use projection models predicting 1970 spatial land-use changes achieved accuracies from 42 to 57 percent. A final modeling strategy was designed, which combines both Markov trend and multivariate spatial projection processes. Landsat-1 image preprocessing included geometric rectification/resampling, spectral-band, and band/insolation ratioing operations. A new, systematic grid-sampled point training-set approach proved to be useful when tested on the four orginal MSS bands, ten image bands and ratios, and all 48 image and map variables (less land use). Ten variable accuracy was raised over 15 percentage points from 38.4 to 53.9 percent, with the use of the 31 ancillary variables. A land-use classification map was produced with an optimal ten-channel subset of four image bands and six ancillary map variables. Point-by-point verification of 331,776 points against a 1972/1973 U.S. Geological Survey (UGSG) land-use map prepared with airphotos and the same classification scheme showed average first-, second-, and third-order accuracies of 76.3, 58.4, and 33.0 percent, respectively
EEG During Pedaling: Evidence for Cortical Control of Locomotor Tasks
Objective: This study characterized the brain electrical activity during pedaling, a locomotor-like task, in humans. We postulated that phasic brain activity would be associated with active pedaling, consistent with a cortical role in locomotor tasks. Methods: Sixty four channels of electroencephalogram (EEG) and 10 channels of electromyogram (EMG) data were recorded from 10 neurologically-intact volunteers while they performed active and passive (no effort) pedaling on a custom-designed stationary bicycle. Ensemble averaged waveforms, 2 dimensional topographic maps and amplitude of the β (13–35 Hz) frequency band were analyzed and compared between active and passive trials. Results: The peak-to-peak amplitude (peak positive–peak negative) of the EEG waveform recorded at the Cz electrode was higher in the passive than the active trials (p \u3c 0.01). β-band oscillations in electrodes overlying the leg representation area of the cortex were significantly desynchronized during active compared to the passive pedaling (p \u3c 0.01). A significant negative correlation was observed between the average EEG waveform for active trials and the composite EMG (summated EMG from both limbs for each muscle) of the rectus femoris (r = −0.77, p \u3c 0.01) the medial hamstrings (r = −0.85, p \u3c 0.01) and the tibialis anterior (r = −0.70, p \u3c 0.01) muscles. Conclusions: These results demonstrated that substantial sensorimotor processing occurs in the brain during pedaling in humans. Further, cortical activity seemed to be greatest during recruitment of the muscles critical for transitioning the legs from flexion to extension and vice versa. Significance: This is the first study demonstrating the feasibility of EEG recording during pedaling, and owing to similarities between pedaling and bipedal walking, may provide valuable insight into brain activity during locomotion in humans
Detecting event-related recurrences by symbolic analysis: Applications to human language processing
Quasistationarity is ubiquitous in complex dynamical systems. In brain
dynamics there is ample evidence that event-related potentials reflect such
quasistationary states. In order to detect them from time series, several
segmentation techniques have been proposed. In this study we elaborate a recent
approach for detecting quasistationary states as recurrence domains by means of
recurrence analysis and subsequent symbolisation methods. As a result,
recurrence domains are obtained as partition cells that can be further aligned
and unified for different realisations. We address two pertinent problems of
contemporary recurrence analysis and present possible solutions for them.Comment: 24 pages, 6 figures. Draft version to appear in Proc Royal Soc
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Detecting abnormality in optic nerve head images using a feature extraction analysis
Imaging and evaluation of the optic nerve head (ONH) plays an essential part in the detection and clinical management of glaucoma. The morphological characteristics of ONHs vary greatly from person to person and this variability means it is difficult to quantify them in a standardized way. We developed and evaluated a feature extraction approach using shiftinvariant wavelet packet and kernel principal component analysis to quantify the shape features in ONH images acquired by scanning laser ophthalmoscopy (Heidelberg Retina Tomograph [HRT]). The methods were developed and tested on 1996 eyes from three different clinical centers. A shape abnormality score (SAS) was developed from extracted features using a Gaussian process to identify glaucomatous abnormality. SAS can be used as a diagnostic index to quantify the overall likelihood of ONH abnormality. Maps showing areas of likely abnormality within the ONH were also derived. Diagnostic performance of the technique, as estimated by ROC analysis, was significantly better than the classification tools currently used in the HRT software – the technique offers the additional advantage of working with all images and is fully automated
Morphology, topography, and optics of the orthokeratology cornea
The goal of this work was to objectively characterize the external morphology, topography, and optics
of the cornea after orthokeratology (ortho-k). A number of 24 patients between the ages of 17 and 30 years
(median ¼ 24 years) were fitted with Corneal Refractive Therapy® contact lenses to correct myopia between
−2.00 and −5.00 diopters (D) (median ¼ −3.41 D). A classification algorithm was applied to conduct an automatic
segmentation based on the mean local curvature. As a result, three zones (optical zone, transition zone,
and peripheral zone) were delimited. Topographical analysis was provided through global and zonal fit to a
general ellipsoid. Ray trace on partially customized eye models provided wave aberrations and retinal
image quality. Monozone topographic description of the ortho-k cornea loses accuracy when compared with
zonal description. Primary (C0
4) and secondary (C0
6) spherical aberration (SA) coefficients for a 5-mm pupil
increased 3.68 and 19 times, respectively, after the treatments. The OZ area showed a strong correlation
with C0
4 (r ¼ −0.49, p < 0.05) and a very strong correlation with C0
6 (r ¼ 0.78, p < 0.01). The OZ, as well as
the TZ, areas did not correlate with baseline refraction. The increase in the eye’s positive SA after ortho-k
is the major factor responsible for the decreased retinal optical quality of the unaccommodated eyeThis work was funded in part by European Fund for Regional
Development (FEDER) through the COMPETE Program and
by the Portuguese Foundation for Science and Technology
(FCT). FCT provided financial support in the framework of
projects PTDC/SAU-BEB/098391/2008, PTDC/SAU-BEB/
098392/2008 and the Strategic Project PEST-C/FIS/UI607/
2011. The authors have no proprietary interest in the methods
and devices described in this manuscript
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