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2019


Semi-supervised learning, causality, and the conditional cluster assumption
Semi-supervised learning, causality, and the conditional cluster assumption

von Kügelgen, J., Mey, A., Loog, M., Schölkopf, B.

NeurIPS 2019 Workshop “Do the right thing”: machine learning and causal inference for improved decision making, December 2019 (poster)

Poster PDF link (url) [BibTex]

2019

Poster PDF link (url) [BibTex]


Optimal experimental design via Bayesian optimization: active causal structure learning for Gaussian process networks
Optimal experimental design via Bayesian optimization: active causal structure learning for Gaussian process networks

von Kügelgen, J., Rubenstein, P. K., Schölkopf, B., Weller, A.

NeurIPS 2019 Workshop “Do the right thing”: machine learning and causal inference for improved decision making, December 2019 (poster)

arXiv Poster link (url) [BibTex]

arXiv Poster link (url) [BibTex]


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Perception of temporal dependencies in autoregressive motion

Meding, K., Schölkopf, B., Wichmann, F. A.

European Conference on Visual Perception (ECVP), 2019 (poster)

[BibTex]


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Phenomenal Causality and Sensory Realism

Bruijns, S. A., Meding, K., Schölkopf, B., Wichmann, F. A.

European Conference on Visual Perception (ECVP), 2019 (poster)

[BibTex]

[BibTex]

2016


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Autofocusing-based correction of B0 fluctuation-induced ghosting

Loktyushin, A., Ehses, P., Schölkopf, B., Scheffler, K.

24th Annual Meeting and Exhibition of the International Society for Magnetic Resonance in Medicine (ISMRM), May 2016 (poster)

link (url) [BibTex]

2016

link (url) [BibTex]


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Novel Random Forest based framework enables the segmentation of cerebral ischemic regions using multiparametric MRI

Katiyar, P., Castaneda, S., Patzwaldt, K., Russo, F., Poli, S., Ziemann, U., Disselhorst, J. A., Pichler, B. J.

European Molecular Imaging Meeting, 2016 (poster)

link (url) [BibTex]

link (url) [BibTex]


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PGO wave-triggered functional MRI: mapping the networks underlying synaptic consolidation

Logothetis, N. K., Murayama, Y., Ramirez-Villegas, J. F., Besserve, M., Evrard, H.

47th Annual Meeting of the Society for Neuroscience (Neuroscience), 2016 (poster)

[BibTex]

[BibTex]


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Multiparametric Imaging of Ischemic Stroke using [89Zr]-Desferal-EPO-PET/MRI in combination with Gaussian Mixture Modeling enables unsupervised lesions identification

Castaneda, S., Katiyar, P., Russo, F., Maurer, A., Patzwaldt, K., Poli, S., Calaminus, C., Disselhorst, J. A., Ziemann, U., Pichler, B. J.

European Molecular Imaging Meeting, 2016 (poster)

link (url) [BibTex]

link (url) [BibTex]


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Statistical source separation of rhythmic LFP patterns during sharp wave ripples in the macaque hippocampus

Ramirez-Villegas, J. F., Logothetis, N. K., Besserve, M.

47th Annual Meeting of the Society for Neuroscience (Neuroscience), 2016 (poster)

[BibTex]

[BibTex]


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Screening Rules for Convex Problems

Raj, A., Olbrich, J., Gärtner, B., Schölkopf, B., Jaggi, M.

2016 (unpublished) Submitted

[BibTex]

[BibTex]


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Hippocampal neural events predict ongoing brain-wide BOLD activity

Besserve, M., Logothetis, N. K.

47th Annual Meeting of the Society for Neuroscience (Neuroscience), 2016 (poster)

[BibTex]

[BibTex]

2014


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Learning Motor Skills: From Algorithms to Robot Experiments

Kober, J., Peters, J.

97, pages: 191, Springer Tracts in Advanced Robotics, Springer, 2014 (book)

DOI [BibTex]

2014

DOI [BibTex]


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Dynamical source analysis of hippocampal sharp-wave ripple episodes

Ramirez-Villegas, J. F., Logothetis, N. K., Besserve, M.

Bernstein Conference, 2014 (poster)

DOI [BibTex]

DOI [BibTex]


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Computational Diffusion MRI and Brain Connectivity

Schultz, T., Nedjati-Gilani, G., Venkataraman, A., O’Donnell, L., Panagiotaki, E.

pages: 255, Mathematics and Visualization, Springer, 2014 (book)

Web [BibTex]

Web [BibTex]


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FID-guided retrospective motion correction based on autofocusing

Babayeva, M., Loktyushin, A., Kober, T., Granziera, C., Nickisch, H., Gruetter, R., Krueger, G.

Joint Annual Meeting ISMRM-ESMRMB, Milano, Italy, 2014 (poster)

[BibTex]

[BibTex]


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Cluster analysis of sharp-wave ripple field potential signatures in the macaque hippocampus

Ramirez-Villegas, J. F., Logothetis, N. K., Besserve, M.

Computational and Systems Neuroscience Meeting (COSYNE), 2014 (poster)

[BibTex]

[BibTex]


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oxel level [18]F-FDG PET/MRI unsupervised segmentation of the tumor microenvironment

Katiyar, P., Divine, M. R., Pichler, B. J., Disselhorst, J. A.

World Molecular Imaging Conference, 2014 (poster)

[BibTex]

[BibTex]

2011


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Optimization for Machine Learning

Sra, S., Nowozin, S., Wright, S.

pages: 494, Neural information processing series, MIT Press, Cambridge, MA, USA, December 2011 (book)

Abstract
The interplay between optimization and machine learning is one of the most important developments in modern computational science. Optimization formulations and methods are proving to be vital in designing algorithms to extract essential knowledge from huge volumes of data. Machine learning, however, is not simply a consumer of optimization technology but a rapidly evolving field that is itself generating new optimization ideas. This book captures the state of the art of the interaction between optimization and machine learning in a way that is accessible to researchers in both fields. Optimization approaches have enjoyed prominence in machine learning because of their wide applicability and attractive theoretical properties. The increasing complexity, size, and variety of today's machine learning models call for the reassessment of existing assumptions. This book starts the process of reassessment. It describes the resurgence in novel contexts of established frameworks such as first-order methods, stochastic approximations, convex relaxations, interior-point methods, and proximal methods. It also devotes attention to newer themes such as regularized optimization, robust optimization, gradient and subgradient methods, splitting techniques, and second-order methods. Many of these techniques draw inspiration from other fields, including operations research, theoretical computer science, and subfields of optimization. The book will enrich the ongoing cross-fertilization between the machine learning community and these other fields, and within the broader optimization community.

Web [BibTex]

2011

Web [BibTex]


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Spatiotemporal mapping of rhythmic activity in the inferior convexity of the macaque prefrontal cortex

Panagiotaropoulos, T., Besserve, M., Crocker, B., Kapoor, V., Tolias, A., Panzeri, S., Logothetis, N.

41(239.15), 41st Annual Meeting of the Society for Neuroscience (Neuroscience), November 2011 (poster)

Abstract
The inferior convexity of the macaque prefrontal cortex (icPFC) is known to be involved in higher order processing of sensory information mediating stimulus selection, attention and working memory. Until now, the vast majority of electrophysiological investigations of the icPFC employed single electrode recordings. As a result, relatively little is known about the spatiotemporal structure of neuronal activity in this cortical area. Here we study in detail the spatiotemporal properties of local field potentials (LFP's) in the icPFC using multi electrode recordings during anesthesia. We computed the LFP-LFP coherence as a function of frequency for thousands of pairs of simultaneously recorded sites anterior to the arcuate and inferior to the principal sulcus. We observed two distinct peaks of coherent oscillatory activity between approximately 4-10 and 15-25 Hz. We then quantified the instantaneous phase of these frequency bands using the Hilbert transform and found robust phase gradients across recording sites. The dependency of the phase on the spatial location reflects the existence of traveling waves of electrical activity in the icPFC. The dominant axis of these traveling waves roughly followed the ventral-dorsal plane. Preliminary results show that repeated visual stimulation with a 10s movie had no dramatic effect on the spatial structure of the traveling waves. Traveling waves of electrical activity in the icPFC could reflect highly organized cortical processing in this area of prefrontal cortex.

Web [BibTex]

Web [BibTex]


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Evaluation and Optimization of MR-Based Attenuation Correction Methods in Combined Brain PET/MR

Mantlik, F., Hofmann, M., Bezrukov, I., Schmidt, H., Kolb, A., Beyer, T., Reimold, M., Schölkopf, B., Pichler, B.

2011(MIC18.M-96), 2011 IEEE Nuclear Science Symposium, Medical Imaging Conference (NSS-MIC), October 2011 (poster)

Abstract
Combined PET/MR provides simultaneous molecular and functional information in an anatomical context with unique soft tissue contrast. However, PET/MR does not support direct derivation of attenuation maps of objects and tissues within the measured PET field-of-view. Valid attenuation maps are required for quantitative PET imaging, specifically for scientific brain studies. Therefore, several methods have been proposed for MR-based attenuation correction (MR-AC). Last year, we performed an evaluation of different MR-AC methods, including simple MR thresholding, atlas- and machine learning-based MR-AC. CT-based AC served as gold standard reference. RoIs from 2 anatomic brain atlases with different levels of detail were used for evaluation of correction accuracy. We now extend our evaluation of different MR-AC methods by using an enlarged dataset of 23 patients from the integrated BrainPET/MR (Siemens Healthcare). Further, we analyze options for improving the MR-AC performance in terms of speed and accuracy. Finally, we assess the impact of ignoring BrainPET positioning aids during the course of MR-AC. This extended study confirms the overall prediction accuracy evaluation results of the first evaluation in a larger patient population. Removing datasets affected by metal artifacts from the Atlas-Patch database helped to improve prediction accuracy, although the size of the database was reduced by one half. Significant improvement in prediction speed can be gained at a cost of only slightly reduced accuracy, while further optimizations are still possible.

Web [BibTex]

Web [BibTex]


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Atlas- and Pattern Recognition Based Attenuation Correction on Simultaneous Whole-Body PET/MR

Bezrukov, I., Schmidt, H., Mantlik, F., Schwenzer, N., Hofmann, M., Schölkopf, B., Pichler, B.

2011(MIC18.M-116), 2011 IEEE Nuclear Science Symposium, Medical Imaging Conference (NSS-MIC), October 2011 (poster)

Abstract
With the recent availability of clinical whole-body PET/MRI it is possible to evaluate and further develop MR-based attenuation correction methods using simultaneously acquired PET/MR data. We present first results for MRAC on patient data acquired on a fully integrated whole-body PET/MRI (Biograph mMR, Siemens) using our method that applies atlas registration and pattern recognition (ATPR) and compare them to the segmentation-based (SEG) method provided by the manufacturer. The ATPR method makes use of a database of previously aligned pairs of MR-CT volumes to predict attenuation values on a continuous scale. The robustness of the method in presence of MR artifacts was improved by location and size based detection. Lesion to liver and lesion to blood ratios (LLR and LBR) were compared for both methods on 29 iso-contour ROIs in 4 patients. ATPR showed >20% higher LBR and LLR for ROIs in and >7% near osseous tissue. For ROIs in soft tissue, both methods yielded similar ratios with max. differences <6% . For ROIs located within metal artifacts in the MR image, ATPR showed >190% higher LLR and LBR than SEG, where ratios <0.1 occured. For lesions in the neighborhood of artifacts, both ratios were >15% higher for ATPR. If artifacts in MR volumes caused by metal implants are not accounted for in the computation of attenuation maps, they can lead to a strong decrease of lesion to background ratios, even to disappearance of hot spots. Metal implants are likely to occur in the patient collective receiving combined PET/MR scans, of our first 10 patients, 3 had metal implants. Our method is currently able to account for artifacts in the pelvis caused by prostheses. The ability of the ATPR method to account for bone leads to a significant increase of LLR and LBR in osseous tissue, which supports our previous evaluations with combined PET/CT and PET/MR data. For lesions within soft tissue, lesion to background ratios of ATPR and SEG were comparable.

Web [BibTex]

Web [BibTex]


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Retrospective blind motion correction of MR images

Loktyushin, A., Nickisch, H., Pohmann, R.

Magnetic Resonance Materials in Physics, Biology and Medicine, 24(Supplement 1):498, 28th Annual Scientific Meeting ESMRMB, October 2011 (poster)

Abstract
We present a retrospective method, which significantly reduces ghosting and blurring artifacts due to subject motion. No modifications to the sequence (as in [2, 3]), or the use of additional equipment (as in [1]) are required. Our method iteratively searches for the transformation, that applied to the lines in k-space -- yields the sparsest Laplacian filter output in the spatial domain.

PDF Web DOI [BibTex]

PDF Web DOI [BibTex]


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Model based reconstruction for GRE EPI

Blecher, W., Pohmann, R., Schölkopf, B., Seeger, M.

Magnetic Resonance Materials in Physics, Biology and Medicine, 24(Supplement 1):493-494, 28th Annual Scientific Meeting ESMRMB, October 2011 (poster)

Abstract
Model based nonlinear image reconstruction methods for MRI [3] are at the heart of modern reconstruction techniques (e.g.compressed sensing [6]). In general, models are expressed as a matrix equation where y and u are column vectors of k-space and image data, X model matrix and e independent noise. However, solving the corresponding linear system is not tractable. Therefore fast nonlinear algorithms that minimize a function wrt.the unknown image are the method of choice: In this work a model for gradient echo EPI, is proposed that incorporates N/2 Ghost correction and correction for field inhomogeneities. In addition to reconstruction from full data, the model allows for sparse reconstruction, joint estimation of image, field-, and relaxation-map (like [5,8] for spiral imaging), and improved N/2 ghost correction.

PDF Web DOI [BibTex]

PDF Web DOI [BibTex]


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Simultaneous multimodal imaging of patients with bronchial carcinoma in a whole body MR/PET system

Brendle, C., Sauter, A., Schmidt, H., Schraml, C., Bezrukov, I., Martirosian, P., Hetzel, J., Müller, M., Claussen, C., Schwenzer, N., Pfannenberg, C.

Magnetic Resonance Materials in Physics, Biology and Medicine, 24(Supplement 1):141, 28th annual scientific meeting of the European Society for Magnetic Resonance in Medicine and Biology (ESMRB), October 2011 (poster)

Abstract
Purpose/Introduction: Lung cancer is among the most frequent cancers (1). Exact determination of tumour extent and viability is crucial for adequate therapy guidance. [18F]-FDG-PET allows accurate staging and the evaluation of therapy response based on glucose metabolism. Diffusion weighted MRI (DWI) is another promising tool for the evaluation of tumour viability (2,3). The aim of the study was the simultaneous PET-MR acquisition in lung cancer patients and correlation of PET and MR data. Subjects and Methods: Seven patients (age 38-73 years, mean 61 years) with highly suspected or known bronchial carcinoma were examined. First, a [18F]-FDG-PET/CT was performed (injected dose: 332-380 MBq). Subsequently, patients were examined at the whole-body MR/PET (Siemens Biograph mMR). The MRI is a modified 3T Verio whole body system with a magnet bore of 60 cm (max. amplitude gradients 45 mT/m, max. slew rate 200 T/m/s). Concerning the PET, the whole-body MR/PET system comprises 56 detector cassettes with a 59.4 cm transaxial and 25.8 cm axial FoV. The following parameters for PET acquisition were applied: 2 bed positions, 6 min/bed with an average uptake time of 124 min after injection (range: 110-143 min). The attenuation correction of PET data was conducted with a segmentation-based method provided by the manufacturer. Acquired PET data were reconstructed with an iterative 3D OSEM algorithm using 3 iterations and 21 subsets, Gaussian filter of 3 mm. DWI MR images were recorded simultaneously for each bed using two b-values (0/800 s/mm2). SUVmax and ADCmin were assessed in a ROI analysis. The following ratios were calculated: SUVmax(tumor)/SUVmean(liver) and ADCmin(tumor)/ADCmean(muscle). Correlation between SUV and ADC was analyzed (Pearson’s correlation). Results: Diagnostic scans could be obtained in all patients with good tumour delineation. The spatial matching of PET and DWI data was very exact. Most tumours showed a pronounced FDG-uptake in combination with decreased ADC values. Significant correlation was found between SUV and ADC ratios (r = -0.87, p = 0.0118). Discussion/Conclusion: Simultaneous MR/PET imaging of lung cancer is feasible. The whole-body MR/PET system can provide complementary information regarding tumour viability and cellularity which could facilitate a more profound tumour characterization. Further studies have to be done to evaluate the importance of these parameters for therapy decisions and monitoring

Web DOI [BibTex]

Web DOI [BibTex]


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Bayesian Time Series Models

Barber, D., Cemgil, A., Chiappa, S.

pages: 432, Cambridge University Press, Cambridge, UK, August 2011 (book)

[BibTex]

[BibTex]


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Support Vector Machines for finding deletions and short insertions using paired-end short reads

Grimm, D., Hagmann, J., König, D., Weigel, D., Borgwardt, KM.

International Conference on Intelligent Systems for Molecular Biology (ISMB), 2011 (poster)

Web [BibTex]

Web [BibTex]


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Handbook of Statistical Bioinformatics

Lu, H., Schölkopf, B., Zhao, H.

pages: 627, Springer Handbooks of Computational Statistics, Springer, Berlin, Germany, 2011 (book)

Web DOI [BibTex]

Web DOI [BibTex]


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Statistical estimation for optimization problems on graphs

Langovoy, M., Sra, S.

Empirical Inference Symposium, 2011 (poster)

[BibTex]


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Transfer Learning with Copulas

Lopez-Paz, D., Hernandez-Lobato, J.

Neural Information Processing Systems (NIPS), 2011 (poster)

PDF [BibTex]

PDF [BibTex]

2001


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Perception of Planar Shapes in Depth

Wichmann, F., Willems, B., Rosas, P., Wagemans, J.

Journal of Vision, 1(3):176, First Annual Meeting of the Vision Sciences Society (VSS), December 2001 (poster)

Abstract
We investigated the influence of the perceived 3D-orientation of planar elliptical shapes on the perception of the shapes themselves. Ellipses were projected onto the surface of a sphere and subjects were asked to indicate if the projected shapes looked as if they were a circle on the surface of the sphere. The image of the sphere was obtained from a real, (near) perfect sphere using a highly accurate digital camera (real sphere diameter 40 cm; camera-to-sphere distance 320 cm; for details see Willems et al., Perception 29, S96, 2000; Photometrics SenSys 400 digital camera with Rodenstock lens, 12-bit linear luminance resolution). Stimuli were presented monocularly on a carefully linearized Sony GDM-F500 monitor keeping the scene geometry as in the real case (sphere diameter on screen 8.2 cm; viewing distance 66 cm). Experiments were run in a darkened room using a viewing tube to minimize, as far as possible, extraneous monocular cues to depth. Three different methods were used to obtain subjects' estimates of 3D-shape: the method of adjustment, temporal 2-alternative forced choice (2AFC) and yes/no. Several results are noteworthy. First, mismatch between perceived and objective slant tended to decrease with increasing objective slant. Second, the variability of the settings, too, decreased with increasing objective slant. Finally, we comment on the results obtained using different psychophysical methods and compare our results to those obtained using a real sphere and binocular vision (Willems et al.).

Web DOI [BibTex]

2001

Web DOI [BibTex]


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Plaid maskers revisited: asymmetric plaids

Wichmann, F.

pages: 57, 4. T{\"u}binger Wahrnehmungskonferenz (TWK), March 2001 (poster)

Abstract
A large number of psychophysical and physiological experiments suggest that luminance patterns are independently analysed in channels responding to different bands of spatial frequency. There are, however, interactions among stimuli falling well outside the usual estimates of channels' bandwidths. Derrington & Henning (1989) first reported that, in 2-AFC sinusoidal-grating detection, plaid maskers, whose components are oriented symmetrically about the signal orientation, cause a substantially larger threshold elevation than would be predicted from their sinusoidal constituents alone. Wichmann & Tollin (1997a,b) and Wichmann & Henning (1998) confirmed and extended the original findings, measuring masking as a function of presentation time and plaid mask contrast. Here I investigate masking using plaid patterns whose components are asymmetrically positioned about the signal orientation. Standard temporal 2-AFC pattern discrimination experiments were conducted using plaid patterns and oblique sinusoidal gratings as maskers, and horizontally orientated sinusoidal gratings as signals. Signal and maskers were always interleaved on the display (refresh rate 152 Hz). As in the case of the symmetrical plaid maskers, substantial masking was observed for many of the asymmetrical plaids. Masking is neither a straightforward function of the plaid's constituent sinusoidal components nor of the periodicity of the luminance beats between components. These results cause problems for the notion that, even for simple stimuli, detection and discrimination are based on the outputs of channels tuned to limited ranges of spatial frequency and orientation, even if a limited set of nonlinear interactions between these channels is allowed.

Web [BibTex]

Web [BibTex]


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The pedestal effect with a pulse train and its constituent sinusoids

Henning, G., Wichmann, F., Bird, C.

Twenty-Sixth Annual Interdisciplinary Conference, 2001 (poster)

Abstract
Curves showing "threshold" contrast for detecting a signal grating as a function of the contrast of a masking grating of the same orientation, spatial frequency, and phase show a characteristic improvement in performance at masker contrasts near the contrast threshold of the unmasked signal. Depending on the percentage of correct responses used to define the threshold, the best performance can be as much as a factor of three better than the unmasked threshold obtained in the absence of any masking grating. The result is called the pedestal effect (sometimes, the dipper function). We used a 2AFC procedure to measure the effect with harmonically related sinusoids ranging from 2 to 16 c/deg - all with maskers of the same orientation, spatial frequency and phase - and with masker contrasts ranging from 0 to 50%. The curves for different spatial frequencies are identical if both the vertical axis (showing the threshold signal contrast) and the horizontal axis (showing the masker contrast) are scaled by the threshold contrast of the signal obtained with no masker. Further, a pulse train with a fundamental frequency of 2 c/deg produces a curve that is indistinguishable from that of a 2-c/deg sinusoid despite the fact that at higher masker contrasts, the pulse train contains at least 8 components all of them equally detectable. The effect of adding 1-D spatial noise is also discussed.

[BibTex]

[BibTex]


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Modeling the Dynamics of Individual Neurons of the Stomatogastric Networks with Support Vector Machines

Frontzek, T., Gutzen, C., Lal, TN., Heinzel, H-G., Eckmiller, R., Böhm, H.

Abstract Proceedings of the 6th International Congress of Neuroethology (ICN'2001) Bonn, abstract 404, 2001 (poster)

Abstract
In small rhythmic active networks timing of individual neurons is crucial for generating different spatial-temporal motor patterns. Switching of one neuron between different rhythms can cause transition between behavioral modes. In order to understand the dynamics of rhythmically active neurons we analyzed the oscillatory membranpotential of a pacemaker neuron and used different neural network models to predict dynamics of its time series. In a first step we have trained conventional RBF networks and Support Vector Machines (SVMs) using gaussian kernels with intracellulary recordings of the pyloric dilatator neuron in the Australian crayfish, Cherax destructor albidus. As a rule SVMs were able to learn the nonlinear dynamics of pyloric neurons faster (e.g. 15s) than RBF networks (e.g. 309s) under the same hardware conditions. After training SVMs performed a better iterated one-step-ahead prediction of time series in the pyloric dilatator neuron with regard to test error and error sum. The test error decreased with increasing number of support vectors. The best SVM used 196 support vectors and produced a test error of 0.04622 as opposed to the best RBF with 0.07295 using 26 RBF-neurons. In pacemaker neuron PD the timepoint at which the membranpotential will cross threshold for generation of its oscillatory peak is most important for determination of the test error. Interestingly SVMs are especially better in predicting this important part of the membranpotential which is superimposed by various synaptic inputs, which drive the membranpotential to its threshold.

[BibTex]

[BibTex]

2000


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Advances in Large Margin Classifiers

Smola, A., Bartlett, P., Schölkopf, B., Schuurmans, D.

pages: 422, Neural Information Processing, MIT Press, Cambridge, MA, USA, October 2000 (book)

Abstract
The concept of large margins is a unifying principle for the analysis of many different approaches to the classification of data from examples, including boosting, mathematical programming, neural networks, and support vector machines. The fact that it is the margin, or confidence level, of a classification--that is, a scale parameter--rather than a raw training error that matters has become a key tool for dealing with classifiers. This book shows how this idea applies to both the theoretical analysis and the design of algorithms. The book provides an overview of recent developments in large margin classifiers, examines connections with other methods (e.g., Bayesian inference), and identifies strengths and weaknesses of the method, as well as directions for future research. Among the contributors are Manfred Opper, Vladimir Vapnik, and Grace Wahba.

Web [BibTex]

2000

Web [BibTex]


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Contrast discrimination using periodic pulse trains

Wichmann, F., Henning, G.

pages: 74, 3. T{\"u}binger Wahrnehmungskonferenz (TWK), February 2000 (poster)

Abstract
Understanding contrast transduction is essential for understanding spatial vision. Previous research (Wichmann et al. 1998; Wichmann, 1999; Henning and Wichmann, 1999) has demonstrated the importance of high contrasts to distinguish between alternative models of contrast discrimination. However, the modulation transfer function of the eye imposes large contrast losses on stimuli, particularly for stimuli of high spatial frequency, making high retinal contrasts difficult to obtain using sinusoidal gratings. Standard 2AFC contrast discrimination experiments were conducted using periodic pulse trains as stimuli. Given our Mitsubishi display we achieve stimuli with up to 160% contrast at the fundamental frequency. The shape of the threshold versus (pedestal) contrast (TvC) curve using pulse trains shows the characteristic dipper shape, i.e. contrast discrimination is sometimes “easier” than detection. The rising part of the TvC function has the same slope as that measured for contrast discrimination using sinusoidal gratings of the same frequency as the fundamental. Periodic pulse trains offer the possibility to explore the visual system’s properties using high retinal contrasts. Thus they might prove useful in tasks other than contrast discrimination. Second, at least for high spatial frequencies (8 c/deg) it appears that contrast discrimination using sinusoids and periodic pulse trains results in virtually identical TvC functions, indicating a lack of probability summation. Further implications of these results are discussed.

Web [BibTex]

Web [BibTex]


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Subliminale Darbietung verkehrsrelevanter Information in Kraftfahrzeugen

Staedtgen, M., Hahn, S., Franz, MO., Spitzer, M.

pages: 98, (Editors: H.H. Bülthoff, K.R. Gegenfurtner, H.A. Mallot), 3. T{\"u}binger Wahrnehmungskonferenz (TWK), February 2000 (poster)

Abstract
Durch moderne Bildverarbeitungstechnologien ist es m{\"o}glich, in Kraftfahrzeugen bestimmte kritische Verkehrssituationen automatisch zu erkennen und den Fahrer zu warnen bzw. zu informieren. Ein Problem ist dabei die Darbietung der Ergebnisse, die den Fahrer m{\"o}glichst wenig belasten und seine Aufmerksamkeit nicht durch zus{\"a}tzliche Warnleuchten oder akustische Signale vom Verkehrsgeschehen ablenken soll. In einer Reihe von Experimenten wurde deshalb untersucht, ob subliminal dargebotene, das heißt nicht bewußt wahrgenommene, verkehrsrelevante Informationen verhaltenswirksam werden und zur Informations{\"u}bermittlung an den Fahrer genutzt werden k{\"o}nnen. In einem Experiment zur semantischen Bahnung konnte mit Hilfe einer lexikalischen Entscheidungsaufgabe gezeigt werden, daß auf den Straßenverkehr bezogene Worte schneller verarbeitet werden, wenn vorher ein damit in Zusammenhang stehendes Bild eines Verkehrsschildes subliminal pr{\"a}sentiert wurde. Auch bei parafovealer Darbietung der subliminalen Stimuli wurde eine Beschleunigung erzielt. In einer visuellen Suchaufgabe wurden in Bildern realer Verkehrssituationen Verkehrszeichen schneller entdeckt, wenn das Bild des Verkehrszeichens vorher subliminal dargeboten wurde. In beiden Experimenten betrug die Pr{\"a}sentationszeit f{\"u}r die Hinweisreize 17 ms, zus{\"a}tzlich wurde durch Vorw{\"a}rts- und R{\"u}ckw{\"a}rtsmaskierung die bewußteWahrnehmung verhindert. Diese Laboruntersuchungen zeigten, daß sich auch im Kontext des Straßenverkehrs Beschleunigungen der Informationsverarbeitung durch subliminal dargebotene Stimuli erreichen lassen. In einem dritten Experiment wurde die Darbietung eines subliminalen Hinweisreizes auf die Reaktionszeit beim Bremsen in einem realen Fahrversuch untersucht. Die Versuchspersonen (n=17) sollten so schnell wie m{\"o}glich bremsen, wenn die Bremsleuchten eines im Abstand von 12-15 m voran fahrenden Fahrzeuges aufleuchteten. In 50 von insgesamt 100 Durchg{\"a}ngen wurde ein subliminaler Stimulus (zwei rote Punkte mit einem Zentimeter Durchmesser und zehn Zentimeter Abstand) 150 ms vor Aufleuchten der Bremslichter pr{\"a}sentiert. Die Darbietung erfolgte durch ein im Auto an Stelle des Tachometers integriertes TFT-LCD Display. Im Vergleich zur Reaktion ohne subliminalen Stimulus verk{\"u}rzte sich die Reaktionszeit dadurch signifikant um 51 ms. In den beschriebenen Experimenten konnte gezeigt werden, daß die subliminale Darbietung verkehrsrelevanter Information auch in Kraftfahrzeugen verhaltenswirksam werden kann. In Zukunft k{\"o}nnte durch die Kombination der online-Bildverarbeitung im Kraftfahrzeug mit subliminaler Darbietung der Ergebnisse eine Erh{\"o}hung der Verkehrssicherheit und des Komforts erreicht werden.

Web [BibTex]

Web [BibTex]

1998


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Masking by plaid patterns: effects of presentation time and mask contrast

Wichmann, F., Henning, G.

pages: 115, 1. T{\"u}binger Wahrnehmungskonferenz (TWK 98), February 1998 (poster)

Abstract
Most current models of early spatial vision comprise of sets of orientation- and spatial-frequency selective filters with our without limited non-linear interactions amongst different subsets of the filters. The performance of human observers and of such models for human spatial vision were compared in experiments using maskers with two spatial frequencies (plaid masks). The detectability of horizontally orientated sinusoidal signals at 3.02 c/deg was measured in standard 2AFC-tasks in the presence of plaid patterns with two-components at the same spatial frequency as the signal but at different orientations (+/- 15, 30, 45, and 75 deg from the signal) and with varying contrasts (1.0, 6.25 and 25.0% contrast). In addition, the temporal envelope of the stimulus presentation was either a rectangular pulse of 19.7 msec duration, or a temporal Hanning window of 1497 msec.Threshold elevation varied with plaid component orientation, peaked +/- 30 deg from the signal where nearly a log unit threshold elevation for the 25.0% contrast plaid was observed. For plaids with 1.0% contrast we observed significant facilitation even with plaids whose components were 75 deg from that of the signal. Elevation factors were somewhat lower for the short stimulus presentation time but were still significant (up to a factor of 5 or 6). Despite of the simple nature of the stimuli employed in this study-sinusoidal signal and plaid masks comprised of only two sinusoids-none of the current models of early spatial vision can fully account for all the data gathered.

Web [BibTex]

1998

Web [BibTex]


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A bootstrap method for testing hypotheses concerning psychometric functions

Hill, N., Wichmann, F.

1998 (poster)

Abstract
Whenever psychometric functions are used to evaluate human performance on some task, it is valuable to examine not only the threshold and slope values estimated from the original data, but also the expected variability in those measures. This allows psychometric functions obtained in two experimental conditions to be compared statistically. We present a method for estimating the variability of thresholds and slopes of psychometric functions. This involves a maximum-likelihood fit to the data using a three-parameter mathematical function, followed by Monte Carlo simulation using the first fit as a generating function for the simulations. The variability of the function's parameters can then be estimated (as shown by Maloney, 1990), as can the variability of the threshold value (Foster & Bischof, 1997). We will show how a simple development of this procedure can be used to test the significance of differences between (a) the thresholds, and (b) the slopes of two psychometric functions. Further, our method can be used to assess the assumptions underlying the original fit, by examining how goodness-of-fit differs in simulation from its original value. In this way data sets can be identified as being either too noisy to be generated by a binomial observer, or significantly "too good to be true." All software is written in MATLAB and is therefore compatible across platforms, with the option of accelerating performance using MATLAB's plug-in binaries, or "MEX" files.

[BibTex]


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Nonlinearities and the pedestal effect

Wichmann, F., Henning, G., Ploghaus, A.

Perception, 27, pages: S86, 1998 (poster)

Abstract
Psychophysical and physiological evidence suggests that luminance patterns are independently analysed in "channels" responding to different bands of spatial frequency. There are, however, interactions among stimuli falling well outside the usual estimates of channels' bandwidths (Henning, Hertz, and Broadbent, (1975). Vision Res., 15, 887-899). We examined whether the masking results of Henning et al. are consistent with independent channels. We postulated, before the channels, a point non-linearity which would introduce distortion products that might produce the observed interactions between stimuli two octaves apart in spatial frequency. Standard 2-AFC masking experiments determined whether possible distortion products of a 4.185 c/deg masking sinusoid revealed their presence through effects on the detection of a sinusoidal signal at the frequency of the second harmonic of the masker-8.37 c/deg. The signal and masker were horizontally orientated and the signal was in-phase, out-of-phase, or in quadrature with the putative second-order distortion product of the masker. Significant interactions between signal and masker were observed: for a wide range of masker contrasts, signal detection was facilitated by the masking stimulus. However, the shapes of the functions relating detection performance to masker contrast, as well as the effects of relative phase, were inconsistent with the notion that distortion products were responsible for the interactions observed.

[BibTex]

[BibTex]

1995


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Image segmentation from motion: just the loss of high-spatial-frequency content ?

Wichmann, F., Henning, G.

Perception, 24, pages: S19, 1995 (poster)

Abstract
The human contrast sensitivity function (CSF) is bandpass for stimuli of low temporal frequency but, for moving stimuli, results in a low-pass CSF with large high spatial-frequency losses. Thus the high spatial-frequency content of images moving on the retina cannot be seen; motion perception could be facilitated by, or even be based on, the selective loss of high spatial-frequency content. 2-AFC image segmentation experiments were conducted with segmentation based on motion or on form. In the latter condition, the form difference mirrored that produced by moving stimuli. This was accomplished by generating stimulus elements which were spectrally either broadband or low-pass. For the motion used, the spectral difference between static broadband and static low-pass elements matched the spectral difference between moving and static broadband elements. On the hypothesis that segmentation from motion is based on the detection of regions devoid of high spatial-frequencies, both tasks should be similarly difficult for human observers. However, neither image segmentation (nor, incidentally, motion detection) was sensitive to the high spatial-frequency content of the stimuli. Thus changes in perceptual form produced by moving stimuli appear not to be used as a cue for image segmentation.

[BibTex]