ϕ-Informational Measures: Some Results and Interrelations.

In this paper, we focus on extended informational measures based on a convex function ϕ: entropies, extended Fisher information, and generalized moments. Both the generalization of the Fisher information and the moments rely on the definition of an escort distribution linked to the (entropic) functional ϕ. We revisit the usual maximum entropy principle-more precisely its inverse problem, starting from the distribution and constraints, which leads to the introduction of state-dependent ϕ-entropies. Then, we examine interrelations between the extended informational measures and generalize relationships such the Cramér-Rao inequality and the de Bruijn identity in this broader context. In this particular framework, the maximum entropy distributions play a central role. Of course, all the results derived in the paper include the usual ones as special cases.

Micro-pursuit: A class of fixational eye movements correlating with smooth, predictable, small-scale target trajectories.

Humans generate ocular pursuit movements when a moving target is tracked throughout the visual field. In this article, we show that pursuit can be generated and measured at small amplitudes, at the scale of fixational eye movements, and tag these eye movements as micro-pursuits. During micro-pursuits, gaze direction correlates with a target's smooth, predictable target trajectory. We measure similarity between gaze and target trajectories using a so-called maximally projected correlation and provide results in three experimental data sets. A first observation of micro-pursuit is provided in an implicit pursuit task, where observers were tasked to maintain their gaze fixed on a static cross at the center of screen, while reporting changes in perception of an ambiguous, moving (Necker) cube. We then provide two experimental paradigms and their corresponding data sets: a first replicating micro-pursuits in an explicit pursuit task, where observers had to follow a moving fixation cross (Cross), and a second with an unambiguous square (Square). Individual and group analyses provide evidence that micro-pursuits exist in both the Necker and Cross experiments but not in the Square experiment. The interexperiment analysis results suggest that the manipulation of stimulus target motion, task, and/or the nature of the stimulus may play a role in the generation of micro-pursuits.

Online mobile C-arm calibration using inertial sensors: a preliminary study in order to achieve CBCT.

PURPOSE: Cone beam computed tomography (CBCT) became increasingly popular over the last years. It allows more accurate diagnosis and treatment planning with a lower effective radiation dose. However, volume reconstruction algorithms require a very precise knowledge of the imaging geometry. Due to mechanical instabilities, mobile C-arms are incompatible with existing tomography algorithms. Therefore, C-arm online calibration is essential in order to achieve an accurate volume reconstruction. METHODS: We present an online calibration method for mobile C-arms. It is based on tracking the detector and the X-ray source of the C-arm using three-axis gyroscopes and accelerometers. It aims to be precise and noninvasive. The performance of the calibration algorithm is evaluated in regard to the precision of the sensors and to whether or not dynamic models are considered. In addition, we present an algorithm which propagate the errors from the positions and orientations estimates to the 2D projections on the detector plane. Thus, we can evaluate the impact of the estimation errors on the acquired images. RESULTS: The experiments are conducted on an experimental C-arm. The reached accuracy is [Formula: see text] for orientation and [Formula: see text] for position. These errors propagate as an error of [Formula: see text] for the 2D projections on the detector plane. CONCLUSIONS: The proposed calibration algorithm achieves an accuracy comparable to the precision of existing calibration methods. The required angle accuracy by CBCT algorithms is reached. However, improvements are needed to achieve the required position precision. The in-plane translations of the X-ray source and the detector are the most crucial parameters to estimate in order to conduct CBCT on mobile C-arms.

Lossless quantum data compression with exponential penalization: an operational interpretation of the quantum Rényi entropy.

Based on the problem of quantum data compression in a lossless way, we present here an operational interpretation for the family of quantum Rényi entropies. In order to do this, we appeal to a very general quantum encoding scheme that satisfies a quantum version of the Kraft-McMillan inequality. Then, in the standard situation, where one is intended to minimize the usual average length of the quantum codewords, we recover the known results, namely that the von Neumann entropy of the source bounds the average length of the optimal codes. Otherwise, we show that by invoking an exponential average length, related to an exponential penalization over large codewords, the quantum Rényi entropies arise as the natural quantities relating the optimal encoding schemes with the source description, playing an analogous role to that of von Neumann entropy.

Microsaccades are modulated by both attentional demands of a visual discrimination task and background noise.

Microsaccades are miniature saccades occurring once or twice per second during visual fixation. While microsaccades and saccades share similarities at the oculomotor level, the functional roles of microsaccades are still debated. In this study, we examined the hypothesis that the microsaccadic activity is affected by the type of noisy background during the execution of a particular discrimination task. Human subjects had to judge the orientation of a tilted stimulus embedded in static or dynamic backgrounds in a forced choice-task paradigm, as adapted from Rucci, Iovin, Poletti, and Santini (2007). Static backgrounds induced more microsaccades than dynamic ones only during the execution of the discrimination task. A directional bias of microsaccades, dictated by the stimulus orientation, was temporally coupled with this period of increased activity. Both microsaccade rates and orientations were comparable across background types after the response time although subjects maintained fixation until the end of the trial. This represents a background-specific modulation of the microsaccadic activity driven by attentional demands. The visual influence of microsaccades on discrimination performances was modeled at the retinal level for both types of backgrounds. A higher simulated microsaccadic activity was necessary for static backgrounds in order to achieve discrimination performance scores comparable to that of dynamic ones. Taken together, our experimental and theoretical findings further support the idea that microsaccades are under attentional control and represent an efficient sampling strategy allowing spatial information acquisition.

A numerical scheme for modeling wavefront propagation on a monolayer of arbitrary geometry.

The majority of models of wavefront propagation in cardiac tissue have assumed relatively simple geometries. Extensions to complicated three-dimensional (3-D) representations are computationally challenging due to issues related both to problem size and to the correct implementation of flux conservation. In this paper, we present a generalized finite difference scheme (GDFS) to simulate the reaction-diffusion system on a 3-D monolayer of arbitrary shape. GDFS is a vertex-centered variant of the finite-volume method that ensures local flux conservation. Owing to an effectively lower dimensionality, the overall computation time is reduced compared to full 3-D models at the same spatial resolution. We present the theoretical background to compute both the wavefront conduction and local electrograms using a matrix formulation. The same matrix is used for both these quantities. We then give some results of simulation for simple monolayers and complex monolayers resembling a human atria.

Study of unipolar electrogram morphology in a computer model of atrial fibrillation.

INTRODUCTION: Electrograms exhibit a wide variety of morphologies during atrial fibrillation (AF). The basis of these time courses, however, is not completely understood. In this study, data from computer models were studied to relate features of the signals to the underlying dynamics and tissue substrate. METHODS AND RESULTS: A computer model of entire human atria with a gross fiber architecture based on histology and membrane kinetics based on the Courtemanche et al. atrial model was developed to simulate paced activation and simulated AF. Unipolar electrograms were computed using a current source approximation at 256 sites in right atrium, to simulate a mapping array. The results show the following: (1) In a homogeneous and isotropic tissue, the presence of highly asymmetric electrograms is rare (<2%), although there is a marked variability in amplitude and symmetry. (2) The introduction of anisotropy increases this variability in symmetry and amplitude of the, electrograms especially for propagation across fibers. The percentage of highly asymmetric electrograms increases to 12% to 15% for anisotropy ratios greater than 3:1. (3) Multiphasic and fractionated electrograms are rarely seen in the model with uniform properties but are more common (15%-17%) in a model including regions with abrupt changes in conductivity. Beat-to-beat variations in the occurrence of multiphasic signals are possible with fixed anatomic heterogeneity, due to beat-to-beat variations in the direction of the wavefront relative to the heterogeneity. CONCLUSION: Analysis of the amplitude and symmetry of unipolar atrial electrograms can provide information about the electrophysiologic substrate maintaining AF.