W. Tedeschi

Shannon entropy applied to the analysis of event-related fMRI time series.

Event-related functional magnetic resonance imaging (ER-fMRI) refers to the blood oxygen level-dependent (BOLD) signal in response to a short stimulus followed by a long period of rest. These paradigms have become more popular in the last few years due to some advantages over standard block techniques. Most of the analysis of the time series generated in such exams is based on a model of specific hemodynamic response function. In this paper we propose a new method for the analysis of ER-fMRI based in a specific aspect of information theory: the entropy of a signal using the Shannon formulation, which makes no assumption about the shape of the response. The results show the ability to discriminate between activated and resting cerebral regions for motor and visual stimuli. Moreover, the results of simulated data show a more stable pattern of the method, if compared to typical algorithms, when the signal to noise ratio decreases.

Non-extensive entropy and the extraction of BOLD spatial information in event-related functional MRI.

Functional magnetic resonance imaging (fMRI) data analysis has been carried out recently in the framework of information theory, by means of the Shannon entropy. As a natural extension, a method based on the generalized Tsallis entropy was developed to the analysis event-related (ER-fMRI), where a brief stimulus is presented, followed by a long period of rest. The new technique aims for spatial localization neuronal activity due to a specific task. This method does not require a priori hypothesis of the hemodynamic response function (HRF) shape and the linear relation between BOLD responses with the presented task. Numerical simulations were performed so as to determine the optimal values of the Tsallis q parameter and the number of levels, L. In order to avoid undesirable divergences of the Tsallis entropy, only positive q values were studied. Results from simulated data (with L = 3) indicated that, for q = 0.8, the active brain areas are detected with the highest performance. Moreover, the method was tested for an in vivo experiment and demonstrated the ability to discriminate active brain regions that selectively responded to a bilateral motor task.

Software for subjective visual vertical assessment: an observational cross-sectional study

UNLABELLED Spatial orientation in relation to the gravitational axis is significantly important for the maintenance of the posture, gait and for most of the humans motor activities. The subjective visual vertical exam evaluates the individuals perception of vertical orientation. OBJECTIVES The aims of this study were (1) to develop a virtual system to evaluate the subjective visual vertical exam, (2) to provide a simple tool to clinical practice and (3) to assess the subjective visual vertical values of healthy subjects using the new software. STUDY DESIGN observational cross-sectional study. METHODS Thirty healthy volunteers performed the subjective visual vertical exam in both static and dynamic conditions. The exam consisted in adjusting a virtual line in the vertical position using the computer mouse. For the static condition, the virtual line was projected in a white background. For the dynamic condition, black circles rotated in clockwise or counterclockwise directions. Six measurements were taken and the mean deviations in relation to the real vertical calculated. RESULTS The mean values of subjective visual vertical measurements were: static -0.372º; ± 1.21; dynamic clockwise 1.53º ± 1.80 and dynamic counterclockwise -1.11º ± 2.46. CONCLUSION This software showed to be practical and accurate to be used in clinical routines.

Tsallis information measure applied to the analysis of EEG signals in a model of the somatosensory system

Abstract The collective behavior of cortical neurons during processes of reorganization after amputation of a digit in a realistic computational model of the somatosensory system is studied with Tsallis entropy measure. The presence of transient events and significant alterations into the level of entropy associated with the reorganization processes in the simulated cortical area during and after the organization of new representational areas show that the application of this kind of analysis may provide interesting insights into the analysis of reorganization processes in cortical areas by means of extracellular field potentials recordings.

Virtual apparatus to study the subjective visual vertical

The study of the human equilibrium has been increasing recently. In this field a developed issue is the study to perceive ones visual vertical. But there is no agreement in the measurement apparatus and protocol for this investigation in the different centers working in this area. In this paper it is proposed a software with the same propose as the usual mechanical apparatus used to visual excitation during equilibrium evaluation. This software can be easily used in different places, and will help to improve the patterns of the evaluation protocols in all clinics and research centers.

Event-Related Functional MRI and Information Theory

Functional magnetic resonance imaging (fMRI) has become one of the main tools for non-invasive assessment of normal human brain functions. Most of the analysis that have been done thus far, either in the clinical environment or for research purposes, have made use of block paradigms, that involves periods of activation alternated with periods of rest. Nevertheless, in the last few years event-related fMRI (ER-fMRI) has become an alternative approach to infer about human brain functions. It consists basically of the presentation of a series of short stimuli, for further following of the brain activation patterns, translated by variations in brain hemoglobin state, resulting in local image contrast changes [1]. Recently, event-related paradigms, also called single trial paradigms, have become more extensively applied to cognitive experiments [2]. Classically the analysis of ER time series is based on the computation of the covariance between the observed averaged signal and an artificial hemodynamic response function. However, choosing the right reference function can become a significant problem. In fact, separate cortical regions can exhibit very different hemodynamic responses, varying its shape and amplitude [3]. Besides the conventional algorithms for analysis of fMRI block paradigms, such as cross correlation and student-t test, here we propose a new technique for analysis of event-related fMRI time series, based on an information measurement dependent on time. The results of the two methods are then compared in a standard visual and motor experiment. More specifically, the method consists in the computation of the Shannon entropy derived from the image time series, generated in a typical ER visual and motor paradigms. The maximum and minimum of the entropies were interpreted based on the temporal evolution of the probability distribution of the states of the system. For the calculation of the entropy dependent on time» one can define the states accessible to the system as the amplitude levels of a signal within a time interval (window) centered at time /. The Shannon entropy is then calculated as [4]:

VOLUME CONDUCTOR MODELLING WITH INCOMPLETE LOW RESOLUTION MAGNETIC RESONANCE TOMOGRAPHY RECORDINGS

Sl \ I \ I . \ K N Modcll ing thc clcctromagnctic propertics l thc thorux in magnctocardiographic (MCG) studies is u s i u l K pcrlnrmcd by thc Boundary Element Method ι Hl M) Mostly, Magnctic Rcsonancc Imaging (MRI) si ,ms arc thc hasis for cxtracting thc coordinatcs for the I U M As MRI is a (timc) cxpensivc tcchnique and Ncanncrs havc a high usc dcmand, a strategy is prescnted in t h is vvork that rcduccs thc costs and thc necd for addit ional MRI Images. This strategy is based on the use o f l o v v resolution and incomplctc MRI image sets of the thorax.