The generalized shrinkage estimator for the analysis of functional connectivity of brain signals

We develop a new statistical method for estimating functional connectivity between neurophysiological signals represented by a multivariate time series. We use partial coherence as the measure of functional connectivity. Partial coherence identifies the frequency bands that drive the direct linear association between any pair of channels. To estimate partial coherence, one would first need an estimate of the spectral density matrix of the multivariate time series. Parametric estimators of the spectral density matrix provide good frequency resolution but could be sensitive when the parametric model is misspecified. Smoothing-based nonparametric estimators are robust to model misspecification and are consistent but may have poor frequency resolution. In this work, we develop the generalized shrinkage estimator, which is a weighted average of a parametric estimator and a nonparametric estimator. The optimal weights are frequency-specific and derived under the quadratic risk criterion so that the estimator, either the parametric estimator or the nonparametric estimator, that performs better at a particular frequency receives heavier weight. We validate the proposed estimator in a simulation study and apply it on electroencephalogram recordings from a visual-motor experiment.Comment: Published in at http://dx.doi.org/10.1214/10-AOAS396 the Annals of Applied Statistics (http://www.imstat.org/aoas/) by the Institute of Mathematical Statistics (http://www.imstat.org

Effects of Two Different Catheter Ablation Techniques on Spectral Characteristics of Atrial Fibrillation

ObjectivesThe aim of this study was to determine the effects of circumferential pulmonary vein ablation (CPVA) and electrogram-guided ablation (EGA) on the spectral characteristics of atrial fibrillation (AF) and the relationship between changes in dominant frequency (DF) and clinical outcome.BackgroundCircumferential pulmonary vein ablation and EGA have been used to eliminate AF. Spectral analysis may identify high-frequency sources.MethodsIn 84 consecutive patients, CPVA (n = 42) or EGA (n = 42) was performed for paroxysmal (n = 49) or persistent (n = 35) AF. During EGA, complex electrograms were targeted. Lead V1and electrograms from the left atrium and coronary sinus were analyzed to determine the DF of AF before and after ablation.ResultsThe left atrial DF was higher in persistent (5.83 ± 0.86 Hz) than paroxysmal AF (5.33 ± 0.76 Hz, p = 0.03). There was a frequency gradient from the left atrium to the coronary sinus (p = 0.02). Circumferential pulmonary vein ablation and EGA resulted in a similar decrease in DF (18 ± 17% vs. 17 ± 15%, p = 0.8). During a mean follow-up of 9 ± 6 months, the change in DF after CPVA was similar among patients with and without recurrent AF. An acute decrease in DF after EGA was associated with freedom from recurrent AF only in patients with persistent AF (19 ± 14% vs. 3 ± 6%, p = 0.02).ConclusionsBoth CPVA and EGA decrease the DF of AF, consistent with elimination of high-frequency sources. Whereas the efficacy of EGA is associated with a decrease in DF in patients with persistent AF, the efficacy of CPVA is independent of changes in DF. This suggests that CPVA and EGA eliminate different mechanisms in the genesis of persistent AF

Differences in atrial fibrillation properties under vagal nerve stimulation versus atrial tachycardia remodeling

Fond : Le substrat de fibrillation auriculaire (FA) vagale et celui secondaire à remodelage par tachycardie auriculaire (RTA) partagent beaucoup des caractéristiques : période réfractaire efficace (PRE) réduite, hétérogénéité accrue de PRE et quelques mécanismes moléculaires communs. Cette étude a comparé les 2 substrats à une abréviation comparable de PRE. Méthodes : Chez chacun de 6 chiens de groupe de stimulation vagal (SV), les paramètres de stimulation cervicale bilatérale de nerves vagaux ont été ajustés pour produire la même PRE moyenne (calculé à 8 sites des oreillettes gauche et droite) avec 6 chiens de groupe de RTA assorti à sexe et poids. Des paramètres électrophysiologiques, la durée moyenne de la fibrillation auriculaire (DAF) et les fréquences dominantes (FD) locales ont étés calculés. Résultats : En dépit des PREs assorties (SV: 80±12msec contre RTA: 79±12msec) la DAF était plus longue (*), l’hétérogénéité de conduction était plus élevée (*), la FD était plus rapide (*) et la variabilité de FD plus grande (*) chez les chiens SV. Les zones de maximum FD qui reflètent les zones d’origine de FA étaient à côté de ganglions autonomes chez les chiens SV. Conclusions : Pour un PRE atriale comparable, la FA secondaire à SV est plus rapide et plus persistante que la FA avec un substrat de RTA. Ces résultats sont consistants avec des modèles de travail suggérant que l'hyperpolarisation SV-induite contribue de façon important à la stabilisation et à l'accélération des rotors qui maintiennent la FA. La similitude de la distribution de FD du groupe vagal avec la distribution des lésions d’ablation après cartographie des électrogrammes atriales fragmentés suggère des nouvelles techniques d’ablation. La distribution des FD entre le SV et le RTA fournit de nouvelles idées au sujet de possible rémodelage neuroreceptorial et indique des différences importantes entre ces substrats de FA superficiellement semblables.Background: Vagal nerve stimulation (VS) and atrial tachycardia remodeled (ATR) atrial fibrillation (AF) substrates share many features: reduced effective refractory period (ERP), increased ERP heterogeneity and some common molecular mechanisms. This study compared VS and ATR substrates at comparable ERP abbreviation. Methods: In each of 6 VS dogs, bilateral cervical VS parameters were adjusted to produce the same mean ERP as a sex and weight matched ATR dog. Electrophysiological parameters, mean duration of AF (DAF) and local dominant frequencies (DF) were determined (before (CTL) and after VS in VS dogs). Results: Despite matched ERPs (VG: 80±12msec vs ATR: 79±12msec) DAF was greater (*), conduction heterogeneity was greater (*), DF was faster (*) and DF variability greater (*) in VS dogs. AF drivers reflected by maximum DF zones were adjacent to autonomic ganglia in VS dogs; there was a tendency (p<0.07) to faster driver zones in the left atrium comparing to the right in ATR dogs. Conclusions: For a comparable atrial ERP, VS AF is faster and more persistent than AF with an ATR substrate. These results are consistent with modeling work suggesting that VS-induced hyperpolarization is an important contributor to AF-maintaining rotor stabilization and acceleration. Similarities in DF distribution in VS dogs with distribution of ablation lesions performed after Complex Fractionated Atrial Electrograms mapping suggests new curative ablation methods. DF distribution differences between VS and ATR provides new ideas about possible neuroreceptorial remodeling and indicates important differences between these superficially similar AF substrates