Y. Biton

Singular value decomposition of optically-mapped cardiac rotors and fibrillatory activity

Our progress of understanding how cellular and structural factors contribute to the arrhythmia is hampered in part because of controversies whether a fibrillating heart is driven by a single, several, or multiple number of sources, and whether they are focal or reentrant, and how to localize them. Here we demonstrate how a novel usage of the neutral singular value decomposition (SVD) method enables the extraction of the governing spatial and temporal modes of excitation from a rotor and fibrillatory waves. Those modes highlight patterns and regions of organization in the midst of the otherwise seemingly-randomly propagating excitation waves. We apply the method to experimental models of cardiac fibrillation in rabbit hearts. We show that the SVD analysis is able to enhance the classification of the heart electrical patterns into regions harboring drivers in the form of fast reentrant activity and other regions of by-standing activity. This enhancement is accomplished without any prior assumptions regarding the spatial, temporal or spectral properties of those drivers. The analysis corroborates that the dominant mode has the highest activation rate and further reveals a new feature: A transfer of modes from the driving to the passive regions resulting in a partial reaction of the passive region to the driving region.

Dynamics of a spiral pair source and its interaction with plane waves

Spiral pair creation and dynamics is a widely occurring phenomenon in nature. It can appear in the heart tissue, causing severe arrhythmia, known as a figure-eight reentry. We consider the appearance of a spiral pair source, its minimal strength for survival, and the possible results of its interaction with a plane wave. In particular, its ability to outlast such an encounter is of interest. We also consider the question of exposing the source to a train of pulses, in terms of the frequency and angle of encounter. Results show different regimes of behavior, e.g. source annihilation, motion of the source away from, or towards the origin of the plane waves, its breaking and multiplication. Relevance of these results to heart arrhythmia and their possible cancellation by external pacing are briefly discussed.

The Weiss–Lapicque and the Lapicque–Blair strength—duration curves revisited

The Weiss–Lapicque and Lapicque–Blair relations connecting the strength of a stimulation pulse and its duration in order to attain a threshold are examined here in the case of the nonlinear FitzHugh–Nagumo (FHN) model system. The relation parameters can easily be derived and explained by analyzing the dynamical behavior at very short and very long pulse durations. This explanation should hold true for most nonlinear biological systems. It is seen that, for the FHN, both relations provide only approximations (albeit good ones) to the actual strength-duration curve.

New Generation Methods of Spiral-Pairs and 3D Patterns

Reaction-Diffusion systems of nonlinear partial differential equations are used to model quite a few scientific phenomena. We have used a specific system, the FitzHugh-Nagumo one, to study new methods of generating spiral pairs and spatiotemporal periodic structures in 2D, and 3D repeat ing scroll rings in excitable media. Results are important in different fields, the electrical information transfer and arr hythmias in the heart, chemical reactions such as the Belousov-Zhabotinsky ones and possibly abnormal brain waves.

Reentry as an Origin for Rotors

The aim of the study is to understand in depth the meaning of “reentry”, and to decipher if and how it can lead to malfunctions of the heart and possibly of the brain. A simple model is used to reveal the mechanism by which a single pulse of action potential rotating around a ring of excitable medium, the latter simulating a reentry circuit, can generate spirals (single and/or double) when the pulse can emerge from and develop outside the ring. Two mechanisms of spiral generation are demonstrated: (1) a mechanism in which a source of single spirals is created at the contact with the core soon after the pulse freeing action, their chirality being due to the sense of the preceding pulse rotation. Interestingly, these spirals, adhering to the core, become “double-spiral patterns” while leaving behind the seeds of the new single spirals. (2) A second possible mechanism, similar to the known “arms encountering methods”, in which a double spiral (a figure of eight) is repeatedly created on the other side of the core. Similar procedures are assumed to occur in the heart, leading to tachycardia and fibrillation and possibly in the brain leading to epilepsy. The exact processes of the hitherto assumed spiral generations by reentry were established. The novel deep understanding of the mechanisms involved in these processes can lead to new methods of treating heart fibrillation (e.g., by judicial ablation).

A simple simulation model for spiral induced epilepsy

regionwas intact, structuralMRI datawere obtained during the pretreatment scanning session. To establish whether tDCS supports re-recruitment of LH structures, lateralisation indices (LI) will be calculated as a measure for the relative contributionof the left and righthemisphere. LIswill be compared for patients receiving tDCS versus patients receiving sham, both pre and post treatment. We will unveil the significantly activated brain areas during both paradigms, in relation to behavioural performance on these language tasks. Results: At present, the analysis is ongoing. Results will be available in March 2017. Discussion: The results of this study will improve our understanding of tDCS-induced language reorganisation, and will contribute to the ongoing discussion among aphasiologists on the roles of the RH and LH in poststroke language reorganisation.

The Baroreflex Mechanism Revisited

We state that the autonomic part of the brain controls the blood pressure (BP) and the heart rate (HR) via the baroreflex mechanism in all situations of human activity (at sleep, at rest, during exercise, fright etc.), in a way which is not, as was hitherto assumed, a mere homeostatic tool or even a resetting device, designed to bring these variables on the road to preset values. The baroreflex is rather a continuous feedback mechanism commanded by the autonomic part of the brain, leading to values appropriate to the situation at hand. Feasibility of this assertion is demonstrated here by using the Seidel–Herzel feedback system outside of its regular practice. Results show indeed that the brain can, and we claim that it does, control the HR and BP throughout life. New responses are demonstrated, e.g., to a sudden fear or apnea. In this event, large BP and HR overshoots are expected before the variables can relax to a new level. Response to abrupt downward change in the controlling parameter shows an undershoot in HR and just a gradual resetting in the BP. The relaxation from sudden external changes to various expected states are calculated and discussed and properties of the Rheos test are explained. Experimental findings for orthostatic tests and for babies under translations and rotations reveal complete qualitative agreement with our model and show no need to invoke the operation of additional body systems. Our method should be the preferred one by the Occam Razor approach. The outcomes may lead to beneficial clinical implication.