False Transients Research Articles

Detecting and correcting false transients in calcium imaging.

Population recordings of calcium activity are a major source of insight into neural function. Large datasets require automated processing, but this can introduce errors that are difficult to detect. Here we show that popular time course-estimation algorithms often contain substantial misattribution errors affecting 10-20% of transients. Misattribution, in which fluorescence is ascribed to the wrong cell, arises when overlapping cells and processes are imperfectly defined or not identified. To diagnose misattribution, we develop metrics and visualization tools for evaluating large datasets. To correct time courses, we introduce a robust estimator that explicitly accounts for contaminating signals. In one hippocampal dataset, removing contamination reduced the number of place cells by 15%, and 19% of place fields shifted by over 10 cm. Our methods are compatible with other cell-finding techniques, empowering users to diagnose and correct a potentially widespread problem that could alter scientific conclusions.

Decontaminate Traces From Fluorescence Calcium Imaging Videos Using Targeted Non-negative Matrix Factorization.

Fluorescence microscopy and genetically encoded calcium indicators help understand brain function by recording large-scale in vivo videos in assorted animal models. Extracting the fluorescent transients that represent active periods of individual neurons is a key step when analyzing imaging videos. Non-specific calcium sources and background adjacent to segmented neurons contaminate the neurons’ temporal traces with false transients. We developed and characterized a novel method, temporal unmixing of calcium traces (TUnCaT), to quickly and accurately unmix the calcium signals of neighboring neurons and background. Our algorithm used background subtraction to remove the false transients caused by background fluctuations, and then applied targeted non-negative matrix factorization to remove the false transients caused by neighboring calcium sources. TUnCaT was more accurate than existing algorithms when processing multiple experimental and simulated datasets. TUnCaT’s speed was faster than or comparable to existing algorithms.

Evaluation of a Constant Parameter Line-Based TWFL Real-Time Testbed

This paper presents a testbed for real-time testing of Traveling-Wave Fault Locators (TWFLs). A Field Programmable Gate Array (FPGA) is used as the main hardware platform for conducting the real-time simulation. To accurately reproduce fault-launched travelling waves, the simulated power system is executed at a 500 ns time-step. The fault locating device is the SEL-T400L relay with a sampling rate capability of 1 MHz. The simulated grid is composed of constant parameter (CP) transmission lines (TLs). The paper presents the capabilities and limitations of such an approach and assesses the suitability of the line model for testing TWFL. Firstly, off-line analyses and validation are performed using EMTP. It is found that the CP line model can be incorporated in the double-ended TWFL testing setups. For the single-ended TWFL testing, it is shown that the CP line model can introduce false transients due to ground propagation mode which can affect testing accuracy. Secondly, real-time experimental simulations are done to elaborate the importance of sub-microsecond time-steps for testing of TWFL.

A numerical study of the effect of gas distribution profile across the pulp–froth interface on flotation performance

A solution to the Laplace equation developed using a semi-analytical method termed the method of false transients was used to study the impact of the distribution of gas across the pulp–froth interface on froth performance. Simulation results indicated that profiling gas across the pulp–froth interface has a huge impact on bubble breakage across the surface of the froth and consequently results in different air recoveries. Further, gas flux distribution across the interface also resulted in changes in both bubble and particle residence time distribution. Comparison of predictions from residence time distribution and air recovery to experimental flotation performance was in partial agreement. The discrepancy between model predictions and experimental results was attributed to the fact that the current model does not take into account important froth-phase sub-processes such as bubble coalescence and froth drainage.

A Direct Numerical Method of Lines Approach for Predicting Primary and Secondary Current Density Distributions: Linear and Nonlinear Boundary Conditions

A direct numerical method of lines approach is presented for solving Laplace’s equation to obtain primary and secondary potential and current density distributions in electrochemical cells. Steady-state heat transfer in solids is governed by Laplace’s equation and has been solved directly by various numerical methods such as the successive over-relaxation method and the implicit alternative direction method (see e.g., Carnahan et al., 1969). Another method that has been used to solve Laplace’s equation is the method of false transients (Schiesser, 1991, 1994). In this method, one adds a time derivative of the dependent variable to Laplace’s equation, uses finite differences to approximate the spatial derivatives, and then solves the resulting system of equations by the method of lines (Davis, 1984; Schiesser, 1991, 1994; Rice and Do, 1995; Schiesser and Silebi, 1997; Cutlip and Shacham, 1998; Constantinides and Mostoufi, 1999;Taylor, 1999; Subramanian and White, 2000a). In our earlier paper, we presented a method for solving Laplace’s equation using a semi analytical method of lines. This method consists of using a central difference approximation for the second-order derivative in one of the spatial directions followed by solving analytically the resulting system of second-order differential equations by an analytical method. That is, the system of second-order, two-point boundary value problems are solved analytically by casting them in first-order form. As of today, solvers have developed to a point where the resulting two point boundary value problems in y (when discretization is used in x) can be directly and efficiently solved numerically. We present and analyze the results of this approach, called Numerical two dimensional method of lines for analyzing current distributions in Hull cell and other geometries of interest for electrochemical engineers. Another key point is that instead of finite difference methods, staggered grid and finite volume methods with variable node spacing in x gives much superior results even compared to state of the art finite element software such as COMSOL. Acknowledgements The work presented herein was funded in part by the Clean energy institute at the University of Washington, Seattle. References Carnahan, B., Luther, H.A., Wilkes, J.O., 1969. Applied Numerical Methods. Wiley, New York.Schiesser, W.E., 1991. The Numerical Method of Lines. Academic Press, New York.Schiesser, W.E., 1994. Computational mathematics in Engineering and Applied Science: ODEs, DAEs and PDEs. CRC Press, Boca Raton.Schiesser, W.E., Silebi, C.A., 1997. Computational Transport Phenomena. Cambridge UniversityPress, New York.Davis, M.E., 1984. Numerical Methods and Modeling for Chemical Engineers. Wiley, New York.Rice, R.G., Do, D.D., 1995. Applied mathematics and modelling for chemical Engineers. John Wiley& Sons Inc., New York.Constantinides, A., Mostoufi, N., 1999. Numerical Methods for Chemical Engineers with Matlab Applications. Prentice-Hall, New Jersey.Cutlip, M.B., Shacham, M., 1998. The numerical method of lines for partial differential equations. CACHE News 47, 18–21.Taylor, R., 1999. Engineering computing with maple: solution of PDEs via method of lines. CACHE News 49, 5–8.Taylor, R., Krishna, R., 1993. Multi Component Mass Transfer. Wiley, New York.Subramanian, V.R., White, R.E., 2000a. Solving differential equations with maple. Chemical Engineering Education 34, 328–336.Subramanian, V.R., White, R.E., 2000b. A semianalytical method for predicting primary and secondary current density distributions: linear and non-linear boundary conditions. Journal of Electrochemical Society 147 (5), 1636–1644.Subramanian, V.R., White, R.E., 2004. Semianalytical method of lines for solving elliptic partial differential equations. Chemical Engineering Science 59, 781–788.

A robust false transient method of lines for elliptic partial differential equations

Elliptic partial differential equations (PDEs) are frequently used to model a variety of engineering phenomena, such as steady-state heat conduction in a solid, or reaction-diffusion type problems. However, computing a solution can sometimes be difficult or inefficient using standard solvers. Techniques have been developed, including the method of lines (Schiesser, 1991), which can solve parabolic PDEs using well developed numerical solvers, but are not directly applicable to elliptic PDEs. The method of false transients overcomes this limitation by arbitrarily introducing a pseudo time derivative to modify the elliptic PDE to a parabolic PDE. However, this technique diverges for certain problems, such as when the solution is an unstable equilibrium point. A Jacobian-based perturbation approach is presented as an alternative for situations when the standard false-transient method fails. Two examples are shown to demonstrate the robustness of the proposed method over the false transient method.

Simulation of transonic flows using quad-core OpenMP Euler, flux modified transonic small disturbance, and Fluent codes

This article investigates what performance can be achieved when executing an aerodynamic code on a quad-core based personal computer with an OpenMP compiler in a Windows environment. Euler equations are solved in two dimensions for simplicity, to predict flow field around an aerofoil on an O-type boundary fitted structured grid. The grid is generated by solving a system of Poisson's equations utilising an efficient method of false transients, coupled with an approximate factorisation technique and variable time steps cycling process. Comparison between the OpenMP Euler, flux modified transonic small disturbance and Fluent results for transonic flow fields, with shock waves, around a NACA0012 aerofoil is presented. References Bailey, D., Barszcz, E., Barton, J., Browning, D., Carter, R., Dagum, L., Fatoohi, R., Fineberg, S., Frederickson, P., Lasinski, T., Schreiber, R., Simon, H., Venkatakrishnan, V., and Weeratunga, S., The NAS Parallel Benchmarks, Contractor Report 203186, NASA, USA, 1994. Engquist, B., and Osher, S., Stable and Entropy Satisfying Approximations for Transonic Flow Calculations, Mathematics of Computation, 34, 149, January 1980, pp. 45--75. Gear, J., Ly, E., and Phillips, N. J. T., A Time Marching, Type-Dependent, Finite Difference Algorithm for the Modified Transonic Small Disturbance Equation, Proceedings of the 21st Congress of the International Council of the Aeronautical Sciences (ICAS98), ICAS and AIAA, Melbourne, Australia, 1998, Paper A98-31513. Hirsch, C., Numerical Computation of Internal and External Flows: Computational Methods for Inviscid and Viscous Flows, Volume 2, John Wiley and Sons, Great Britain, 1992. Jin, H., Frumkin, M., and Yan, J., The OpenMP Implementation of NAS Parallel Benchmarks and its Performance, Technical Report NAS-99-011, NASA, Moffett Field, USA, 1999. Liu, Y., and Vinokur, M., Nonequilibrium Flow Computations 1: An Analysis of Numerical Formulations of Conservation Laws, Contractor Report 177489, NASA, California, USA, 1988. Lock, R. C., Test Cases for Numerical Methods in Two-Dimensional Transonic Flows, Report Number 575, AGARD, 1970. Ly, E., Improved Approximate Factorisation Algorithm for the Steady Subsonic and Transonic Flow over an Aircraft Wing, Proceedings of the 21st Congress of the International Council of the Aeronautical Sciences (ICAS98), ICAS and AIAA, Melbourne, Australia, 1998, Paper Number A98-31699. Ly, E., and Nakamichi, J., Time-Linearised Transonic Computations Including Entropy, Vorticity and Shock Wave Motion Effects, The Aeronautical Journal, November 2003, pp. 687--695. Ly, E., and Norrison, D., Automatic Elliptic Grid Generation by an Approximate Factorisation Algorithm, ANZIAM Journal, 48 (CTAC2006), pp. C188--C202, July 2007. Murman, E., Analysis of Embedded Shock Waves Calculated by Relaxation Methods, AIAA Journal, 12, 5, May 1974, pp. 626--633. Pulliam, T., Kutler, P., and Rossow, V., Harvard Lomax: His Quiet Legacy to Computational Fluid Dynamics, 14th AIAA Computational Fluid Dynamics Conference, Norfolk, VA, June 1999. Steger, J.L., and Warming, R. F., Flux Vector Splitting of the Inviscid Gasdynamic Equations with Application to Finite Difference Methods, Technical Memorandum TM-78605, NASA, California, USA, 1979. Van Leer, B., Flux-Vector Splitting for the Euler Equations, Proceedings of the 8th International Conference on Numerical Methods in Fluid Dynamics, Aachen, West Germany, 1982.

Application of method of false transients to generate smooth grids around a body in motion

A time marching finite difference scheme incorporating an efficient method of false transients, an approximate factorisation technique and a time steps cycling process, is presented for solution of a system of Poisson's equations. The solution to the equations provides a smooth three dimensional boundary fitted grid around a body in motion. The scheme required much less computational effort than that required by other iterative schemes. In closure, examples of a static grid around an aircraft tailplane and a dynamic grid around a flapping wing are presented. References Ly, E., Improved Approximate Factorisation Algorithm for the Steady Subsonic and Transonic Flow over an Aircraft Wing, in Proceedings of the 21st Congress of the International Council of the Aeronautical Sciences (ICAS98), AIAA and ICAS, Melbourne, Australia, Sep. 1998, Paper A98-31699. Ly, E., and Gear, J. A., Time-Linearized Transonic Computations Including Shock Wave Motion Effects, Journal of Aircraft, 39, 6, Nov./Dec. 2002, pp. 964--972, http://mams.rmit.edu.au/3j9p7fh8xj2o1.pdf Ly, E., and Nakamichi, J., Time-Linearised Transonic Computations Including Entropy, Vorticity and Shock Wave Motion Effects, The Aeronautical Journal, Nov. 2003, pp. 687--695. Ly, E., and Norrison, D., Automatic Structured Grid Generation by an Approximate Factorisation Algorithm, Mathematics and Statistics Research Report 2006/02, School of Mathematical and Geospatial Sciences, RMIT University, Melbourne, Australia, July 2006, 28 pages, http://mams.rmit.edu.au/1354efwypnzn.pdf Ly, E., and Norrison, D., Automatic Elliptic Grid Generation by an Approximate Factorisation Algorithm, ANZIAM Journal, 48 (CTAC 2006), pp. C188--C202, July 2007. http://anziamj.austms.org.au/ojs/index.php/ANZIAMJ/article/view/48 Ly, E., and Norrison, D., Generating Elliptic Grids in Three Dimensions by a Method of False Transients, ANZIAM Journal, 49 (EMAC2007), pp. C170--C183, Nov. 2007. http://anziamj.austms.org.au/ojs/index.php/ANZIAMJ/article/view/313 Mathur, J. S., and Chakrabartty, S. K., An Approximate Factorization Scheme for Elliptic Grid Generation with Control Functions, Numerical Methods for Partial Differential Equations, 10, 6, 1994, pp. 703--713. Thompson, J. F., Thames, F. C., and Mastin, C. W., Boundary-Fitted Curvilinear Coordinate Systems for Solution of Partial Differential Equations on Fields Containing any Number of Arbitrary Two-Dimensional Bodies, NASA Contractor Report CR-2729, Washington DC, USA, July 1977, 253 pages. Warming, R. F., and Beam, R. M., On the Construction and Application of Implicit Factored Schemes for Conservation Laws, in SIAM-AMS Proceedings, 11, USA, 1978, pp. 85--129. Abbott, I. H., and von Doenhoff, A. E., Theory of Wing Sections, Dover Publications Inc., New York, USA, 1959. Catherall, D., Optimum Approximate-Factorization Schemes for Two Dimensional Steady Potential Flows, AIAA Journal, 20, 8, 1982, pp. 1057--1063. Gear, J. A., Time Marching Approximate Factorization Algorithm for the Modified Transonic Small Disturbance Equation, Research Report Number 6, Department of Mathematics, RMIT University, Melbourne, Australia, Feb. 1996, 11 pages. Hoffmann, K. A., Computational Fluid Dynamics for Engineers, Engineering Educational System, Texas, USA, 1989.

OpenMP performance with an Euler code on an Intel based personal computer

Personal computers of the past were limited to running computational fluid dynamics codes in serial mode. With the advent of multicore technology and suitable compilers, personal computers nowadays can execute codes in a parallel fashion similar to that of supercomputers and cluster computer systems. This article investigated what performance can be achieved when executing an aerodynamic code on an Intel quad core based personal computer with an OpenMP compiler in a Windows environment. The code solved the Euler equations to find the flowfield around a NACA0012 aerofoil on an O-type boundary fitted structured grid system. A speedup of up to 350% was obtainable in double precision accuracy. The reduced computation time means that, for small scale problems, more accurate Euler codes can replace commonly used transonic small disturbance codes. For larger problems, this information serves as a reference for developers of hybrid MPI/OpenMP algorithms for cluster computer systems. References Advanced Micro Devices (AMD), AMD FireStream 9170: Industry's First GPU with Double-Precision Floating Point, 2007, http://ati.amd.com/products/streamprocessor/specs.html Allmaras, S. R., Venkatakrishnan, V., and Johnson, F. T., Farfield Boundary Conditions for 2-D Airfoils, AIAA Paper 2005-4711, AIAA, 2005. Bailey, D., Barszcz, E., Barton, J., Browning, D., Carter, R., Dagum, L., Fatoohi, R., Fineberg, S., Frederickson, P., Lasinski, T., Schreiber, R., Simon, H., Venkatakrishnan, V., and Weeratunga, S., The NAS Parallel Benchmarks, National Aeronautics and Space Administration (NASA), Contractor Report 203186, USA, 1994, http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970014936_1997021577.pdf Dong, S., and Karniadakis, G. E., Dual-Level Parallelism for High-Order CFD Methods, Parallel Computing, 30, Jan. 2004, pp. 1--20. Hirsch, C., Numerical Computation of Internal and External Flows (Volume 2): Computational Methods for Inviscid and Viscous Flows, John Wiley and Sons, Great Britain, 1992. Hoffmann, K. A., Computational Fluid Dynamics for Engineers, Engineering Educational System, Texas, USA, 1989. Intel, Tera-scale Computing Research Program, c.2007, http://www.intel.com/research/platform/terascale/ Jin, H., Frumkin, M., and Yan, J., The OpenMP Implementation of NAS Parallel Benchmarks and its Performance, National Aeronautics and Space Administration (NASA), Technical Report NAS-99-011, Moffett Field, USA, 1999. Liu, Y., and Vinokur, M., Nonequilibrium Flow Computations 1: An Analysis of Numerical Formulations of Conservation Laws, National Aeronautics and Space Administration (NASA), Contractor Report 177489, California, USA, 1988, http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880021000_1988021000.pdf Lock, R. C., Test Cases for Numerical Methods in Two-Dimensional Transonic Flows, Advisory Group for Aerospace Research and Development (AGARD), Report Number 575, 1970. Ly, E., Improved Approximate Factorisation Algorithm for the Steady Subsonic and Transonic Flow over an Aircraft Wing, Proceedings of the 21st Congress of the International Council of the Aeronautical Sciences (ICAS98), ICAS and AIAA, Melbourne, Australia, 1998, Paper A98-31699. Ly, E., and Gear, J. A., Time-Linearized Transonic Computations Including Shock Wave Motion Effects, Journal of Aircraft, 39, 6, Nov/Dec. 2002, pp. 964--972. Ly, E., and Nakamichi, J., Time-Linearised Transonic Computations Including Entropy, Vorticity and Shock Wave Motion Effects, The Aeronautical Journal, Nov. 2003, pp. 687--695. Ly, E., and Norrison, D., Automatic Elliptic Grid Generation by an Approximate Factorisation Algorithm, ANZIAM Journal, 48 (CTAC2006), pp. C188--C202, July 2007, http://anziamj.austms.org.au/ojs/index.php/ANZIAMJ/article/view/48. Ly, E., and Norrison, D., Generating Elliptic Grids in Three Dimensions by a Method of False Transients, ANZIAM Journal, 49 (EMAC2007), pp. C170--C183, Nov. 2007, http://anziamj.austms.org.au/ojs/index.php/ANZIAMJ/article/view/313. Nvidia, The Era of the Personal Supercomputer, 2007, http://www.nvidia.com/content/events/siggraph_2007/supercomputing.html Pope, S. B., A Perspective on Turbulence Modeling, Modeling Complex Turbulent Flows, Kluwer Academic Publishers, The Netherlands, 1999. Pulliam, T., Kutler, P., and Rossow, V., Harvard Lomax: His Quiet Legacy to Computational Fluid Dynamics, 14th AIAA Computational Fluid Dynamics Conference, Norfolk, VA, 28 June-1 July, 1999, http://people.nas.nasa.gov/ pulliam/mypapers/lomax_la.ps Steger, J. L., and Warming, R. F., Flux Vector Splitting of the Inviscid Gasdynamic Equations with Application to Finite Difference Methods, National Aeronautics and Space Administration (NASA) Technical Memorandum 78605, California, USA, 1979, http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790020779_1979020779.pdf Thompson, J. R., Warsi, Z. U. A., and Mastin, C. W., Numerical Grid Generation: Foundations and Applications, Elsevier Science Publishing Co. Inc., New York, USA, 1985. Van Leer, B., Flux-Vector Splitting for the Euler Equations, Proceedings of the 8th International Conference on Numerical Methods in Fluid Dynamics, Aachen, West Germany, 1982.

Generating elliptic grids in three dimensions by a method of false transients

A finite difference method based scheme incorporating a method of false transients and an approximate factorisation technique is presented for solution of a system of Poisson's equations used for grid generation. A time step cycling process with repeated endpoints is incorporated into the scheme to enhance the convergence rate. The scheme required much less computational effort than all other numerical schemes compared in this article, to obtain a high quality grid over a body (converged solution) in three dimensions. Although, the superiority of the scheme has been demonstrated for a grid generation problem, it may be applied for other problems requiring the numerical solution of a set of similar partial differential equations. References Ly, E., and Gear, J. A., Time-Linearized Transonic Computations Including Shock Wave Motion Effects, Journal of Aircraft , 39 , 6, Nov-Dec 2002, pp. 964--972. Ly, E., and Nakamichi, J., Time-Linearised Transonic Computations Including Entropy, Vorticity and Shock Wave Motion Effects, The Aeronautical Journal , Nov. 2003, pp. 687--695. Ly, E., and Norrison, D., Automatic Elliptic Grid Generation by an Approximate Factorisation Algorithm, ANZIAM Journal , 48 (CTAC2006), pp. C188--C202, July 2007. Mathur, J. S., and Chakrabartty, S. K., An Approximate Factorization Scheme for Elliptic Grid Generation with Control Functions, Numerical Methods for Partial Differential Equations , 10 , 6, 1994, pp. 703--713. Thompson, J. F., Thames, F. C., and Mastin, C. W., Boundary-Fitted Curvilinear Coordinate Systems for Solution of Partial Differential Equations on Fields Containing any Number of Arbitrary Two-Dimensional Bodies , NASA Contractor Report CR-2729, Washington DC, USA, July 1977, 253 pages. Warming, R. F., and Beam, R. M., On the Construction and Application of Implicit Factored Schemes for Conservation Laws, in SIAM-AMS Proceedings , 11 , USA, 1978, pp. 85--129. Catherall, D., Optimum Approximate-Factorization Schemes for Two-Dimensional Steady Potential Flows, AIAA Journal , 20 , 8, 1982, pp. 1057--1063. Gear, J. A., Time Marching Approximate Factorization Algorithm for the Modified Transonic Small Disturbance Equation, Research Report Number 6, Department of Mathematics, RMIT University, Melbourne, Australia, Feb. 1996, 11 pages. Hoffmann, K. A., Computational Fluid Dynamics for Engineers , Engineering Educational System, Texas, USA, 1989. Ly, E., Improved Approximate Factorisation Algorithm for the Steady Subsonic and Transonic Flow over an Aircraft Wing, in Proceedings of the 21st Congress of the International Council of the Aeronautical Sciences (ICAS98) , AIAA and ICAS, Melbourne, Australia, Sep. 1998, Paper A98-31699.

Automatic elliptic grid generation by an approximate factorisation

A procedure for automatic numerical generation of a structured grid system with coordinate lines coinciding with all boundaries of a general two-dimensional region containing a body of arbitrary shape is presented. The solution procedure incorporated the method of false transients and the approximate factorisation algorithm, where a sequence of time steps is cycled in a geometric fashion with repeated endpoints, and has a capability for clustering grid lines close to the body. The procedure requires significantly much less computational effort to obtain a converged solution than a point or line successive over-relaxation iterative scheme. Although, the superiority of the presented algorithm has been demonstrated for the grid generation problem, it can be utilised for other problems requiring the solution of a set of elliptic partial differential equations of similar nature.

Simulations of transonic flow fields around an elastic arrow wing

Transonic flows are characterised by the presence of adjacent regions of subsonic and supersonic flow, usually with shock waves present at the downstream supersonic-subsonic interface. An in-house fortran90 computer code, tranflow3d, which is robust and capable of simulating complex nonlinear flow fields around a double-swept-back semispan supersonic transport wing model (of high sweep angle) at transonic speeds is presented. The code has acceptable turnaround times on current high performance personal computers. The flow is governed by the general frequency, nonlinear, transonic small disturbance equation, subject to nonreflecting, far field, boundary conditions. Numerical solution procedure based on finite difference method, method of false transients and approximate factorisation technique is utilised. Chordwise distributions of steady pressure coefficient and local Mach numbers are computed, and compared with those obtained from wind tunnel measurements.

Clinical Utility of Telemetered Intracardiac Electrograms in Diagnosing a Design Dependent Lead Malfunction

Inhibition of pacemakers due to false signals from malfunctioning pacing leads has been previously reported. Three cases are reported in which inappropriate pauses were observed shortly following implantation. In all three cases, active fixation leads with an electronically active screw and ring tip electrode were used. All leads were manufactured by Oscor Medical, St. Petersburg, Florida. In case one, models PY-61 (ventricle) and PY-51 (atrium) were used. In case two, models PY-61 were used in the ventricle and the atrium. In case three, models PY-52 were used both in the atrium and ventricle. Thresholds at the time of implantation were acceptable. In cases one and two, inappropriate pauses were noted following paced ventricular beats. In case three, inappropriate inhibition of the ventricular output was noted intermittently after some of the paced atrial beats. Spurious signals were identified as the cause of the apparent oversensing problem by using electrocardiograms with annotated telemetry information and noninvasively telemetered intracardiac electrograms. The amplitude of the spurious signals varied between 2 and 14 mV. In all cases, the problems resolved with time. A mechanism for the generation of false transients is proposed and the value of telemetered intracardiac electrograms is discussed.