I F Gorodnitsky | PubMed Health

PubMed2002

Variability in AC amplifier distortions: estimation and correction.

AC amplifiers can introduce significant distortions into the low frequency and DC components of recorded electrophysiological data such as event-related potentials (ERPs). Methods for correcting such distortions (i.e., estimating the waveform of the original data) after the data have been amplified and recorded rely on an accurate estimate of the amplifier's time constant (TC). We show that the filter characteristics of AC amplifiers in at least some commercially available ERP recording instruments may vary considerably across individual channels, even when each houses an identical AC amplifier circuit. Clearly, distortion correction methods must take this variability into account. We propose an empirical means of estimating the correct TC value. This approach yields more accurate correction than those based on TCs calculated analytically.

Algorithms↗

PubMed1995

Neuromagnetic source imaging with FOCUSS: a recursive weighted minimum norm algorithm.

The paper describes a new algorithm for tomographic source reconstruction in neural electromagnetic inverse problems. Termed FOCUSS (FOCal Underdetermined System Solution), this algorithm combines the desired features of the two major approaches to electromagnetic inverse procedures. Like multiple current dipole modeling methods, FOCUSS produces high resolution solutions appropriate for the highly localized sources often encountered in electromagnetic imaging. Like linear estimation methods, FOCUSS allows current sources to assume arbitrary shapes and it preserves the generality and ease of application characteristic of this group of methods. It stands apart from standard signal processing techniques because, as an initialization-dependent algorithm, it accommodates the non-unique set of feasible solutions that arise from the neuroelectric source constraints. FOCUSS is based on recursive, weighted norm minimization. The consequence of the repeated weighting procedure is, in effect, to concentrate the solution in the minimal active regions that are essential for accurately reproducing the measurements. The FOCUSS algorithm is introduced and its properties are illustrated in the context of a number of simulations, first using exact measurements in 2- and 3-D problems, and then in the presence of noise and modeling errors. The results suggest that FOCUSS is a powerful algorithm with considerable utility for tomographic current estimation.

Algorithms↗