Location Of Equivalent Sources Research Articles

Downward Continuation and Transformation of Total‐Field Magnetic Anomalies Into Magnetic Gradient Tensors Between Arbitrary Surfaces Using Multilayer Equivalent Sources

AbstractThe equivalent source method offers a potential means for continuing and manipulating magnetic data between arbitrary surfaces. However, it ultimately suffers from a limited continuation distance and significant errors, which restrict its applicability in practice. To address this problem, we reformulate the equivalent source method with a partial differential equation framework. A multilayer equivalent susceptibility method is innovatively derived based on this approach. The location of equivalent sources is designed for adaptive estimation through the inverse procedure. We introduce nonlinear optimization, regularization, and constraining technologies in inversion to improve the stability and accuracy of the solution. Synthetic, real airborne, and ground data are tested in experiments. The results show that the new method significantly improves the capability and accuracy of the downward continuation and transformation of irregularly located data between arbitrary undulating surfaces.

The use of equivalent source models for reduced order simulation in room acoustics

The sound field in a room can be predicted by using the Boundary Element Method if the motion of the source is known along with the impedance of the room surfaces and furnishings: however, these computations are very time consuming, especially at high frequencies. It is known that the total sound field in a room consists of three components: (1) free space source radiation; (2) the incoming sound field reflected from the room surfaces; and (3) the outgoing sound field scattered from the source surface, particularly if it is large: e.g., a flat screen television. For the purpose of fast computation, an equivalent source method (ESM) can be adopted in which two sets of equivalent sources represent the incoming and scattered sound field components, respectively. When the free space component is known, the parameters of the ESM can be estimated by least squares approximations of the impedance boundary conditions on the room and source surfaces. Accuracy and computational load can be balanced by the choice of source order and the constraints placed on equivalent source locations. Simulation results will be presented to compare the performances of different ESM configurations.

Localization of sources of brain alpha/theta/delta activity and the influence of the mode of spontaneous mentation

A method is described that accounts for multichannel brain field data (EEG) epochs, after transformation into the frequency domain, by a single oscillating dipole source in terms of phase angles. The method produces a potential distribution for each frequency period (FFT dipole approximation). These maps can be subjected to conventional equivalent dipole source fittings in terms of amplitudes. The authors studied the equivalent source locations for the different temporal EEG frequency bands (delta/theta/alpha) in 12 normal subjects during the collection of reports of spontaneous thoughts. Some of the thought reports were classed into two modes, 'visual imagery' and 'abstract', and the associated equivalent source locations during the 2 s immediately prior to these reports were computed. Different equivalent source locations were found for the different spectral components of the EEG, implying that different neural generator populations generate the different frequencies.

Nonlinear least squares optimization applied to the method of fundamental solutions for eddy current problems

The authors describe the use of a nonlinear least squares optimization technique for adaptively determining the equivalent source locations when the method of fundamental solution is used for 2D eddy current problems. The main advantage of the method is that guesswork in locating the sources is eliminated. Two examples of applying the method are given. One is the case of two circular conductors (one magnetic and the other nonmagnetic). The second is the case of a single conductor having a rectangular cross section. In both cases, the conductors carry an impressed current. Results are compared with those obtained without the use of nonlinear least squares optimization. It is shown that in both cases the source points can easily go across the boundaries during the optimization procedure. >

Selective listening modifies activity of the human auditory cortex.

We have studied the effect of selective listening on the neuromagnetic evoked activity of the human auditory cortex. In the word categorization experiment the stimuli were 5-letter words, each beginning with (k). Half of them were targets, i.e., names of animals or plants, and half other meaningful Finnish words. In the duration discrimination experiment equiprobable tones of 425 ms (targets) or 600 ms duration were presented. In both experiments the interstimulus interval (ISI) was 2.3 s and the stimuli of the two classes were presented randomly. Subjects either ignored the stimuli (reading condition) or counted the number of targets (listening condition). The magnetic field over the head was measured with a 7-channel 1st-order SQUID-gradiometer. The stimuli evoked a transient response followed by a sustained field. The transient response did not differ between the two conditions but the sustained field was significantly larger in the listening than reading condition; the increase began 120-200 ms after stimulus onset and continued for several hundred milliseconds. The equivalent source locations of both transient and sustained responses agreed with activation of the supratemporal auditory cortex. In the dichotic listening experiment 25-ms square-wave stimuli were presented randomly and equiprobably either to the left or to the right ear at an ISI of 0.8-1 s, either alone or in presence of a speech masker. Counting the stimuli of either ear resulted in differences between responses to relevant and irrelevant sounds. The difference began 140-150 ms after stimulus onset and peaked at 200-240 ms. During monaural speech masking, N100m was larger for attended than ignored stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)