The dielectric properties of tissues have been extracted from the literature of the past five decades and presented in a graphical format. The purpose is to assess the current state of knowledge, expose the gaps there are and provide a basis for the evaluation and analysis of corresponding data from an on-going measurement programme.

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The dielectric properties of biological tissues: I. Literature survey

There has been tremendous advances in our ability to produce images of human brain function. Applications of functional brain imaging extend from improving our understanding of the basic mechanisms of cognitive processes to better characterization of pathologies that impair normal function. Magnetoencephalography (MEG) and electroencephalography (EEG) (MEG/EEG) localize neural electrical activity using noninvasive measurements of external electromagnetic signals. Among the available functional imaging techniques, MEG and EEG uniquely have temporal resolutions below 100 ms. This temporal precision allows us to explore the timing of basic neural processes at the level of cell assemblies. MEG/EEG source localization draws on a wide range of signal processing techniques including digital filtering, three-dimensional image analysis, array signal processing, image modeling and reconstruction, and, blind source separation and phase synchrony estimation. We describe the underlying models currently used in MEG/EEG source estimation and describe the various signal processing steps required to compute these sources. In particular we describe methods for computing the forward fields for known source distributions and parametric and imaging-based approaches to the inverse problem.

Electromagnetic brain mapping

This committee was appointed by the SPR Board to provide recommendations for publishing data on electrodermal activity (EDA). They are intended to be a stand-alone source for newcomers and experienced users. A short outline of principles for electrodermal measurement is given, and recommendations from an earlier report (Fowles et al., ) are incorporated. Three fundamental techniques of EDA recording are described: (1) endosomatic recording without the application of an external current, (2) exosomatic recording with direct current (the most widely applied methodology), and (3) exosomatic recording with alternating current-to date infrequently used but a promising future methodology. In addition to EDA recording in laboratories, ambulatory recording has become an emerging technique. Specific problems that come with this recording of EDA in the field are discussed, as are those emerging from recording EDA within a magnetic field (e.g., fMRI). Recommendations for the details that should be mentioned in publications of EDA methods and results are provided.

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Publication recommendations for electrodermal measurements.

Gyri-precise head model of transcranial direct current stimulation: Improved spatial focality using a ring electrode versus conventional rectangular pad

A standardized boundary element method volume conductor model

The paper traces the history of, and tabulates determinations of the electrical resistivity of blood, other body fluids, cardiac muscle, skeletal muscle, lung, kidney, liver, spleen, pancreas, nervous tissue, fat, bone, and other miscellaneous tissues. Where possible, the conditions of measurement are given.

The specific resistance of biological material—A compendium of data for the biomedical engineer and physiologist

The low frequency impedance of bare silver electrodes in contact with physiological saline was found to exhibit capacitive reactance and by the deposition of chloride the impedance became resistive in nature. It was found that a chloride deposit of 100–500 mA.sec/cm2 of electrode area provided the lowest electrode-electrolyte impedance. Prolongation of chloriding beyond this range increased the electrode-electrolyte impedance at all frequencies but did not alter the resistive nature of the impedance. To achieve a chloride deposit which is proportional to the product of mA and sec it was found that a minimum chloriding current density of 5 mA/cm2 of electrode area should be used.

Optimum electrolytic chloriding of silver electrodes.

This paper examines the type of distortion appearing in the canine and human ECG when electrodes having a small surface area are employed with a recording apparatus having various resistive input impedances.

The relationship between input impedance and electrode area in recording the Ecg.

A comparison is made of the linear and non-linear least squares characterizations of the relationship between the transthoracic impedance change (ΔZ) and the respired air volume (ΔV) with regard to electrode location, somatotype, body position and type of breathing for seven normal, male, human subjects. The electrode locations which provided the largest ΔZ signal and those which produced the least error in the values of ΔV as determined from the measured values of ΔZ were also studied.

A comparison of linear and non-linear characterizations of impedance spirometry data

The high impedance of glass micropipettes is widely recognized and appropriate attention is given to the amplifier input impedance required to reproduce bioelectric events recorded with these electrodes. However, similar precautions are often ignored when using metal microelectrodes, because it is often assumed that their impedance is much lower than micropipettes of comparable size. This paper reports on studies of the relationship between metal microelectrode area and amplifier input impedance required to record faithfully the action potential of frog skeletal muscle. The distortion of the EMG, as the amplifier input impedance was intentionally lowered in discrete steps, is quantitated in terms of the resultant per cent tilt of a square wave inserted along with the action potential. In addition, the impedance vs. frequency relationship is plotted for two metal microelectrodes of 100 and 1600 μ2 in area. This relationship shows the increased impedance at lower frequencies resulting from the capacitance produced by the electrical double layer. Results show that the minimum input impedance required for proper EMG recording ranges from 88 MΩ for steel electrodes of 500 μ2 in area to 1 megohm for electrodes of 125,000 μ2.

L. E. Baker

Papers

The specific resistance of biological material—A compendium of data for the biomedical engineer and physiologist

Optimum electrolytic chloriding of silver electrodes.

The relationship between input impedance and electrode area in recording the Ecg.

A comparison of linear and non-linear characterizations of impedance spirometry data

The relationship between electrode area and amplifier input impedance in recording muscle action potentials