Functional magnetic resonance imaging (fMRI) is currently the mainstay of neuroimaging in cognitive neuroscience. Advances in scanner technology, image acquisition protocols, experimental design, and analysis methods promise to push forward fMRI from mere cartography to the true study of brain organization. However, fundamental questions concerning the interpretation of fMRI data abound, as the conclusions drawn often ignore the actual limitations of the methodology. Here I give an overview of the current state of fMRI, and draw on neuroimaging and physiological data to present the current understanding of the haemodynamic signals and the constraints they impose on neuroimaging data interpretation.

What we can do and what we cannot do with fMRI

t. This paper surveys some of the work our group has done in electrical impedance tomography.

Electrical Impedance Tomography

Characterization of heterogeneous systems by dielectric spectroscopy

In vivo measurement of cortical impedance spectrum in monkeys: implications for signal propagation.

Chapter 6 – determination of biological impedances1

At low frequencies, the normally difficult measurement of the dielectric properties of conducting materials is severely compromised by electrode polarization. This problem at the electrode‐sample interface arises from the modulation of the normal dc boundary potential by the passage of alternating current. A solution, permitting a conductance resolution of 1:105 for frequencies between 10 cps and 1 kc, is to use a second noncurrent‐carrying pair of electrodes to measure the voltage across the sample and to employ a null technique for obtaining the required precision. Several four‐electrode null techniques are proposed, each having certain relative merits. In all cases the resolution in capacitance is shown to be governed by a combination of the resolution in conductance, the sample properties, and the frequency of the measurement.

Four‐Electrode Null Techniques for Impedance Measurement with High Resolution

A technique is outlined to control the stray field of electrolytic cells in a predictable and simple manner. The technique is particularly useful in biological impedance work and in determinations of the dielectric constant of solutions of high conductivity. It is based on a plot of the apparent dielectric constant as a function of electrode distance. In a logarithmic presentation the apparent dielectric constant appears as the sum of the true dielectric constant and two linear functions, representative of electrode polarization and stray field component. A few examples are given to illustrate the technique and its limitations.

Herman P. Schwan

Papers

Four‐Electrode Null Techniques for Impedance Measurement with High Resolution

Simple Technique to Control the Stray Field of Electrolytic Cells