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Biomedical subjects

S J Swithenby

Publications and source records attributed to S J Swithenby.

7 recordsLinked to original sources
PubMed1998

Neural processing of human faces: a magnetoencephalographic study.

This is a whole head magnetoencephalographic (MEG) study of the neural processing of briefly presented images of human faces in 14 normal subjects. The experiments involved three tasks of increasing complexity, involving image categorisation, image comparison and the identification of emotion. The analyses were based on average responses to repeated stimuli in the different image categories. These averages were processed to give numerical measures of the power within defined regions and latency spans. The only statistically significant difference in these data between the response to faces and other images is in the right occipito-temporal channels at a latency of 140 ms. The face-specific response is largely independent of the task. Source modelling suggests an extended source in the ventral occipito-temporal region. The analysis supports the notions of both face-specificity and right hemisphere dominance for all image types at early latencies.

Adult↗
PubMed1993

A distributed quasi-static ionic current source in the 3-4 day old chicken embryo.

We report measurements of slowly varying magnetic field patterns close to fertilized eggs of the chicken Gallus domesticus during the first few days of incubation. These fields are generated by ionic currents within the egg that are associated with the development of the embryo. Since they are very weak (no greater than tens of pT) and vary over distances of a few millimetres, it has been necessary to develop specialized instrumentation and analysis techniques. We describe the use of high-spatial-resolution SQUID magnetometers to measure the field patterns and appropriate imaging algorithms to model the current sources responsible for producing the fields. Our results provide strong evidence for a distributed source in the extra-embryonic membranes. There is also indication of a more localized source within the embryo itself.

Algorithms↗
PubMed1993

Identification of discrete regions of activity using correlation coefficient scanning of distributed current maps.

Biomagnetic measurements of the brain are often analysed in terms of a number of discrete primary generators. In this paper we describe an objective method of identifying the number of approximate location of such generators. The method is based on the matching of an instrument-independent representation of the data with a template whose pattern is characteristic of a localized primary source. The method is shown to be insensitive to severe noise and to be capable of resolving closely spaced generators.

Brain Mapping↗
PubMed1988

Non-invasive monitoring of ionic current flow during development by SQUID magnetometry.

The ionic currents flowing in developing organisms produce weak magnetic fields that can be detected using SQUID magnetometers. The method is non-invasive and dc recording is possible. To date SQUID magnetometers have mainly been used in human studies. The features of the technique are described and the prospects of extending its use to developmental studies are discussed. Feasible instrumental specifications are indicated. A recent SQUID magnetometer investigation of ionic current flow in the developing chick in ovo is summarised as an illustration of the magnetometer method. The paper as a whole argues that magnetometry is a useful alternative or adjunct to electrode-based experiments on the electrophysiology of developing organisms.

Animals↗
PubMed1987

Analysis of magnetoencephalographic data using the homogeneous sphere model: empirical tests.

The external magnetic fields produced by artificial current dipoles placed in conducting-gel filled spheres, partial spheres and human skulls have been measured. Using a homogeneously conducting sphere model, the dipole parameters have been estimated. Although good quality data fitting is obtainable for a range of positions of the centre of the sphere used in the modelling, the predictions for the dipole obtained with those different assumed positions differ considerably. It is found that the most reliable method of obtaining accurate estimates of the dipole parameters is by taking an average of the predictions of several of the best-fit analyses. The information about the accuracy of the predictions that is obtainable from the fitting routine's residual error parameter is discussed.

Brain↗
PubMed1985

Macroscopic ionic currents within the human leg.

Recent research on developing and healing tissues suggests that small quasi-DC ionic currents (of magnitude 10-20 microA) may play a controlling role in the initiation and organisation of growing tissues, but the difficulties of measuring such small currents have led to confusing results. Sensitive magnetometry provides a method of demonstrating and, to some extent, locating such currents. A SQUID magnetometer system has been built and used to investigate the magnetic fields around the uninjured human leg. Analysis of the magnetic fields reveals the presence of slowly changing macroscopic current loops (of magnitude up to 12 microA) within the leg. These currents are broadly similar in all subjects, and show day-to-day reproducibility in individuals. They change predictably with time of muscle relaxation (over an hour), and revert to the original form on muscular exertion. These currents are of significance when considering the therapeutic use of injected current for the healing of non-union in bone.

Electromagnetic Fields↗