The purpose of this study was to quantify the distortion of electrical fields by skull foramina using an in vitro model. Extracranial voltage generated by current dipoles located inside a human calva immersed in saline were measured when a 4-mm hole was open and when it was blocked with paraffin wax. Dipoles were located either along the internal surface of the bone (superficial dipoles) or at increasing distances from the bone (deep dipoles). With the hole open, extracranial signals had a substantially greater amplitude than with the hole blocked. The locations of the largest voltage values recorded outside the skull depended on the distance of the recording electrode from the hole rather than on the location of the internal dipole. For superficial dipoles, voltage values with the hole open were as much as 116 times greater than when the hole was blocked. Furthermore, when the hole was open, the largest extracranial signals were seen at the hole even when the dipole was 5 to 6 cm away from the hole. The effects of skull holes were less prominent for deep dipoles than for superficial dipoles. Skull discontinuities can be major determinants for the distribution of extracranial EEG signals. These results have implications for EEG interpretation and for source localization.

Algorithms for low-pass and high-pass three-pole recursive Butterworth filters of a given cut-off frequency have been developed. A band-pass filter can be implemented by sequential application of algorithms for low- and high-pass filters. The algorithms correspond to infinite impulse-response filters that have been designed by applying the bilinear transformation to the transfer functions of the corresponding analog filters, resulting in a recursive digital filter with seven real coefficients. Expressions for filter coefficients as a function of the cut-off frequency and the sampling period are derived. Filter performance is evaluated and discussed. As in the case of their analog counterparts, their transfer function shows marked flattening over the pass band and gradually higher attenuation can be seen at frequencies above or below the cut-off frequency, with a slope of around 60 dB/decade. There is a 3 dB attenuation at the cut-off frequency and a gradual increase in phase shift over one decade above or below the cut-off frequency. Low-pass filters show a maximum overshoot of 8% and high-pass filters show a maximum downwards overshoot of approximately 35%. The filter is mildly under-damped, with a damping factor of 0.5. On an IBM 300GL personal computer at 600 MH with 128 MB RAM, filtering time with MATLAB 5.2 running under Windows 98 is of the order of 50 ms for 60000 samples. This will be adequate for on-line electroencephalography (EEG) applications. The simplicity of the algorithm to calculate filter coefficients for an arbitrary cut-off frequency can be useful to modern EEG laboratories and software designers for electrophysiological applications.

Objectives-To study the variability, topography, polarity, duration, and incidence of interictal epileptiform discharges (EDs) in the scalp EEG and electrocorticogram (ECoG) from 16 patients with temporal lobe epilepsy who underwent surgical treatment.Methods-Preoperative scalp EEGs during quinalbarbitone induced sleep were compared with preresection ECoGs obtained under general anaesthesia. The analysis was based on the initial ECoG record obtained before activation by intravenous thiopentone, and the EEG during stages I and II of sleep.Results-On the scalp, 15 patients had a single discharge pattern, spikes were predominantly negative, EDs were of largest amplitude at the anterior temporal electrode in 13 patients and mean discharge incidence was 4.0 (SD 4.2) discharges/ min. In ECoG recordings, nine patients had two independent ECoG patterns, the polarity of spikes was negative, positive-negative, or positive, the site of maximal amplitude varied greatly between subjects, discharge incidence was 7.3 (SD 3.9) discharges/min. There was no relation between the topography of the largest spikes on the scalp and in the ECoG. In 14 patients, scalp spikes showed statistically significant longer duration on the scalp than in the ECoG. In seven patients who had frequent widespread ECoG discharges, averaging spikes across ECoG channels generated spiky patterns of duration similar to that of scalp spikes.Conclusion-It seems that, in temporal lobe epilepsy, scalp discharges originate from widespread ECoG discharges and tend to produce a stereotyped pattern on the scalp with largest amplitudes at the anterior temporal electrodes. This is probably due to local anatomical peculiarities in the brain coverings, such as skull discontinuities, rather than to the location of neuronal generators within the temporal lobe. Due to spatiotemporal averaging, widespread cortical discharges which become asynchronous during propagation appear with increased duration and blunted waveform in the EEG, whereas sharply localised phenomena such as positive focal spikes are not recorded from the scalp.

EEG/VEP and fMRI responses to periodic visual stimulation are reported. The purpose of these experiments was to look for similar patterns in the time series produced by each method to help understand the relationship between the two. The stimulation protocol was the same for both sets of experiments and consisted of five complete cycles of checkerboard pattern reversal at 1.87 Hz for 30 s followed by 30 s of a stationary checkerboard. The fMRI data was analyzed using standard methods, while the EEG was analyzed with a new measurement of activation-the VEPEG. Both VEPEG and fMRI time series contain the fundamental frequency of the stimulus and quasi harmonic components-an unexplained double frequency commonly found in fMRI data. These results have prompted a reappraisal of the methods for analyzing fMRI data and have suggested a connection between our findings and much older published invasive electrophysiological measurements of blood flow and the partial pressures of oxygen and carbon dioxide. Overall our new analysis suggests that fMRI signals are strongly dependant on hydraulic blood flow effects. We distinguish three categories of fMRI signal corresponding to: focal activated regions of brain tissue; diffuse nonspecific regions of steal; and major cerebral vessels of arterial supply or venous drainage. Each category of signal has its own finger print in frequency, amplitude, and phase. Finally, we put forward the hypothesis that modulations in blood flow are not only the consequence but are also the cause of modulations in functional activity.

In a previous study we showed that fast moving stimuli activate V5, an area specialized for motion, at very short latencies through a pathway that reaches it without passing through V1. Using the same technique of visual evoked responses, we have tested our conclusions by studying patient GY, whose V1 is damaged but whose V5 is intact. In spite of the contralateral hemi-blindness due to his V1 lesion, GY has a residual visual capacity that allows him to perceive, consciously, fast but not slow moving stimuli presented in his affected hemifield. By stimulating GY's 'blind' hemifield and comparing the responses with those obtained from normal subjects, we were able to study the relative contribution of V1 and V5 to the visual evoked response to motion in normal subjects. We found that GY's early response to fast motion is preserved and correlates with activity elicited in control subjects over area V5, while slow motion, pattern offset, and pattern reversal stimuli failed to elicit responses in GY. The results confirm our previous conclusions: namely, that the early part of the motion evoked response is generated in area V5 and that signals reach this area through a dynamically parallel pathway that bypasses area V1. They go on to demonstrate that neurophysiological activity in the prestriate cortex correlates with the conscious visual perception of motion.

Medical imaging has, in the past 25 years, been revolutionized by the introduction and subsequent widespread clinical use of X-ray computerized tomography (CT). CT led the way for the introduction of several new tomographic methods that use physical techniques as diverse as position annihilation and nuclear magnetic resonance to obtain high spatial resolution reconstructed images of both human anatomy and more recently, human biological function. The central theme running throughout this revolution is the tomographic principle. This article describes the key elements in present-day medical imaging using tomography as the unifying concept. X-ray CT, position emission tomography (PET) and magnetic resonance imaging (MRI) are described as far as possible in non-mathematical terms. Most space is devoted to MRI, the newest of the methods and the one which probably holds most promise for the future in terms of its range of application.

Published clinical evidence has led us to hypothesize that there are parallel pathways which lead to the striate (V1) and prestriate cortex in the human brain. We have used the technique of visually evoked EEG coupled to magnetoencephalography (MEG) to test our hypothesis, by detecting the timing of arrival of signals into these visual areas, using published PET evidence to guide us in the location of the evoked response sources. We found that, if the moving stimulus has a speed of 22 degrees s(-1), signals arrive in V5 before V1; with speeds of <6 degrees s(-1), signals arrive in V1 first. We conclude that, in addition to the classical picture of a sequential input to prestriate cortex through V1, there is also a fast parallel input which by-passes V1. The parallelism manifests itself only as a function of the characteristics of the visual stimulus, a phenomenon we describe as dynamic parallelism. The results obtained help us explain the residual motion vision of patients with lesions in V1 or V5.

The hypothesis that focal scalp EEG and MEG interictal epileptiform activity can be modelled by single dipoles or by a limited number of dipoles was examined. The time course and spatial distribution of interictal activity recorded simultaneously by surface electrodes and by electrodes next to mesial temporal structures in 12 patients being assessed for epilepsy surgery have been studied to estimate the degree of confinement of neural activity present during interictal paroxysms, and the degree to which volume conduction and neural propagation take part in the diffusion of interictal activity. Also, intrapatient topographical correlations of ictal onset zone and deep interictal activity have been studied. Correlations between the amplitudes of deep and surface recordings, together with previous reports on the amplitude of scalp signals produced by artificially implanted dipoles suggest that the ratio of deep to surface activity recorded during interictal epileptiform activity on the scalp is around 1:2000. This implies that most such activity recorded on the scalp does not arise from volume conduction from deep structures but is generated in the underlying neocortex. Also, time delays of up to 220 ms recorded between interictal paroxysms at different recording sites show that interictal epileptiform activity can propagate neuronally within several milliseconds to relatively remote cortex. Large areas of archicortex and neocortex can then be simultaneously or sequentially active via three possible mechanisms: (1) by fast association fibres directly, (2) by fast association fibres that trigger local phenomena which in turn give rise to sharp/slow waves or spikes, and (3) propagation along the neocortex. The low ratio of deep-to-surface signal on the scalp and the simultaneous activation of large neocortical areas can yield spurious equivalent dipoles localised in deeper structures. Frequent interictal spike activities can also take place independently in areas other than the ictal onset zone and their interictal propagation to the surface is independent of their capacity to trigger seizures. It is concluded that: (1) the deep-to-surface ratios of electromagnetic fields from deep sources are extremely low on the scalp; (2) single dipoles or a limited number of dipoles are not adequate for surgical assessment; (3) the correct localisation of the onset of interictal activity does not necessarily imply the onset of seizures in the region or in the same hemisphere. It is suggested that, until volume conduction and neurophysiological propagation can be distinguished, semiempirical correlations between symptomatology, surgical outcome, and detailed presurgical modeling of the neocortical projection patterns by combined MEG, EEG, and MRI could be more fruitful than source localization with unrealistic source models.

Interictal epileptiform activity recorded by scalp EEG, foramen ovale electrodes and MEG is discussed. Gross differences in waveform between the electric and magnetic records are discussed in the light of intracranial depth recordings.

Specific heat and magnetization measurements have been made in the temperature range 1.7-5.2 K on ternary spin-glass alloys based on Cu-0.25 at.% Mn and Cu-4 at.% Mn. Changes in both the thermal and the magnetic properties with the concentration of a third element (Pt or Al) provide evidence for the importance of the distribution of interaction strengths in determining the details of spin-glass phenomena. A simple scaling hypothesis applied to the thermal data suggests that, whereas the addition of Al to Cu-Mn preserves the binary alloy interaction distribution, the addition of Pt introduces new terms that may be attributable to the Dzyaloshinsky-Moriya interaction.

An ASSEMBLER routine is described for data aquisition and "on-line" averaging, artefact rejection and graphic display of data on a personal microcomputer (IBM compatible). The user determines the number of input channels, sampling frequency, number of samples, input range, stimulation frequency (epoch frequency) and the number of epochs to be acquired and averaged. Data from each epoch are scanned in search of saturating artefacts and will be added to previous epochs if none is found. Data are then graphically displayed as voltage versus time before acquiring next epoch. Display options can be defined by the user at run time by means of the keyboard and include: display of last epoch, display of the average, storage screen and refreshing screen after every epoch. High data transfer rates and program speed allows for high stimulation rates in the presence of on line graphic display. The computer then behaves as a multichannel digital oscilloscope with access to large memory buffers, disk storage, high averaging capabilities, artefact rejection and wide potential for data analysis. Its applications to the recording of magnetic and electric evoked responses are illustrated. The program is available from the authors.

A detailed analysis is given of the procedures involved in balancing second order axial gradiometers using superconducting vanes. Building on this analysis a very simple practical procedure emerges that provides a means of quantifying the initial imbalance and correcting it in an iterative manner. The method is illustrated by comparing experimental data with numerical calculations. The effects of test field inhomogeneities and eddy current sources on the balancing procedure and the degree of noise rejection obtained are discussed.

A simple closed approximate expression for the vector potential produced by a circular current loop that is accurate to better than 0.05% is described. Applications of this expression to problems in biomagnetism are described.

The magnetisation and the field dependence of the transport critical current have been studied in detail at 77 K in three well characterised samples of polycrystalline YBa2Cu3O7. Measurements have been made over a wide range of field, from below 10 mu T to above 5 T. The differences in the Jcs between samples are about a factor of 3, but are reflected only in a rather subtle fashion in the magnetisations. There is a strong dependence of Jc on field history, particularly at fields between 10 and 500 mT. Most of the observed behaviour at low fields, less than about 100 mT, can be understood in terms of a network model of a lattice of weak links, provided that the field dependence of the weak-link critical currents is itself included. Such improvement in Jc as can be gained by careful fabrication of polycrystalline YBa2Cu3O7 seems to involve either the enhancement of a relatively small fraction of macroscopic current paths, or possibly intergranular flux pinning through entanglement with intragranular pinned flux.

It is shown that the empirical expression delta M/ delta H=X(0)-( beta /2)log(1+H2/K2) provides a very close fit to experimental spin-glass magnetic isotherms both at and above the freezing temperature Tg. Above Tg the parameter K is shown to vary in a very similar manner to Xnl(T) as reported by previous researchers. At Tg, however, K is found to be finite. It is argued that this result demonstrates that there are no divergent quantities in the spin-glass magnetic isotherm.

Calorimetric measurements on the hysteresis loop of the spin glass CuMn show that the energy changes take place on those legs of the loop which correspond to reversals of the remanent magnetisation and that it is always heating which occurs, irrespective of the direction of the applied magnetic field. Cycling through successive hysteresis loops causes large initial changes in sample heating and loop area, with both of these quantities tending to a temperature-independent limit. Although the limit cycle is displaced from the origin, the thermal energy changes on both irreversible legs are approximately equal, indicating a change of internal magnetic energy on reversal of the remanence. It is argued that the energy released in the initial magnetisation reversals is attributable to irreversible rearrangements of the relative spin directions and that it is only in the limit cycle that energy changes represent exclusively the intrinsic irreversible processes of magnetisation reversal.