Recording and interpretation of cerebral magnetic fields

Bilateral electrical stimulation of a congenitally-deaf ear and of an acquired-deaf ear

Two identical multichannel intracochlear prostheses were implanted in the same patient. The first prosthesis, implanted in the congenitally-deaf right ear, elicited clear sound perception but no speech recognition. After 2 years, a second prosthesis, implanted in the acquired-deaf left ear, enabled the patient to understand speech without lip-reading. Brainstem and middle-latency evoked potentials were similar with electrical stimulation of both ears and resembled those evoked by acoustic stimuli in subjects with normal hearing. Cortical electric and magnetic responses differed for right- and left-sided electrical stimulation suggesting that stimulation of the congenitally-deaf ear elicited an abnormal activation of the auditory cortex. These results suggest that only cortical responses were affected by the different histories of deafness of the ears.

EEG versus MEG localization accuracy: theory and experiment

We first review the theoretical and computer modelling studies concerning localization accuracy of EEG and MEG, both separately and together; the source is here a dipole. The results show that, of the three causes of localization errors, noise and head modelling errors have about the same effect on EEG and MEG localization accuracies, while the results for measurement placement errors are inconclusive. Thus, these results to date show no significant superiority of MEG over EEG localization accuracy. Secondly, we review the experimental findings, where there are again localization accuracy studies of EEG and MEG both separately and together. The most significant EEG-only study was due to dipoles implanted in the heads of patients, and produced an average localization error of 20 mm. Various MEG-only studies gave an average error of 2-3 mm in saline spheres and 4-8 mm in saline-filled skulls. In the one study where EEG and MEG localization were directly compared in the same actual head, again using dipoles implanted in patients, the average EEG and MEG errors of localization were 10 and 8 mm respectively. The MEG error was later confirmed by a similar (but MEG-only) experiment in another study, using a more elaborate MEG system. In summary, both theory and experiment suggests that the MEG offers no significant advantage over the EEG in the task of localizing a dipole source. The main use of the MEG, therefore, should be based on the proven feature that the MEG signal from a radial source is highly suppressed, allowing it to complement the EEG in selecting between competing source configurations.(ABSTRACT TRUNCATED AT 250 WORDS)

Gender differences in source location for the N100 auditory evoked magnetic field

Auditory evoked magnetic fields were recorded in response to contralateral stimulation over the right hemisphere in 6 adult males and 6 adult females. The data were fit to a model of a current-dipole source in a homogeneous sphere and 5 parameters of the dipole were computed--3 spatial coordinates, orientation, and strength. When average values for the dipole parameters were compared between sexes, it was found that the current source for the N100m is located more than 1 cm posterior in females and is oriented pointing more downward. These findings were replicated in separate measurement sessions. Viewing of individual magnetic resonance images did not reveal a corresponding anatomical disparity in the location of the primary auditory cortex which is assumed to produce the N100m. Therefore, functional organization of the auditory cortex may be different for the sexes.

Advantages and limitations of magnetic source imaging

The term "magnetic source image" (MSI) describes the distribution of neuronal activity in the brain that can be deduced from measurements of the field pattern it produces across the scalp. The signals which provide the basis for an MSI are obtained from the magnetoencephalogram (MEG) which is conventionally recorded with superconducting detectors. Advances in MSI techniques during the past decade have revealed numerous aspects of the functional organization of human sensory systems that were previously unknown. In addition, studies of spontaneous signals, such as those in the alpha bandwidth, have identified specific cortical areas that support rhythmic activity. Extensions of this approach to cognitive research are able to determine the active cortical areas where spontaneous activity is suppressed when a person is engaged in a task such as mental imagery and auditory memory recall. Because only the component of the intracellular current tangential to the overlying skull contributes to the extracranial field, a confined source--modeled as a current dipole--has a characteristic field pattern that simplifies the pattern recognition problem of identifying the underlying sources. This advantage is illustrated by the identification of simultaneously active sources in auditory primary and association cortex. Their separate localization makes it possible to characterize their functional differences. Because the source strength in an MSI may be inferred without knowledge of the electrical conductivities of intervening tissue, it is also possible to estimate the extent of cortical involvement. From the tangential source strength in an MSI, it is possible in most cases to determine the total source strength by taking account of the orientation of the cortical surface. This provides an objective, quantitative measure of the strength of neuronal activity. At present, the major limitation in more extensive use of MSI is the cost of instrumentation. While it requires no contact with the head, and measurements can commence within a few minutes of the arrival of the subject or patient, the present cost of a large array of sensors is two to three million dollars.

Seeing faces activates three separate areas outside the occipital visual cortex in man

We have examined magnetic cortical responses of 15 healthy humans to 46 different pictures of faces. At least three areas outside the occipital visual cortex appeared to be involved in processing this input, 105-560 ms after the stimulus onset. The first active area was near the occipitotemporal junction, the second in the inferior parietal lobe, and the third in the middle temporal lobe. The source in the inferior parietal lobe was also activated by other simple and complex visual stimuli.

MEG versus EEG localization test using implanted sources in the human brain

It is believed that the magnetoencephalogram (MEG) localizes an electrical source in the brain to within several millimeters and is therefore more accurate than electroencephalogram (EEG) localization, reported as 20 mm. To test this belief, the localization accuracy of the MEG and EEG were directly compared. The signal source was a dipole at a known location in the brain; this was made by passing a weak current pulse simulating a neural signal through depth electrodes already implanted in patients for seizure monitoring. First, MEGs and EEGs from this dipole were measured at 16 places on the head. Then, computations were performed on the MEG and EEG data separately to determine the apparent MEG and EEG source locations. Finally, these were compared with the actual source location to determine the MEG and EEG localization errors. Measurements were made of four dipoles in each of three patients. After MEGs with weak signals were discounted, the MEG average error of localization was found to be 8 mm, which was worse than expected. The average EEG error was 10 mm, which was better than expected. These results suggest that the MEG offers no significant advantage over the EEG in localizing a focal source. However, this does not diminish other uses of the MEG.

Magnetoencephalography in the study of epilepsy

A brief review is given about the basic principles of magnetoencephalography (MEG), a noninvasive brain research method in which weak magnetic fields are detected outside the human head with SQUID (Superconducting Quantum Interference Device) magnetometers. The active brain areas, producing the signal, are modelled by current dipoles, which are assumed to be situated in a spherically symmetric volume conductor. The locations of these "equivalent dipoles" can be found, in the optimal case, with a precision of a few millimeters. The new multichannel magnetometers allow measurements of spontaneous brain activity without EEG-triggered averaging. The 3-dimensional locations of superficial epileptogenic foci can be determined with respect to external landmarks on the skull and to known generator areas of evoked responses in the brain. Examples are given about MEG recordings of epileptic patients.

Magnetoencephalography and epilepsy research

The theory of magnetoencephalography (MEG) and its application to epilepsy research are reviewed briefly. The MEG prediction appears to agree in general with regions where epileptiform discharges are found on the electrocorticogram. MEG appears to have a somewhat better localizing capability than EEG, although MEG may well miss the tangential component of magnetic fields. Thus, the combination of MEG and EEG may be more fruitful than either one alone.

Localization of epileptic foci using a large-area magnetometer and functional brain anatomy

We used a large-area, 7-channel, first-order superconducting quantum interference device (SQUID) gradiometer to preoperatively determine the sites of epileptic foci in 2 patients with intractable temporal lobe seizures. The equivalent dipoles for the epileptic spikes were located with respect to external landmarks of the skull and in relation to the generation sites of magnetic auditory evoked responses. It was also possible, for the first time, to determine the location of the equivalent source using simultaneously measured data from seven locations only. The sites of the equivalent dipoles, in the right temporal lobe, agreed with the electrocorticographic and depth electrode recordings made during the operation.

Separate finger representations at the human second somatosensory cortex

We recorded neuromagnetic responses of the second somatosensory cortex in healthy humans. Cutaneous electrical stimulation of fingers elicited a response around 100 ms, with a field pattern agreeing with activation of the second somatosensory cortex in the upper bank of the Sylvian fissure. In an oddball paradigm, with standards presented to the thumb and deviants (10%) to the middle finger, or vice versa, the second somatosensory cortex responses to deviants were almost three times as high in amplitude as those to standards. A similar amplitude enhancement was obtained when the deviants were presented in the absence of the intervening standards but with the same interstimulus interval. The results indicate that an accurate functional representation of different body areas is maintained at the human second somatosensory cortex.

Spatially Distributed Cortical Excitation Patterns of Auditory Processing during Contralateral and Ipsilateral Stimulation

Utilizing the high spatial and temporal resolution of magnetoencephalography in conjunction with magnetic resonance images, the current study explored the underlying electrical patterns of cortical excitation during both contralateral and ipsilateral auditory stimulation. Instead of studying only the peaks of the N100 component of the evoked magnetic field, a 30-msec window was chosen about the area where the peaks occurred and the intracranial sources generating that component were estimated at successive 5-msec intervals. Results indicated that the sources for both contralateral and ipsilateral conditions were best represented as a continuous movement of activation in an anterior–inferior direction along the superior surface of the temporal lobe. Although the peak magnetic fields of the N100 to contralateral stimulation were of shorter latency and higher amplitude, the generating sources of both had very similar time-dependent movement patterns, and comparisons of source localizations were dependent on the latency at which they were contrasted.

Realistic conductivity geometry model of the human head for interpretation of neuromagnetic data

In this paper, the computational and practical aspects of a realistically-shaped multilayer model for the conductivity geometry of the human head are discussed. A novel way to handle the numerical difficulties caused by the presence of the poorly conducting skull is presented. Using our method, both the potential on the surface of the head and the magnetic field outside the head can be computed accurately. The procedure was tested with the multilayer sphere model, for which analytical expressions are available. The method is then applied to a realistically-shaped head model, and it is numerically shown that for the computation of B, produced by cerebral current sources, it is sufficient to consider a brain-shaped homogeneous conductor only since the secondary currents on the outer interfaces give only a negligible contribution to the magnetic field outside the head. Comparisons with the sphere model are also included to pinpoint areas where the homogeneous conductor model provides essential improvements in the calculation of the magnetic field outside the head.