High resolution EEG: 124-channel recording, spatial deblurring and MRI integration methods.

This paper describes a method for increasing the spatial detail of the EEG and for integrating physiological data with anatomical models based on magnetic resonance images (MRIs). This method includes techniques to efficiently record EEG data from up to 124 channels, to measure 3-D electrode positions for alignment with MRI-derived head models, and to estimate potentials near the outer convexity of the cortex using a spatial deblurring technique which uses a realistic model of the structure of the head and which makes no assumptions about the number or type of generator sources. The validity of this approach has been initially tested by comparing estimated cortical potentials with those measured with subdural grid recordings from two neurosurgical patients. The method is illustrated with somatosensory steady-state evoked potential data recorded from 5 healthy subjects. Results suggest that deblurred 124-channel topographic maps, registered with a subject's MRI and rendered in 3 dimensions, provide better spatial detail than has heretofore been obtained with scalp EEG recordings. The results also suggest that the potential for EEG as a functional neuroimaging modality has yet to be fully realized.

Seeing through the skull: advanced EEGs use MRIs to accurately measure cortical activity from the scalp.

There is a vast amount of untapped spatial information in scalp-recorded EEGs. Measuring this information requires use of many electrodes and application of spatial signal enhancing procedures to reduce blur distortion due to transmission through the skull and other tissues. Recordings with 124 electrodes are now routinely made, and spatial signal enhancing techniques have been developed. The most advanced of these techniques uses information from a subject's MRI to correct blur distortion, in effect providing a measure of the actual cortical potential distribution. Examples of these procedures are presented, including a validation from subdural recordings in an epileptic patient. Examples of equivalent dipole modeling of the somatosensory evoked potential are also presented in which two adjacent fingers are clearly separated. These results demonstrate that EEGs can provide images of superficial cortical electrical activity with spatial detail approaching that of O15 PET scans. Additionally, equivalent dipole modeling with EEGs appears to have the same degree of spatial resolution as that reported for MEGs. Considering that EEG technology costs ten to fifty times less than other brain imaging modalities, that it is completely harmless, and that recordings can be made in naturalistic settings for extended periods of time, a greater investment in advancing EEG technology seems very desirable.

Beyond topographic mapping: towards functional-anatomical imaging with 124-channel EEGs and 3-D MRIs.

A functional-anatomical brain scanner that has a temporal resolution of less than a hundred milliseconds is needed to measure the neural substrate of higher cognitive functions in healthy people and neurological and psychiatric patients. Electrophysiological techniques have the requisite temporal resolution but their potential spatial resolution has been not realized. Here we briefly review progress in increasing the spatial detail of scalp-recorded EEGs and in registering this functional information with anatomical models of a person's brain. We describe methods and systems for 124-channel EEGs and magnetic resonance image (MRI) modeling, and present first results of the integration of equivalent-dipole EEG models of somatosensory stimulation with 3-D MRI brain models.

The magnetoencephalographic localisation of source-systems in the brain: early and late components of event related potentials.

The application of magnetoencephalography (MEG) to the analysis of sources in the brain responsible for early and late components of evoked potentials is discussed. Representative data are presented and discussed which demonstrate localisation of sources assuming single equivalent dipoles. Distributed systems as sources for some steady-state responses are discussed in relation to the broader issue of the usefulness of equivalent single dipole models. These issues are related to the use of MEG for the analysis of source systems influenced by alcohol and other drugs.

Magnetic localisation of intracranial dipoles: simulation with a physical model.

Current dipoles energized by isolated sources were located in known positions inside a human skull filled with an electrically conductive medium. Maps of the measured electrical and magnetic fields confirmed the predicted relationships between those fields for both single and multiple dipoles. Two methods of dipole localisation were compared: the peak-location method which used only the locations of the maximum and minimum recorded values, and a least-squares iterative method which found the parameters for a dipole such that the sum of squared differences between the recorded and predicted data was minimized. Also, in an attempt to account for some of the error due to the non-sphericity of the head, the measured distance from the centre of the skull to each recording position was used in the dipole calculations. This last technique resulted in the smallest 3-dimensional location error (averaging 3.5 mm) for the least-squares method, even when no recording positions were near the actual field extrema and the peak-location method therefore produced much greater error. Also investigated were combinations of two dipoles for which the magnetic field maps appeared similar to those for a single dipole and comparisons were made to determine how well single and double dipole models could account for the recorded data.

The use of a squid third order spatial gradiometer to measure magnetic fields of the brain.

It appears to be clear from the results that the third order gradiometer is able to detect small biomagnetic signals from the brain which are related to evoked potentials and spontaneous electrical activity. The instrument operates reasonably well within a noisy environment, however further development is necessary to balance the first gradient. We intend to pursue this direction with software systems. Some of the data presented suggest that components of MEG evoked activity may change independently of EEG. One interpretation which may derive from this is that the same current dipoles are probably not responsible for the entire configuration of evoked fields. This interpretation is consistent with EEG evidence which indicates that analogous components in the evoked potential may vary independently as a function of stimulus parameters and information processing. Perhaps a model of magnetic dipoles due to small current loops would be more compatible with the electrophysiological data.

Movement-related potentials preceding toe plantarflexion and dorsiflexion.

The cerebral potentials associated with self-paced rapid dorsiflexion and plantarflexion of the left toes were compared in the same experiment using 10 subjects 5 of whom were female and 5 of whom were male. The EEG was recorded from C3, C1, Cz, C2 and C4 including computed bipolar recordings C4/C3 and C2/C1. For both kinds of movements, the Bereitschaftspotential (BP) or readiness potential was bilaterally symmetrical in the first half of the foreperiod. In the later foreperiod, an ipsilateral preponderance of negativity (IPN) developed. The direction of toe movement had no effect on the IPN. The etiology of the paradoxical BP side preponderance prior to toe movements is discussed which can probably be referred to the unique dipole orientation of a generator situated on the mesial cortical surface in the depth of the interhemispherical fissure.

Magnetic fields of the human brain (Bereitschaftsmagnetfeld) preceding voluntary foot and toe movements.

Voluntary movements are preceded by a slow electrical potential of the brain (Bereitschafts-potential, BP) or readiness potential. The BP is accompanied by a magnetic field shift of similar time characteristics (Bereitschaftsmagnetfeld, BM). The BM preceding volitional right foot or toe movements was recorded from anterior, posterior, and lateral positions of the scalp using a SQUID (Superconducting Quantum Interference Device) third-order gradiometer. Controls were implemented to reduce head movements, which were simultaneously recorded with a mechanograph. The results showed that movements of the lower extremities are also preceded by a BM. However, contrary to finger movements, BMs with field lines directed into the head were found predominantly for foot movements and exclusively for toe movements. The BM preceding foot movements was maximum over a position 2 cm left of the vertex, i.e., contralateral to the movement. Two centimeters right of the vertex it was smaller, thus exhibiting a normal contralateral preponderance and not sharing the paradoxical side preponderance of the electrical BP preceding foot or toe movements. The BM preceding toe movements was only apparent at the vertex and was smaller than the one preceding foot movement. This may suggest a source that is located still deeper in the brain than with foot movements.

Magnetic fields of the human brain accompanying voluntary movement: Bereitschaftsmagnetfeld.

A slow magnetic field shift has been detected in the human brain occurring in the foreperiod of a voluntary finger movement. This magnetic field accompanies a slow negative electrical cerebral potential which occurs in the same foreperiod, the Bereitschaftspotential (BP) of Kornhuber and Deecke. The present report is the first of a magnetic field associated with the BP, and has been named the Bereitschaftsmagnetfeld (BM) or readiness magnetic field. The BM is oriented with the field lines directed out of the head in the pre-rolandic region and with the field lines directed into the head in post-rolandic areas, suggesting a source in the sensorimotor area for the contralateral hand. Distribution of the magnetic fields has so far not revealed a source in the fronto-central midline where the BP is recorded maximally. The time course and morphology of the BP and BM are similar, but they have different topography over the skull.

A weight illusion produced by lifting movements.

The perceived relative heaviness of objects was manipulated by instructing 13 subjects to lift them gently or vigorously, as was predicted by the motor theory of weight judgment and the known behavior of proprioceptors. The time-order error was shown to be significantly related to lifting movements.