H. Weinberg

A MEG analysis of the P300 in visual discrimination tasks

Based on recent research that indicated that P300 scalp topography varies as a function of task and/or information to be processed, this study examined scalp-recorded magnetic fields correlated with the P300 by means of whole-head magnetoencephalography. Subjects performed two discrimination tasks, in which targets, defined on either object or spatial characteristics of the same visual stimuli, had to be discriminated. Based on the across-subject root mean square (RMS) functions a sequence of 4 components could be identified in both tasks, N1m, P3m, and two later components, which, based on their estimated neuronal sources, were classified as representing motor processes during and following the manual responses to target stimuli. Reliable between-task differences in source localization were obtained for the P3m component, but not for the other components. Inferior-medial sources were found for the P3m evoked by both spatial and object targets, with these sources being located about 3.5 cm more anterior for object targets. These results suggest that different neuronal sources, possibly located in subcortical regions in the vicinity of the thalamus, contribute to the P3m evoked by target stimuli defined by either object or spatial stimulus characteristics.

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.

Topographic Distribution of the 40 Hz Auditory Evoked-Related Potential in Normal and Aged Subjects

SummaryGalambos, Makeig and Talmachoff (1981) described what they called the 40 Hz event-related potential (ERP). This steady-state response is an EEG following response to repetitive auditory stimulation which becomes sinusoidal in form and maximal in amplitude at rates between 35 and 45 Hz. The present study was designed to examine the scalp topography of the 40 Hz ERP in order to complement previous magnetoencephalographic studies which implicate auditory cortex in the generation of the response. In addition, this study was designed to collect normative data on an aged sample in order to assess the effects of aging on the response. 40 Hz ERPs were recorded from a group of seven audiometrically and neurologically normal elderly subjects (mean age = 69.6 years) and a younger group of five normal adults (mean age = 38.0 years), using 1000 Hz tones presented binaurally at 40 per second. A 21 channel recording system was used to obtain a comprehensive picture of the scalp distribution of the response. Recorded ERPs were Fourier transformed to enhance the signal-to-noise ratio. No significant differences were found in phase or amplitude of the 40 Hz ERP between the two age groups, indicating that the normal aging process does not have an effect on this response. Topographic maps of the 40 Hz ERP showed reversals of electrode potential in temporal regions, supporting an interpretation of bilateral sources in temporal cortex. The data presented in this study complement previous studies of the 40 Hz event-related magnetic field and support the position that temporal cortex is involved in the generation of the response.

The frontal CNV: Its dissimilarity to CNVs recorded from other sites

Two new techniques were used to study the distribution of the CNV recorded from the scalp of humans. One technique was for the suppression of eye movements and the elimination of them fron the average data. The other technique was for the recognition of differences in the form of CNVs recorded from different sites. The latter technique utilized an average vertex CNV as a template pattern against which CNVs from the vertex, left and right frontal, central and parietal sites were matched. The results showed that the frontal CNV was different in form from the vertex, central and parietal Cnvs and that the vertex, central and parietal CnvS were similar in form. An analysis of the amplitudes of CNVs showed that the frontal CNVs were significantly smaller than the CNVs from all other sites, that the vertex CNV was significantly larger than the parietal or central CNVs and that the central CNV was significantly larger than the parietal CNV. There were no differences in amplitude between hemispheres within Condition A responding with the right hand) or Condition B (responding with the left hand) or between Conditions A and B. The results are discussed with respect to their possible theoretical implications.

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.

Measurements of extremely low frequency brain magnetic fields associated with four-tone memory processes

Recently magnetic-field measurements of the brain have become a useful tool to study higher brain function. The purpose of this study is to propose source models of both the memorizing process and the recognition process. The components of magnetic fields perpendicular to the surface of the head were measured by using a whole cortex type of DC SQUID with a third-derivative gradiometer. The authors observed the extremely slow components of MEG activities during both the memorizing process and the recognition process. They estimated the sources using two current dipoles, and compared the two cases.

The biomagnetic inverse problem: some theoretical and practical considerations

A method is described which estimates the location of current dipoles in the brain. The estimate establishes the best fit, according to stated criteria, between observed magnetic fields measured with a third-order gradiometer and fields predicted from one or two dipoles. The method takes into account corrections for the number, spatial separation and size of the gradiometer coils, and for the inclination of the axis of the gradiometer to the radius from a computed origin in the head to the centre of the sensing (first) coil, or to the position of observation on the head. The method also provides for a means to establish the best initial guess for a dipole location and contains procedures for estimating the positions of more than two dipoles in appropriate cases. Several relevant theoretical considerations are discussed and a number of applications of the method are described.

Functional imaging of brain responses to repetitive sensory stimulation: sources estimated from EEG and SPECT.

SummaryThis study examined the effect of modality of stimulation on two measures of cerebral function: (a) the scalp distribution of sensory evoked potentials and (b) the cerebral distribution of radiolabelled HMPAO. Steady-state stimulation in the auditory, somatosensory or visual modality was presented to six subjects. Scalp potentials were measured from a distribution of electrodes, and the radiopharmaceutical was injected through an indwelling intravenous catheter midway through the stimulation/ recording session. Equivalent dipole sources estimated from the spatial distribution of the scalp potentials were found to be consistent with regions of high HMPAO uptake as imaged by Single Photon Emission Computed Tomography (SPECT).

The contingent negative variation (CNV) and speech production: slow potentials and the area of Broca.

The lateral distribution of the CNV was investigated during conditions that required normal male and female subjects to vocalize simple English words in a CNV paradigm. In addition to a non-verbal or comparison condition (Tone-Clicks/Button Press), two word conditions presented stimulus words in either S1 or S2 positions (Word-Tone/Speak; Tone-Word/Speak) and a third condition required a one-word association to the stimulus words (Word-Tone/Speak Association). Monopolar scalp activity was collected from sites that included a location approximating Brocas speech area on the left hemisphere, a homologous comparison site on the right hemisphere, frontal sites (F3, F4) and the vertex (Cz). Although several subjects showed some asymmetrical activity in some of the word conditions, amplitude measures based on CNV averages indicated no consistent lateralization effects over the left hemisphere prior to word vocalizaiton. Negative pretrial shifting was suspected in those conditions in which words appeared as S1 signals.

MEG and EEG evoked by verbal cognitive tasks

We observed the brain electrical activities associated with verbal cognitive processes by measuring both EEGs and MEGs evoked by visually cognitive tasks. English words, nonsense words and random dots were presented. We discussed the source estimation at latencies of 150 msec and 360 msec using MEG data. At 150 msec, two equivalent current dipoles are localized in or near the primary visual cortex. The dipole sources for recognition of English words at 360 msec were estimated in both hemispheres, and the dipole moment in the left hemisphere was larger than the dipole moment of the right hemisphere. However, we could not find sources common to all subjects.