Stephen B. Baumann

Localization of the P3 sources using magnetoencephalography and magnetic resonance imaging

In this study, two related issues were addressed: first, whether the P3 component of auditory evoked responses, obtained in the context of an oddball paradigm, and its magnetoencephalographically recorded counterpart (P3m) are generated by the same intracranial sources; and, second, whether these sources, modeled as equivalent current dipoles, can be localized in particular brain structures using magnetic resonance imaging. The study involving 8 normal adult subjects resulted in the following findings. (1) Both the similarities and differences in wave form characteristics of the simultaneously recorded P3 and P3m can be best accounted for by common intracranial sources. (2) Several successively activated single-dipolar sources, rather than a single source, account for the entire evolution of the P3m component. (3) Most of these sources were localized in the vicinity of the auditory cortex in all subjects, although some sources appeared to be in deeper structures, possibly the lateral thalamus. (4) The successive activation of sources followed an orderly medial-to-lateral course. These results suggest that activity responsible for the surface-recorded P3 (and P3m) component may be initiated in deep structures, but it quickly spreads over and is sustained in areas near the auditory cortex.

Neuromagnetic evidence of a dynamic excitation pattern generating the N100 auditory response

Evoked magnetic field recordings were used to localize multiple sources of the negative component of cortical responses to auditory stimuli. The negative cortical component of the auditory evoked response, often called the N100, has traditionally been of interest due to its sensitivity to both stimulation parameters and cognitive variables. Results indicate that this component appears to reflect spreading activation of adjacent cortical columns within the primary projection area of the temporal lobe, extending anteriorly for about 1 cm following the downward slope of the superior surface of the lobe.

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 anteriorinferior 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.

Late magnetic fields and positive evoked potentials following infrequent and unpredictable omissions of visual stimuli

Randomized and infrequent omissions during presentation of a steady train of visual stimulation produced distinctive wave forms of both the magnetic fields and electrical potentials. Electrical potentials at Pz showed a positive peak in response to the omitted stimuli which occurred on the average 445 msec after the time when a stimulus was anticipated. Analyses of the magnetic wave forms indicated that at least two separate sources appear to be active coincident with the electrical positive peak. One source localized in the occipital lobes in the vicinity of the visual cortex while the other source was located in the medial aspects of the temporal lobe or even deeper in the lateral thalamus. Judging from the calculated direction of current flow it appeared that the deep source would contribute greater potentials in the frontal areas of the scalp while the source in the occipital area would contribute to more posterior placement of electrodes, especially at Pz.

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.

Source localization of two evoked magnetic field components using two alternative procedures

SummaryThe purpose of this study was to compare the relative efficacy of two methods in assessing the location of the sources of the N100 and P200 components of evoked magnetic fields (EMFs) to transient tone stimuli. EMFs to left ear stimulation, containing both components, were recorded over the right hemisphere of six normal subjects. The magnetic scalp distributions calculated at several adjacent time points, covering the duration of each components peak, were used to estimate the source parameters of each component. Good estimates of the source of both components were obtained from all magnetic field distributions. The averaged spatial parameters derived from all distributions of each component as well as the parameters derived from the distribution that gave the best source estimate for each component were projected onto magnetic resonance images of subjects head. It was found that the source of each component is located on the superior surface of the temporal lobe and that the source of the P200 component is anterior to the N100 source in all subjects using both procedures.

Intersession replicability of dipole parameters from three components of the auditory evoked magnetic field

SummaryThe replicability of dipole localizations between sessions in an unselected group of subjects was studied. Auditory evoked magnetic fields (AEMFs) in response to contralaterally and ipsilaterally presented 1 kHz tone bursts were recorded from the right hemisphere of 12 subjects with normal hearing in two replicate sessions several days apart. Three long-latency components of the AEMF were studied, occurring at latencies near 50 msec (P1m), near 100 msec (N1m) and near 165 msec (P2m). A spherical model of the head was used to fit equivalent-current dipoles to the data. Statistical analysis of dipole parameters revealed virtually no differences between the two testing sessions. The variability between sessions had a mean absolute difference of 3 to 10 mm for the spatial parameters. Comparison of dipole parameters between components showed that there was a replicable, but nonsignificant, trend for a difference in the location of the N1m from contralateral vs. ipsilateral stimulation, and a statistically significant confirmation that the P2m is located anterior to the N1m for contralateral stimulation. Magnetic resonance images from each subject were used to locate the dipoles near the primary auditory cortex in the Sylvian fissure.

Replicability for Localization of the P1, N1, and P2 Components of the Auditory Evoked Response in an Unselected Group of Subjects

With few exceptions (Yamamoto et al, 1988) intrasubject replications have not been performed to test the reliability of dipole localization. Furthermore, although several studies have examined either the P1, N1 or P2 components of the auditory evoked magnetic field (EMF), most studies have localized one component in only a few subjects. Consequently, it is difficult to determine whether separate sources are responsible for these components. Also, only a few studies (Reite et al, 1988; Yamamoto et al, 1988) have included magnetic resonance images (MRIs) of each subject’s brain onto which the location of the computed dipoles can be superimposed. We have begun a systematic effort to address these issues by testing a series of subjects at least twice and by superimposing dipole locations onto individual MRIs.

Nonstationary Dynamics of Sequential Magnetic Dipole Source Changes Associated with N100 Auditory Evoked Responses

The source of the N100 component of auditory evoked magnetic fields (EMFs) has been localized in the auditory cortex in a number of MEG studies (see Hari, 1987 or Naatanen & Picton, 1987 for review). In those studies, equivalent dipole parameters were calculated for the magnetic field at a particular single latency during the time course of the N100 component (i.e. the latency at which the N100 reaches the highest amplitude at the two extrema or the one at which the isofield maps appear to have most distinct dipolar pattern). Yet, good dipolar patterns are often observed for a range of latencies reflecting the fact that not all EMFs recorded at the extrema reach maximal peak amplitude at precisely the same time. This suggested the possibility that the N100 may not be due to a stationary equivalent dipole but to a series of sources, a suggestion which has been supported by previous evoked potential studies (e.g. Wood and Wolpaw, 1982). In this experiment, we investigated that possibility as well as the question as to whether the N100 sources of EMFs resulting from ipsilateral and contralateral ear stimulation are distinct.