The studies above employed MEG to examine auditory brain activity. Given the potential utility of millisecond scale electrophysiological sensitivity, along with a typically-argued sensitivity of MEG and EEG to common neural generators, it is worth considering which of the above experimental approaches might be accomplished using EEG, given that EEG is a much less expensive technology. In addition, cognitive and clinical EEG labs increasingly obtain high-density EEG, recording from 64, 128, or even more electrodes, thus providing spatial coverage of the head similar to that obtained with current whole-head MEG systems. Despite the ability to obtain spatial coverage similar to MEG, however, fundamental physical considerations of electric and magnetic fields, as well as the location and orientation of primary and secondary auditory sources, underlie the different capabilities and limitations of MEG and EEG. The following paragraphs briefly illustrate differences between EEG and MEG which, in the auditory studies above, generally argue for the use of MEG over EEG.
A primary argument supporting the preferential use of MEG over EEG is the more straightforward identification of left and right hemisphere auditory activity. The measured signals (electrical potentials in EEG and magnetic fields in MEG) are distant from the neural generators and separated by tissues of the brain, CSF, skull, and scalp. These tissues have widely differing electrical conductivities, which mediate the scalp recorded electrical potentials and make them extremely sensitive to tissue composition and geometry. Thus, recorded electric potentials do not have a simple model to the source generator, distorted by the various intervening tissues. The magnetic permeabilities of these tissues are however similar, such that the pattern of magnetic fields recorded at the sensor positions does not depend significantly upon the intervening tissue composition. As such, simpler models may legitimately be used for source estimation. In casual parlance, some refer to the head and brain as “transparent” to magnetic fields. Thus the measures of M100 source asymmetry as discussed above might be expected to be less easily resolved using EEG compared to MEG.
Of importance to many of the applications and methodological strategies discussed above is the ability to identify, with appropriate precision, the latency of left and right hemisphere ERP/ERF components. Fundamental differences in the sensitivity of EEG and MEG often lead to a decision to choose one over the other modality, or to perform simultaneous recordings. For example, for recording auditory responses from the brainstem (e.g. BAEP, brainstem auditory evoked potentials), the superior sensitivity of EEG makes it a better choice than MEG. Specifically, although such far-field potentials are easily recorded with EEG, given the distance of the sources from the scalp they are difficult to record using MEG. On the other hand, MEG is well suited for studying individual superior temporal gyrus (STG) sources (e.g., see Edgar et al., 2003
, Huang et al., 2003
), which are typically active ~50–100ms post stimulus. With EEG, bilateral STG sources from the left and right hemisphere generate a maximum electric potential distribution on the top of the head (near Cz), and a minimum potential somewhere near the chin and neck area, a region where electrodes are generally not placed. Accordingly, only one pole of the electric field is measured, and when considering bilateral STG sources, recorded activity from mid- and near-midline vertex electrodes will reflect the combined activity from the two STG sources. Such conditions make localization and study of the individual STG generators using the traditional EEG montages difficult and indeed lead to temporally blurred “combined” responses if temporal morphology differences exist between individual hemispheric responses.
An example of the difficulty of separately scoring left and right primary auditory activity with EEG is shown in . 275-channel MEG and 60-channel EEG were recorded from a single subject. shows MEG and EEG responses to a 200 Hz tone presented to the right ear for right MEG temporal sensors (top), left MEG temporal sensors (middle), and electrode Cz (bottom). The peak of the left hemisphere 100 ms contralateral response is observed at 116 ms (first solid vertical line). As expected, the right hemisphere ipsilateral response is delayed, occurring ~20 ms later (second solid vertical line). Maximal 100 ms activity at Cz is observed at 119 ms. Although the latency of Cz response is similar to the latency of the MEG left hemisphere response, Cz does not provide a clean measure of the left hemisphere activity, as left and right hemisphere activity linearly sum at the EEG midline sites. As the right hemisphere response is not observed as a distinct Cz peak, differentiating left from right hemisphere activity at Cz is not possible. Whereas EEG topography plots indicate greater right hemisphere activity at right hemisphere EEG sites (suggesting a better measure of right hemisphere activity could be obtained at more lateral EEG sites), the EEG topography plots do not indicate a similar lateral shift that would allow for easy identification of a distinct left hemisphere response. In contrast, as shown in the MEG topography plots, left and right MEG auditory activity is observed as distinct dipolar fields over each hemisphere.
Figure 4 MEG and EEG responses to a 200Hz sinusoidal tone presented to the right ear of a healthy adult volunteer. 275-channel MEG and simultaneous 64-channel EEG was used. Responses are shown for right MEG temporal sensors (top), left MEG temporal sensors (middle), (more ...)
This 200Hz dataset provides an example of the difficulty inherent in using EEG voltage maps to assess auditory activity. Although interpreting voltage maps is problematic, assessing left and right EEG activity may be possible. For EEG, an alternative approach to locating generators of early auditory left and right hemisphere activity is to improve the spatial resolution of scalp EEG. To this end, high-resolution EEG methods such as skull current density estimates by means of a surface Laplacian algorithm are useful (Perrin et al., 1989
). The surface Laplacian is the second spatial derivative of the voltage distribution in tissue and estimates the volume current flow out of the brain through the skull into the skin (also referred to as current source density or scalp current density maps). shows the surface Laplacian for the 100 ms EEG auditory response. In contrast to the EEG potential map (), clear source and sink peaks are observed over each hemisphere, reflecting the existence of superficial and focal left and right hemisphere activity (because the STG sources are primarily tangential (in a fissure)), the surface Laplacian is weaker than a surface Laplacian obtained from a radial source (Srinivasan, 2005
Figure 5 The surface Laplacian emphasizes superficial, localized sources. The surface Laplacian for the 100 ms EEG auditory response shows clear source and sink peaks over each hemisphere. As such, in this subject, separate scoring of left and right STG activity (more ...)
Whereas most EEG studies compare control and patient potential activity only at midline sites (e.g., Fz, Cz, and Pz), high-resolution EEG methods require a more dense sampling of the head surface. Giard et al. (1994)
used high-resolution EEG to distinguish left and right 100 ms STG activity, and radially oriented frontal activity also was observed. The frontal finding in the Giard et al. (1994)
study highlights a limitation of MEG. As MEG is somewhat insensitive to radially oriented neural generators (Lewine & Orrison, 1995
), radial frontal sources may invisible to MEG. In addition to early 100 ms activity, a radial STG source detected with EEG is present at approximately 140 ms (Wolpaw and Penry, 1975
; Picton et al., 1999
). Such findings indicate the need, in some instances, to obtain simultaneous EEG and MEG.
A complete review of the strengths and limitations of high-resolution EEG techniques is beyond the scope of this chapter. Srinivasan (2005)
provides a detailed but accessible review of high-resolution EEG. One comment, however, is in order. For high-resolution EEG, electrode spacing of ~2 cm is thought to be ideal (Srinivasan et al., 1999). Given the availability of 64 or more channels EEG systems in many clinical and research labs, the use of high-resolution analysis to examine auditory processes in controls and patients is possible. Whole-head EEG (required for high-resolution EEG), however, requires placing many electrodes on the surface of the head. In some subjects with ASD, this may not be feasible. In particular, as correct placement of a whole-head EEG cap may take up to thirty minutes and may not be feasible in subjects hypersensitive to touch.
MEG has an advantage in this regard, as MEG is less physically invasive (requiring the placement of only 3–4 head coils). This advantage comes, however, at a cost. Because MEG sensors are located at a distance from the head, subjects need to stay still during the entire MEG exam so the position of their head with respect to MEG sensors remains constant. As a typical auditory task lasts 10 to 20 minutes, many patient subjects may be unable to hold still during the exam. Although the newest generation of MEG systems provide the ability to correct for head motion, current head motion correction procedures are robust only for a limited range of movement (perhaps 2.5 cm or less).