Source localization of averaged and single EEG spikes using the electric dipole model

Studying spontaneous EEG activity with fMRI

The multifaceted technological challenge of acquiring simultaneous EEG-correlated fMRI data has now been met and the potential exists for mapping electrophysiological activity with unprecedented spatio-temporal resolution. Work has already begun on studying a host of spontaneous EEG phenomena ranging from alpha rhythm and sleep patterns to epileptiform discharges and seizures, with far reaching clinical implications. However, the transformation of EEG data into linear models suitable for voxel-based statistical hypothesis testing is central to the endeavour. This in turn is predicated upon a number of assumptions regarding the manner in which the generators of EEG phenomena may engender changes in the blood oxygen level dependent (BOLD) signal. Furthermore, important limitations are posed by a set of considerations quite unique to 'paradigmless fMRI'. Here, these issues are assembled and explored to provide an overview of progress made and unresolved questions, with an emphasis on applications in epilepsy.

Spatial mislocalization of EEG electrodes -- effects on accuracy of dipole estimation

OBJECTIVE

The estimation of cortical current activity from scalp-recorded potentials is a complicated mathematical problem that requires fairly precise knowledge of the location of the scalp electrodes. It is expected that spatial mislocalization of electrodes will introduce errors in this estimation. The present study uses simulated and real data to quantify these errors for dipole current sources in a spherical head model.

METHODS

A 3-dimensional digitizer was used to locate the positions of 31 scalp electrodes placed on the head according to the 10-20 system in 10 normal subjects. Dipole localizations were performed on auditory evoked potentials (AEPs) collected from these subjects.

RESULTS

Computer simulations with several dipole source configurations suggest that errors in locations and orientations on the order of 5 mm and 5 degrees, respectively, are possible for electrode mislocalizations of about 5 degrees. In actual experimental settings, digitized electrode positions were typically mislocalized by an average of about 4 degrees from their standard 10-20 positions on a spherical model. These differences in electrode positions translated to mean differences of about 8 mm in dipole locations and 5 degrees in dipole orientations.

CONCLUSIONS

Dipole estimation errors due to electrode mislocalizations are within the limits of errors due to other modeling approximations and noise.

Computer-based electroencephalography: technical basics, basis for new applications, and potential pitfalls

EEG has been recorded on paper-based analog systems for over 50 years. In the past 5 years, computer-based digital systems have become more widely used. Digital systems eliminate some artifacts that plagued analog recordings but introduce subtle new problems including aliasing and dynamic range. Digital systems allow reformatting of the same EEG segment using different gain, filter and montage settings. The digital signal allows for measurement and computations on the EEG, leading to applications such as power spectrum, topographic mapping, and spike or seizure detection.