Cortical Functional Mapping by High-Resolution EEG - Quantitative EEG Analysis Methods and Clinical Applications - page 357

Figure 13.1 Integration between EEG scalp electrode positions and a realistic head model gener-
ated using the T1-weighted MR images of the subject. When MRIs of a subject's head are not avail-
able, it is still possible to coregister the electrode positions employed for the EEG recording with an
average head model, a standard head taken as an average of the MRIs of 150 subjects. Such an aver-
age head model is available from the McGill University Web site.
13.2 The Solution of the Linear Inverse Problem: The Head Models
and the Cortical Source Estimation
The ultimate goal of any EEG recording is to produce information about the brain
activity of a subject during a particular sensorimotor or cognitive task. When the
EEG activity is mainly generated by circumscribed cortical sources (i.e.,
short-latency evoked potentials/magnetic fields), the locations and strengths of
these sources can be reliably estimated by the dipole localization technique [3, 4]. In
contrast, when the EEG activity is generated by extended cortical sources (i.e.,
event-related potentials/magnetic fields), the underlying cortical sources can be
described using a distributed source model with spherical or realistic head models
[5-7]. With this approach, typically thousands of equivalent current dipoles cover-
ing the cortical surface modeled and located at the triangle center are used, and their
strengths are estimated using linear and nonlinear inverse procedures [7, 8]. Taking
into account the measurement noise n , supposed to be normally distributed, an esti-
mate of the dipole source configuration that generated a measured potential b can
be obtained by solving the linear system: