1 Basics of EEG: Generation, Acquisition, and Applications of EEG
5
potentials in a small cortical area can induce extracranial electric fields large enough
to be measured on the scalp surface [1]. According to Hämäläinen et al. [10], the
current density on the cortical surface is approximately 100 nA/mm
2 . When numerous cortical neurons within a small area are activated synchronously, a unidirectional
neuronal current flow is formed. Figure 1.1 depicts the comparison between action
potential and postsynaptic potential, as well as a schematic illustration of the generation of the unidirectional neuronal current flow.
The unidirectional neuronal currents, which can be approximately modeled as
equivalent current dipoles (ECDs) in EEG source imaging problems [6] (see Chap. 5
for more details), are called primary or impressed currents [22]. Since the human
body is filled with electrically conductive media, the extracellular currents induced by
the primary currents can flow even to the farthest part of the human body. These extracellular currents are known as secondary, volume, or return currents [22]. According
to the electromagnetic theories, the flow of the secondary currents results in nonuniform potential distributions on the scalp. The measurement of the potential difference
between two distant scalp locations over time is the EEG.
Because the EEG measures dynamic changes in potential differences originating
from the secondary current flows, precise evaluation of conductivity profiles of the
volume conductors, i.e., different tissue compartments inside the head, is important,
not only to understand the underlying mechanisms of the EEG, but also to build a
precise head model to calculate electric field quantities generated by primary neuronal currents (this process is called forward calculation). A human head can be
roughly modelled with four different regions: brain, cerebrospinal fluid (CSF), skull,
and scalp. Table 1.1 shows the typical conductivity values when the conductivity of
each region is assumed to be isotropic (having uniform conductivity in all directions)
and homogeneous [9]. The most notable point in the conductivity profile shown in
Table 1.1 is that the conductivity value of the skull is even smaller than those of the
other tissues. Because of the poor electrical conductivity of the skull, the secondary
currents are severely distorted and/or attenuated before they are delivered to the scalp
surface. Since the tissue conductivity is an important factor affecting the reliability
and accuracy of EEG source imaging, anisotropic conductivity characteristics are
sometimes considered. For example, the skull has an anisotropic conductivity property, approximately 0.014 and 0.0107 S/m for the directions normal and tangential to
the skull surface, respectively [2]. White matter tissues also have an anisotropic conductivity property: the white matter conducts secondary currents much better along
a fiber direction than in its transverse directions [31]. In practice, however, a rough
approximation of the human head structure as piecewise isotropic and homogeneous
volume conductors (e.g., brain, CSF, skull, and scalp) is most widely used. More
detailed discussion of this topic is provided in Chap. 5.
5
potentials in a small cortical area can induce extracranial electric fields large enough
to be measured on the scalp surface [1]. According to Hämäläinen et al. [10], the
current density on the cortical surface is approximately 100 nA/mm
2 . When numerous cortical neurons within a small area are activated synchronously, a unidirectional
neuronal current flow is formed. Figure 1.1 depicts the comparison between action
potential and postsynaptic potential, as well as a schematic illustration of the generation of the unidirectional neuronal current flow.
The unidirectional neuronal currents, which can be approximately modeled as
equivalent current dipoles (ECDs) in EEG source imaging problems [6] (see Chap. 5
for more details), are called primary or impressed currents [22]. Since the human
body is filled with electrically conductive media, the extracellular currents induced by
the primary currents can flow even to the farthest part of the human body. These extracellular currents are known as secondary, volume, or return currents [22]. According
to the electromagnetic theories, the flow of the secondary currents results in nonuniform potential distributions on the scalp. The measurement of the potential difference
between two distant scalp locations over time is the EEG.
Because the EEG measures dynamic changes in potential differences originating
from the secondary current flows, precise evaluation of conductivity profiles of the
volume conductors, i.e., different tissue compartments inside the head, is important,
not only to understand the underlying mechanisms of the EEG, but also to build a
precise head model to calculate electric field quantities generated by primary neuronal currents (this process is called forward calculation). A human head can be
roughly modelled with four different regions: brain, cerebrospinal fluid (CSF), skull,
and scalp. Table 1.1 shows the typical conductivity values when the conductivity of
each region is assumed to be isotropic (having uniform conductivity in all directions)
and homogeneous [9]. The most notable point in the conductivity profile shown in
Table 1.1 is that the conductivity value of the skull is even smaller than those of the
other tissues. Because of the poor electrical conductivity of the skull, the secondary
currents are severely distorted and/or attenuated before they are delivered to the scalp
surface. Since the tissue conductivity is an important factor affecting the reliability
and accuracy of EEG source imaging, anisotropic conductivity characteristics are
sometimes considered. For example, the skull has an anisotropic conductivity property, approximately 0.014 and 0.0107 S/m for the directions normal and tangential to
the skull surface, respectively [2]. White matter tissues also have an anisotropic conductivity property: the white matter conducts secondary currents much better along
a fiber direction than in its transverse directions [31]. In practice, however, a rough
approximation of the human head structure as piecewise isotropic and homogeneous
volume conductors (e.g., brain, CSF, skull, and scalp) is most widely used. More
detailed discussion of this topic is provided in Chap. 5.