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32 S. Viswanathan
biology. Therefore, it can be useful to highlight two distinct perspectives on why
neuroanatomy is practically relevant for EEG.
EEG Signal Generation by the Brain The first perspective is centered on EEG as
an electrophysiological measure that originates in the brain (from Buzsaki et al.,
2012) (see Chap. 1: “EEG in Context: Past, Present, and Future”). Several electrical
charact
eristics of the EEG signal depend on neuroanatomical features. For example,
the EEG signal arises due to micro-anatomical features such as the laminar (i.e.,
layered) structure of cortical gray matter and the neuron distribution and orientation
in these layers. This electrical origin determines the EEG signal’s high temporal
sensitivity but also its small magnitud e (in the microvolt range). Furthermore, the
spatial resolution of EEG is affected by features such as the convoluted shape of the
cerebral gray matter relative to the various layers that separate the brain from the
scalp. Therefore, an understanding of the EEG signal is incomplete without a role for
anatomical features. The role of these features is an important part of advanced
analyses methods such as source analysis that use detailed models of brain and head
anatomy to estimate the neural sources of measured EEG (see Chap.
Source
Analysis”).
20: “EEG
The Brain as Part of the Nervous System The second perspective comes from the
applic
ation of EEG to gain information about nervous system function (and dysfunction) in relation to various capabilities (e.g., language, movement, perception,
etc.) and physiological states (e.g., sleep, wakefulness, arousal, rest) in different
populations (Cohen,
treated
as an extended network to detect, process, and use information about the
2017). In this application context, the nervous system is often
environment and the organism’s own state. For instance, the human nervous system
is broadly subdivided into the central nervous system (CNS) (comprising the brain
and spinal cord) and the peripheral nervous system (PNS) (Fig. 3.1a). Typical
lities (such as perception and action) require extensive interactions within
capabi
Fig. 3.1 (a) Organization of the central and peripheral nervous systems. The CNS consists of the
brain and spinal cord. The PNS consists of an elaborate system of nerves that connect the periphery
to the CNS (read more in Chap.
the pathway of the optic nerve (part of the PNS) that carries visual information from the retina to the
occipital lobe at the posterior of the brain
4: “Basic Anatomy: Peripheral Nervous System”). (b) Schematic of
3 Basic Anatomy: Central Nervous System 33
and between the CNS and PNS (see example below). Therefore, understanding the
relevance of EEG signals requires the broad anatomical context of the nervous
system. This anatomical context is also important for multimodal approaches such
as the simultaneous recording of EEG with functional magnetic resonance imaging
(fMRI) (Warbrick,
as transcranial magnetic stimulation (TMS) (Lioumis & Rosanova,
such
As an example, consider a common scenario where EEG is recorded while
participants are presented with a visual stimulus. A typical assumption is that the
early EEG responses to the stimulus would be measurable at channel locations at the
back of the head (e.g., Oz, O1, O2, PO1, PO2). This assumption depends on
anatomical considerations. Specifically, sensory information received by the eyes
is transmitted via the optic nerve (a part of the PNS) to the posterior regions of the
brain (a part of the CNS) (Fig.
the posterior brain is the hub for visual sensory processing, and it is even
that
referred to as the visual cortex. Therefore, taking anatomy into account can be
valuable in planning what and where to measure, and how to interpret the results.
Motivated by the latter perspective, in this chapter (and the next), we provide a
atic guide on how to include a neuroanatomical perspective in your EEG
pragm
research. This chapter will focus on the central nervous system (CNS). The next
chapter (Chap.
Here, we provide an overview of a few useful concept s related to the CNS
anatom
y. Major technological advances over the past few decades have made
neuroanatomical information conveniently accessible via a variety of digital tools.
Familiarity with basic concepts can allow these tools to be used to include neuroanatomical considerations in an EEG study.
2022), and when combining EEG with brain stimulation methods
2022).
3.1b). Decades of functional studies have established
4) will focus on the role of the peripheral nervous system (PNS).
3.2 General Organization
The central nervous system consists of several structures. Figure 3.2a shows its
hierarchical organization and its major structures. These structures vary considerably
in relative size. For example, when we consider the external appearance of the brain,
what is prominently visible is the cerebral cortex. A variety of structures (such as the
thalamus and basal nuclei) are below the surface and referred to broadly as subcortical structures. Each of these structures contains further structural organization. We
recommend that readers consult an elementary textbook on neuroanatomy for more
information.
The cerebru
(Fig. 3.2b left panel) consists of two folded sheets of gray matter (the cortex) that
are
organized into two hemispheres (left and right). The white matter consists of
myelinated fibers that connect the neurons in the gray matter both locally and across
the brain. These structural connections have an extended organization, as illustrated
for the corpus callosum in Fig.
m is of measurement-related relevance to EEG. The cerebrum
3.2b (right panel).
34 S. Viswanathan
Fig. 3.2 (a) Chart showing the hierarchical organization of the human brain. (Adapted based on
Ward,
2015). The inset shows the main lobes of the human brain. (b) Left: Gray-scale MRI image of
adult human brain along three standard orthogonal planes (sagittal, coronal, axial) of a coordinate
an
system with an origin at the center of the brain. The gray matter and white matter are shown with
darker and lighter shades of gray respectively. Right: Fiber tracts of the corpus callosum linking the
left and right hemispheres. Tracts are colored by segment. (From Rosenbloom & Pfefferbaum,
2008). (c) Electrode position layout on the scalp according to the international 10–20 system
The two hemispheres are approximately mirror-symmetric along the mid-line.
Therefore, many structures have a left and right version. The raised surfaces are
referred to as gyri (singular: gyrus), and the lowered surfaces are referred to as sulci
(singular: sulcus). Based on certain prominent sulci typically observed across individuals, each hemisphere can be further subdivided into prominent lobes (Fig.
These lobes are the frontal lobe (behind the eyes), the temporal lobe (side of
inset).
3.2a,
the head or temples), the occipital lobe (lower back of the head), the parietal lobe
(upper back of the head), and the insula (under the frontal and temporal lobes).
Although individual brains share a similar species-typical organization, there can
considerable variability between individuals in the size of the brain and the exact
be
shape and location of the different structures. Therefore, standardized systems and
tools have been developed (and continue to be developed) to precisely specify the
locations of structures and features across individuals to increase the precision and
reproducibility of neuroscientific findings. An important concept to describe brain
locations is a brain atlas (discussed next).
3 Basic Anatomy: Central Nervous System 35
3.3 Finding Your Way Around: Brain Atlases
3.3.1 International 10–20 System
A useful starting point to understand a brain atlas is to consider the 10–20 system
used for EEG (Jasper, 1958). This convention defines standard locations on the scalp
for the placement of electrodes to accommodate inter-individual variation and
increase reproducibility. The 10–20 system uses distinctive landmarks on the skull
(nasion, inion, pre-auricular points) that can be typically identified for every individual (Fig.
locations
The coarse correspondence between the skull and the cerebral lobes is used to name
the scalp locations. Several extensions of this basic system have been develo ped,
such as the 10–10 and 10–5 system (Oostenveld & Praamstra,
The 10–20 system (and its variants) is based on an assumed geometric scaling
relations
ular head will be proportionally larger on a larger head and smaller on a smaller head.
Thus, anatomically comparable locations can be identified on the scalps of two
different people with different-sized heads. This geometric scaling rationale is also
the core concept in relating brain locations across individuals.
3.2c). The distances between these landmarks are then used to define
on the scalp using relative sizes specified as percentages (i.e., 10%, 20%).
2001).
hip. The assumption is that the distance between two locations on a partic-
3.3.2 Talairach Atlas and MNI Coordinates
Two major stereotactic systems for the brain are the Talairach coordinate system and
the Montreal Neurological Institute (MNI) coordinate system. They were initially
developed for neurosurgery where there is a demand for high precision in defining
the location of brain structures.
Previously, a physical frame was attached to the skull based on selected land-
s. Once secured, it was possible to move precisely to specific 3D points relative
mark
to this frame. These were called stereotactic frames. However, due to the imprecise
correspondence between the structure of the skull and that of the brain, the Talairach
coordinate system was developed based on landmarks directly defined in the brain. It
is defined by using two brain structures as landmarks (the anterior-commissure
(AC) and posterior-commissure (PC), which are white-matter bundles that connect
the right and left hemispheres) and a plane through the mid-sagittal line (
neuroima
ac.uk/imaging/MniTalairach). Using the resulting 3D coordinate system, the loca-
tion
coordinates.
system on each brain slice so that the location of structures can be precisely
specified. Using this approach, Talairach and Tournoux (Harary & Cosgrove,
ge.usc.edu/brainstorm/CoordinateSystems, https://imaging.mrc-cbu.cam.
of different structures within a single brain can be specified by its 3D
As shown
in Fig. 3.3a, the key idea is to place a grid defined by this coordinate
https://
36 S. Viswanathan
Fig. 3.3 (a) Example of a Talairach coordinate grid placed on a brain slice. (From Woodward et al.,
2008). (b) Example of overlaid geographic maps with different information enabled by a shared
coordinate system. (c) Map of cytoarchitecture properties at different locations across the brain as
described by Brodmann (
1909), now referred to as Brodmann areas
2019; Talairach & Tournoux, 1988) developed a detailed atlas of the entire brain
using a single post-mortem brain, that is, a template brain. Importantly, since the
procedure to define the coordinate system was based on landmarks, a comparable
coordinate system could be defined for any individual brain. Then a geometric
re-scaling could be used to bring the individual-specific coordinate system into
correspondence with the standard template brain (i.e., similar to the scaling assump-
–2
tions of the EEG 10
le, a lesion observed
examp
0 system described above). With such an approach, for
on an individual’s brain can be described relative to
standardized coordinates on the template brain.
The advent of MRI and digital representations of the brain have dramatically
increased the ease and precision of this approach. To increase its generality across
individuals, the Montreal Neurological Institute (MNI) coordinates were developed
using MRI images obtained from a large number of individuals (Evans et al.,
These
template brains serve as a shared standard to help increase the reproducibility
1993).
of research.
3.3.3 Enhanced Atlases: Cyto-anatomy,
Connectivity-Anatomy
The coordinate framework described above has enabled the mapping of diverse
types of anatomical information that can then be overlaid much like different
information on a geographic map (Fig. 3.3b). Some examples of these other anatomical
features are:
3 Basic Anatomy: Central Nervous System 37
. Cytoarchitecture: The use of cellular features (cell distribution, myelin, etc.) and
their variation to define “areas.” A famous example is the work of Brodmann
(
1909), who identified variations in cellular structure across the brain (Fig. 3.3c).
Brodmann areas and gradations in myelin distribution (myeloarchitecture)
These
define a set of locations that do not correspond to the sulcus/gyrus coarse structure
(Eickhoff et al., 2018; Fan et al., 2016; Amunts et al., 2020; Abdollahi et al.,
2014; Glasser et al., 2016).
. Tractography: The distribution and organization of white-matter tracts (Nozais
et
al., 2021; Salvalaggio et al., 2020; Thiebaut de Schotten et al., 2020).
. RS-Connectivity: Areas reliably identifiable by patterns of resting state connec-
(Eickhoff et al., 2018; Fan et al., 2016).
tivity
. Gyral patterns (Desikan et al., 2006).
Recommended Reading A major convenience for the practicing cognitive neuroscien
tist is the availability of detailed digital atlases. Several atlases are available on
the web with interactive interfaces. As an example, see the Ebrain atlas:
ebrains.eu/tools/human-brain-atlas. The following publications provide detailed lists
available atlases and access information
of
. Amunts and Zilles (2015): Architectonic Mapping of the Human Brain beyond
Brodma
. Eickhoff et al. (2018): Imaging-Based Parcellations of the Human Brain.
nn.
https://www.
3.3.4 Accessing and Using Atlases
Magnetic Resonance Imaging (MRI) has greatly simplified access to anatomical
information. For example, the images in Fig. 3.2b were created using a MRIcroGL
(https://www.nitrc.org/projects/mricrogl/) (Rorden & Brett, 2000) an open-source
softwar
e to view MRI anatomical scans of the brain. Familiarity with viewing and
navigating MRI images can provide you with access to various freely available
atlases and anatomical resources.
In brief, different imaging protocols provide access to different aspects of anatomical
Gradient-Echo (MPRAGE), Diffusion Tensor Imaging, Diffusion Spectrum
Imaging).
Representations
. Volume: The
features including connectivity structure (Magnetization-Prepared Rapid
default is to acquire the data as a 3D-volume (x,y,z coordinates).
The brain can be explored as slices passing through a particular point in this 3D
volume (see Fig. 3.2b). In this view, it is possible to see the difference between
gray
and white matter, and to see the subcortical structures.
38 S. Viswanathan
. Surface: A more advanced representation of the brain is the use of surfaces.
Obtaining a surface from the acquired MRI volumes involves additional computational steps (typically performed in the background by various software). A
surface representation allows features to be examined closely including by
“inflating” the surface. Two points might be close together within the skull, but
their neural relationship might be distant. This is of particular relevance to EEG
measurement.
. Overlay: As described above, atlases can be placed on top of anatomical image
within a common coordinate system to assess different kinds of infor mation and
features of the anatomical image.
With these basic concepts, tools, and the resources described above, it is possible
to acquire a working familiarity with brain anatomy on your own.
3.4 Putting into All Together
As discussed earlier, understanding the relevance of EEG signals can be valuably
informed by considering the broad anatomical context of the neural p rocesses being
investigated.
The following is a sketch of how your experimental study can be enriched by
consi
dering anatomy explicitly by considering the specific structures that might be
engaged, the locations of these structures in the brain, and the manner in which they
interact.
Identify During research planning, consider the cognitive processes being engaged
by
an experimental task (e.g., stimulus perception, attentional selection, decisionmaking, response selection, response execution). These functional processes can be
used to estimate the expected anatomical networks and structures that might be
engaged in performing this task (e.g., a lateralized motor network or a somatosensory network). This estimation step can benefit from the use of different functional
atlases described above. It can be of particular value in patient popula tions where
further anatomical information might be available.
The full anatomical scope includes the role of the peripheral nervous system. For
le, a “simple” task of viewing a stimulus and responding to it, requires the
examp
engagement of the CNS with the PNS to receive sensory information and to generate
movements of a peripheral effector (e.g., a finger).
Formulate
and where activity in the brain is likely to be engaged. This information can be used
to ensure that the experiment is designed to acquire relevant information. For
example, if the networks are engaged differently between experimental conditions
would the acquisition design (e.g., number of electrodes, their placement) be capable
of detecting these differences.
The estimated anatomical networks provide a prior expectation of when
3 Basic Anatomy: Central Nervous System 39
Peripheral Sensors The expected anatomical networks in the experiment could
indicate a role for peripheral physiological measures (e.g., electromyography
(EMG), electrocardiography (ECG), eye-tracking) dependi ng on the phenomena
being investigated. In simplistic terms, these sensors can help track the timing and
magnitude of information arriving to the brain, and the output of information from
the brain to the periphery.
Apply These considerations can inform the selection of appropriate EEG measures
analyze the effects of interest: their electrode locations, lateralization, and timing,
to
especially in coordination with peripheral sensors. It can also inform the interpretation of the obtained findings.
3.5 Conclusion
Here we have sought to highlight the value of including a neuroanatomical perspective to your EEG study. Although neuroanatomy is a large and complex topic, there
are several tools and resources that can help you increase your familiarity with this
topic. This can have many concrete benefits in better design and analysis strategies
for your EEG studies. Finally, considering the nervous system as an extended
interacting network that includes the brain as well as the peripheral nervous system
can enrich how a study is formulated. The companion chapter on the peripheral
nervous system provides further information about how the CNS and PNS interact to
achieve various functional capabilities.
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Chapter 4
Basic Anatomy: Peripheral Nervous System
Cilia Jaeger
Abstract This chapter provides an overview of the neuroanatomy of the peripheral
nervous
tomical system that enables information exchange between the brain and the body. It
transmits information from sensory receptors to the brain, relays motor commands
from the brain to the body, and regulates homeostasis. This chapter introduces the
anatomy and function of the peripheral nervous system and outlines its two main
domains: the somatic nervous system and the autonomic nervous system. The
chapter illustrates how sensory information is detected, relayed to the brain, and
how this communication produces specific body functions or responses.
Keywords Peripheral nervous system · Somatic nervous system · Autonomic
nervous
system. The peripheral nervous system (PNS) consists of a complex ana-
system · Receptors
4.1 Introduction
Chapter 3 (“Basic Anatomy: Central Nervous System”) introduced the underlying
neuroanatomy that forms the basis of the brain activity recorded in electroencephalography (EEG). Much of the neural activity that we record in EEG is connected to
processing sensory information that is received from the peripheral nervous system
(PNS). For example, a visual evoked response originates from visual information
being detected by special receptors in the eye and relayed to the brain. Similarly, the
brain can send information to the peripheries to elicit a behavioral response, such as
voluntary muscle contraction. While EEG does not directly measure the underlying
physiology of the peripheral nervous system, it is important to understand how these
systems function together to respond to stimuli in the environment and elicit
behavior. This chapter provides an overview of the anatomy and function of the
C. Jaeger (*)
Brain Products GmbH, Gilching, Germany
e-mail:
cilia.jaeger@brainproducts.com
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2026
T.
Warbrick (ed.), The EEG Handbook,
https://doi.org/10.1007/978-3-032-20450-9_4
41