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52 C. Jaeger
1. Receptors
detects hot stimulus
Fig. 4.4 Reflex arc and neurofeedback in electrophysiology. Nociceptors in the skin of the hand
detect sudden temperature changes. This is converted to an electrical signal that is transmitted along
the afferent sensory pathway to the dorsal horn. Interneurons in the vertebrae then transmit the
electrical signal to the efferent motor pathway. The signal is transmitted to the neuromuscular
junction, which causes contraction of the muscles in the forearm, effectively pulling the hand away
from a hot source. Electrodes on the forearm can be used to measure muscle contraction in response
to the nociceptive stimulus. The painful stimulus is also transmitted to the CNS, where it is
consciously perceived and can be modulated. For example, in neurofeedback training, the pain
response can be mindfully downregulated. The CNS transmits an inhibitory signal along efferent
pathways that activate inhibitory neurons in the ventral horn and inhibit muscle contraction of the
forearm
Sensory neurons
Motor neurons
3. Integration
Interneurons
descending pathway in the spinal cord to inhibitory interneurons in the ventral horn
that synapse onto the motor neurons of the hand and suppress their excitability.
In an experimental setting, neurofeedback can be used in combination with EEG
and EMG to modulate the response to a painful stimulus. The response to pain can be
measured with EMG to measure the muscular response of pulling the hand away and
the reaction time between administering a painful heat stimulus and muscle contraction. Pain perception can also be detected with EEG electrodes placed over the
sensorimotor cortex or prefrontal cortex. Figure
real-tim
e feedback of their perceived pain perception. They are asked to suppress
4.4 shows a participant receiving
their pain-related brain activity and subsequently suppress their hand pulling away
from the hot, painful stimulus. Alongside the EEG, feedback from EMG can be used
to see how well the participant is able to downregulate their pain response and
muscle activity in the forearm. This example of a paradigm is often used in studies
investigating chronic pain (Kern et al.,
2024).
4.8 Concluding Remarks
This chapter covers the basic anatomy of the peripheral nervous system and connects
the nervous system to other systems in the body, such as the muscular system and
endocrine system. It is essential to understand how the CNS and PNS work together
4 Basic Anatomy: Peripheral Nervous System 53
to understand how function, cognition, and behavior are linked. Peripheral physiology can interact with central electrophysiology. Autonomic activity, such as changes
in skin conduction or heart rate have shown to be coupled to brain states (Huang
et al.,
2018), which are covered in detail in Chap. 6 (“Brain States”). Incorporating
perip
heral measures into EEG research can provide contextual information on body
and brain interactions and behavior.
References
Huang, J., Ulke, C., Sander, C., Jawinski, P., Spada, J., Hegerl, U., & Hensch, T., 2018. Impact of
brain arousal and time-on-task on autonomic nervous system activity in the wake-sleep transi-
tion. BMC Neuroscience, 19(1), 18. Available at:
PMC5896037/. Accessed 27 Sept 2025.
Iturriaga, R., Alcayaga, J., Chapleau, M. W., & Somers, V. K. (2021). Carotid body chemorecep-
tors:
Physiology, pathology, and implications for health and disease. Physiological Reviews,
101(3), 1177–1235. Available at:
Accessed
Kern, M., Sperlich, B., & Rief, W. (2024). Evaluating the effectiveness of neurofeedback in chronic
pain
https://doi.org/10.3389/fpsyg.2024.1369487. Accessed 27 Sept 2025
Koop, L. K. & Tadi, P. (2025). Neuroanatomy, sensory nerves. In StatPearls [Internet]. StatPearls
Publishing.
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Leclerc, A., & Wray, A. (2023). Sensory Receptors. In StatPearls [Internet]. StatPearls Publishing.
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McCorry, L. K. (2007). Physiology of the autonomic nervous system. American Journal of
Ph
articles/PMC1959222/. Accessed 27 Sept 2025.
OpenStax. (2022).
Available at: https://openstax.org/books/anatomy-and-physiology-2e/pages/14-introduction.
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management: A narrative review. Frontiers in Psychology, 15, 1369487. Available at:
Sept 2025.
armaceutical Education, 71(4), 78. Available at:
Available at: https://www.ncbi.nlm.nih.gov/books/NBK539846/. Accessed
at:
https://www.ncbi.nlm.nih.gov/books/NBK539861/. Accessed 23 June 2025.
The Somatic Nervous System. In Anatomy and physiology 2e. OpenStax.
27 Sept 2025.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8526340/.
https://www.ncbi.nlm.nih.gov/pmc/articles/
https://www.ncb i.nlm.nih.gov/pmc/
Chapter 5
EEG in the Context of Human Physiology
Michael Hoppstädter
Abstract Electroencephalography (EEG) as a noninvasive measurement technique
recorded from the scalp. Hence, it does not measure any signal directly at cell level
is
but rather the signal that reaches the electrodes. Therefore, EEG should be considered in the context of human physiology which interacts with and influences the
EEG signal. This chapter discusses aspects of macroscopic head anatomy and
physiology that affect the EEG signal. The focus will be on ocular, muscular and
skin potentials, and vascular activity, and how they influence the EEG signal.
Keywords Ocular potentials · Muscle artifacts · Sweating · Blood circulation ·
Volume
5.1 Introduction
conduction
What does physiology mean? As with many technical terms, physiology comes from
old Greek, and it roughly translates to the study of nature. Compared to anatomy as
the study of the structure of the body, physiology refers to the processes occurring in
the cells and tissues of the living organism.
EEG and Physiology In a nutshell, the electroencephalography (EEG) signal
originate
s mostly from synchronous excitatory postsynaptic potentials. These are
largely generated by pyramidal neurons in the cortex (see Chap. 2: “What Is EEG?”).
Thus,
EEG itself is a consequence of physiological processes at the cell level. The
signal then travels from the cortex through different tissues of the head until it is
eventually picked up by the sensors on the scalp. As other physiological processes
happen at the same time and in the same place, it makes sense to look at the bigger
picture. Consider that the EEG signal can be influenced by the physical properties of
M. Hoppstädter (✉)
Brain Products GmbH, Gilching, Germany
e-mail:
michael.hoppstaedter@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_5
55
56 M. Hoppstädter
the tissues that it permeates. Also, the human body generates other electromagnetic
signals which will interact with the EEG and have an influence on what is finally
measured.
We should therefore think about the wider context in which EEG arises when we
interpret the signal. This will also help us to differentiate between what in our
recording is true EEG and what is due to other physiological signals. In the following
sections, we will consider some specific physiological processes and how they
manifest in the EEG data.
5.2 Head Anatomy and Signal Propagation
A Brief and Macroscopic Anatomy of the Head Starting from the inside out, the
brain is a conglomera te of billions of nerve cells (the grey matter) and their processes
(i.e., axons and dendrites, the white matter), while the whole structure is suppor ted
through glia cells. Clusters of varying cell compositions form different organs in the
brain that serve specialized functions. The outer boundary of the brain is the cerebral
cortex, a heavily folded structure made up of multiple layers (for a more detailed
account of brain anatomy, please see Chap.
”). The brain is encased by several layers of skin, the so-called meninges.
System
There are also spaces between the meninges and within the brain which are not filled
with air but liquor, for example, the four ventricles . As brain cells need to be
supplied with oxygen, the brain is permeated with many blood vessels and larger
blood-filled cavities (the sinuses). Travelling further toward the outside, we reach the
skull which is covered in muscles that enable movement of the head and face.
Finally, the head is covered in skin. It is not a uniform organ, but consists of multiple
layers, whereby the epidermis (the outermost layer) creates a physical and chemical
barrier. The skin also includes other structures (so-called skin appendages) such as
sweat and sebaceous glands and hairs (see Fig.
3: “Basic Anatomy: Central Nervous
5.1).
Signal Pro
EEG propagates through conductive materials and is measured from a distance
(Rutkove,
detailed
equal
liquor has a conductivity that is roughly five times that of grey matter (McCann et al.,
2019). Another crucial point is that the EEG does not travel in a direct way from its
source
Since electrical activity is not just emitted from one source but from many, the
signals also mix on their way. This means that an EEG sensor placed at any scalp
location will not o nly record the EEG signa l from the neuron populations in the
cortical patch directly below it but also from the surrounding area. The signals from
nearby neurons will certainly make up a large portion of the EEG at that electrode,
but signals from more distant parts of the brain will also contribute. Thus, overall, the
pagation and Volume Conduction As an electromagnetic signal, the
2007). This is usually referred to as volume conduction (for a more
description, see Chap.
in their composition and neither in their physical properties. For instance,
to the head surface, but it spreads evenly from the cortex in all directions.
2: “What Is EEG?”). The different tissues are not
5 EEG in the Context of Human Physiology 57
Fig. 5.1 Simplified schematic of the tissues surrounding the brain
location of the source, its orientation, and the conductivity of the different tissues
between the source and the electrode will have an effect on what EEG signal is
measured at the scalp (Jochmann et al.,
2011; Vorwerk et al., 2024).
Volume conduction is also the reason why EEG effects, like event-related potentials
(ERPs), do not only appear at single electrodes but show instead rather widespread
distributions over the scalp. Neighboring electrodes will display a similar effect, with
amplitudes being largest in the vicinity of the origin of the effect (e.g., visual evoked
potentials are largest at lateral occipital electrodes, which sit above the visual cortex).
And since the generators of EEG usually form an electrical dipole, it is also common
to see the inverse effect on the opposite side of the head, that is, a left hemisphere
negativity accompanied by a right hemisphere positivity (Scherg et al.,
The cell
s that emit the EEG signal are not directly in contact with our recording
2019).
electrodes, but they are part of this complex context of neuroanatomy. This means
that the EEG signal will have traversed all the aforementioned tissues via volume
conduction on its way from the cortex to the electrode. To fully understand what
signal is recorded, it is thus necessary to consider all the elements that interact and
potentially influence the EEG signal along its path.
58 M. Hoppstädter
5.3 How EEG Is Influenced by Other Physiological Signals
There are a few physiological signals that play a bigger role in EEG recordings, and
you should consider these when working with EEG. We will look at signals that you
will most likely encounter in any standard EEG dataset. In this chapter, we will focus
on discussing the origin of those signals (from an anatomical and physiological
perspective) and how they interfere with the EEG signal. More detailed information
on these artifacts and how they can be handled in your recorded data can be found in
Chaps. 15 (“Getting Clean EEG Data: Artifacts and How to Prevent Them”) and 17
(“EEG Pre-processing and Artifact Handling”).
5.3.1 The Eyes and Ocular Potentials
Let us first consider the different parts of the human eye. The largest part is the
eyeball, which lets the light pass through the lens (in the front) toward the retina
(in the back). On top of the lens sits the cornea, which refracts the light and focuses it
on the retina. The eyelid is meant to protect the eyeball and to distribute tear fluid
evenly through blinking. Six eye muscles enable fine movements and rotations of the
eyeball (Trepel,
onment without being reliant on head movem ents.
envir
The eyeball forms an electrical dipole that is positively charged in the front and
negati
vely charged in the back (Malmivuo & Plonsey,
movem
ent will cause changes in the electric field of the eyeball. When the eyelid is
closed during blinking, the contact between the eyelid and the cornea creates a
current flow toward the scalp (Croft & Barry, 2000). Blinking leads to an electrical
potent
ial in the range of 0.4–1 mV (Malmivuo & Plonsey, 1995), which is much
r than what we usually observe with EEG. As with EEG, these ocular potentials
large
reach the EEG electrodes on the scalp through volume conduction (Lins et al.,
The
largest effects are visible in electrodes in the vicinity of the eyes, such as
frontopolar (e.g., Fp1 and Fp2) and lateral frontal channels (e.g., F9 and F10).
However, ocular potentials also travel to more distant sensors, whereby they become
smaller with increasing distance. It is still possible to see ocular potentials at central
electrodes and sometimes beyond.
As blin
inevitably show up in any EEG recording of awake participants. In contrast, eye
movements can be avoided more easily than blinks (e.g., when participants are
guided to focus on the center of a presentation screen), but occasional small
movements are still likely to happen. Since their magnitude is much larger than
more subtle effects like ERPs, ocular potentials would likely conceal the EEG
activity of interest. Therefore, they must be handled as part of data processing.
2008). This allows us to flexibly shift the gaze to locations in our
1995). Therefore, any eye
1993).
king cannot be suppressed over longer time periods, eye blinks will
5 EEG in the Context of Human Physiology 59
5.3.2 Facial Muscles and EMG
The human skull is lined with many muscles which allow us to perform a variety of
smaller and larger movements. These are most notably the muscles of mastication
(four muscles) and the facial or mimetic muscles (more than 20 muscles across five
groups: ocular, nasal, oral, auricular, scalp, and neck). The muscles of mastication
connect to the lower jaw to support the opening and closing of the jaw and chewing.
Whereas the mimetic muscles connect to the skin and allow fine-grained facial
expressions such as smiling, wrinkling the nose, or closing the eyelids.
Activity in facial muscles can have small to detrimental effects on concurrent
EEG
depending on which muscles are involved. Generally, facial muscle activity
generates electrical potentials in a wide frequency range, roughly from 20 to 200 Hz
(Muthukumaraswamy, 2013). Hence, the lower end of the muscle activity spectrum
aps with EEG frequencies of interest in the beta and gamma frequency bands.
overl
Strong artifacts can occur in the case of teeth clenching or chewing, which can easily
affect all scalp channels and obscure EEG activity entirely (Luck, 2014). Some
artifact
s are likely to b e localized to specific electrodes or regions. For example, a
rather common issue is tension in neck muscles that leads to persistent highfrequency noise in single, mostly temporal electrodes. Smaller facial movements
such as smiling or moving the jaw can also lead to artifacts across lateral electrodes,
while movements of the nose or the forehead usually affect more frontally locat ed
channels.
As strong muscle activity can obscure the underlying EEG activity, it is a good
idea to instruct participants to minimize body and facial movements during EEG
recordings. However , it is difficult to avoid this completely, and thus, muscle
artifacts need to be addressed during data processing.
5.3.3 Sweat Glands and Skin Potentials
The skin is made up of several layers, embedded skin organs, and hair. It has both
protective and sensory functions (mechanical, thermal, and nociceptive). Skin and
hair form a physical barrier that a researcher has to overcome through thorough cap
preparation to collect high-quality EEG data.
Another issu
dermis and subcutis) across the whole body, and drain sweat via ducts to the surface
(Trepel,
on, sweating might increase due to high temperature in the environment or
situati
physical activity (e.g., measurement during physical exercise). Since sweat secretion
is mediated via the sympathetic nervous system (Benedek & Kaernbach,
also
Chap. 4: “ Basic Anatomy: Peripheral Nervous System”), psychological stress
also lead to increased sweat secretion, for example, when participants are
can
presented with emotionally arousing stimuli.
e arises from sweat glands, which sit in deeper skin layers (the
2008). Their main function is thermoregulation. In an EEG recording
2010; see
60 M. Hoppstädter
Sweat is mostly made up of water, but it also contains trace amounts of minerals
such as sodium and chlorine. Therefore, sweating changes the conductivity of the
skin (Boucsein,
electrodes
are at a higher risk of being affected by sweating. The result is a slow drift that
produces low-frequency oscillations with large amplitude variations (Kappenman &
Luck, 2010). Thus, sweat artifacts can pose a serious problem when they contaminate
the EEG since they cannot be easily removed or attenuated. They are best
avoided in the first place by optimizing recording conditions.
The effect of sweat glands on skin conductivity can also be a signal of interest
electrodermal activity (EDA) is measured as a marker of emotional arousal
when
(Boucsein, 2012). This signal is usually not recorded from the scalp but from skin
areas
on the hand or foot (Boucsein et al.,
combined with EEG recordings in emotion research.
2012), and this becomes a problem if it happens under the EEG
. Electr odes on hairless and thus more exposed areas, such as the forehead,
2012). EDA measurements are often
5.3.4 Blood Vessels and Heartbeat
The head’s main blood supply is bilaterally driven via two arteries, the arteria
carotis interna and arteria vertebralis, which then branch out again into three further
arteries (the arteria cerebri anterior, media, and posterior, again left and right) to
transport oxygenated blood to dedicated brain areas. In contrast to other internal
organs where large blood vessels enter the organ at specific points, these large
afferent vessels run on the brain surface along the sulci. De-oxygenated blood is
transported back via a separate system of veins that discharge into larger sinuses
leading (via the vena jugularis interna) back to the heart (Trepel,
As the
propagated throughout the body and can also be picked up by EEG. This cardiac
field artifact coincides with the R-peak of the cardiac cycle. However, at the same
time, the EEG can be impacted in a more direct way by blood circulation. Due to the
rhythmic movement of blood being pumped through the body, it is possible to pick
up this signal with EEG electrodes placed on top or close to the pulsating arteries
running below the scalp surface. These so-called pulsatility artifacts will be visible
in the recorded EEG activity as a wave- or spike-like artifact at the frequency of the
heartbeat, but with a roughly constant delay of 200 ms (Kern et al.,
artifacts can affect the signal at single electrodes, but it is also possible that they enter
all EEG sensors if the contaminated channel is part of the reference. This can happen,
for instance, when you are using a mastoid reference as these electrodes are usually
placed above the arteria auricularis posterior (a smaller artery branching off the
arteria carotis externa), which runs between the mastoid process and the ear canal
(Luck,
therefore, older participants are especially prone to showing pulse artifact contamination in the EEG.
heart itself is an electrical organ, the effects of the cardiac cycle are
2014). Higher blood pressure and thinner skin can increase this effect, and
2008).
2013). These
5 EEG in the Context of Human Physiology 61
5.4 Conclusion
EEG provides a measure of the electrical activity of the brain. However, we do not
measure directly from the pyramidal cells (as with invasive depth electrodes) or from
the cortex (as with electrocorticography) but from the scalp surface. Therefore, we
need to consider what else will influence the signal on the way from its source to the
sensor.
This chapter has put the EEG signal into context with human anatomy and
ology. We have learned about the different tissues through which the EEG
physi
signals must travel via volume conduction, and the effects of other physiological
signals that can overlap with EEG at the sensor. Some of the artifacts that we
discussed can be prevented to some extent. Those artifacts that we cannot avoid
will have to be treated during data processing. Some can be attenuated nicely so that
we can keep the data for analysis. However, we might also lose data due to more
severe artifacts. Thus, reading this chapter should form the basis for a better
understanding of artifact prevention and handling.
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