The brain uses a sophisticated communication and command-and-control system that monitors and manages interactions between roughly 100 billion neurons, each with 5,000-10,000 synaptic connections, for as many as 500 trillion synapses in adults (Breedlove & Watson, 2020).
Neuroplasticity, the remodeling of neurons and neural networks with experience, is responsible for learning and memory and makes neurofeedback training possible.
The simplest way to divide the cortex is into the frontal and posterior cortex. The frontal cortex (frontal lobe) specializes in action, ranging from cognition, emotion, and autonomic control to movements and speech. The posterior cortex (parietal, temporal, and occipital lobes) is concerned with perception and memory. The frontal cortex and posterior cortex, along with subcortical structures and the peripheral nervous system, provide the hierarchically arranged feedback loops that allow us to interact with our environment to achieve goals successfully.
The prefrontal cortex (PFC) (cortex rostral to the motor association cortex) directs the cognitive and emotional processes, called perception-action cycles, that adapt (and preadapt) us to our environment. The PFC predicts and creates the future. Working in cooperation with networked brain structures, the PFC marshals its executive functions of planning, attention, working memory, and decision-making to develop innovative and sophisticated actions to pursue future goals (Fuster, 2015).
BCIA's HRV Biofeedback Blueprint does not cover the Central Nervous System.
This unit covers
Neurons, What Is the EEG?, Local Versus Global Decisionmaking, and Connectivity.
Please click on the podcast icon below to hear a full-length lecture.
Neurons
We can divide neurons into sensory, motor, and interneurons.
Sensory neurons are specialized for sensory intake. They are called afferent because they transmit sensory information towards the central
nervous system (brain and spinal cord). The graphic below is courtesy of leavingcertbiology.net.
Motor neurons convey commands to glands, muscles, and other neurons. They
are called efferent because they transmit information towards the periphery. Graphic courtesy of leavingcertbiology.net.
Interneurons provide the integration required for decisions, learning and
memory, perception, planning, and movement.
They have short processes, analyze incoming information, and distribute their analysis with other neurons in their network. Interneurons are entirely confined to the central nervous system (CNS), account for most of its neurons, and comprise most of the brain (Breedlove & Watson, 2020). Local interneurons analyze small amounts of information provided by neighboring neurons. Relay interneurons connect networks of local interneurons from separate regions. These interconnections enable diverse functions like perception, learning and memory, and executive functions like planning (Carlson & Birkett, 2021). Graphic courtesy of leavingcertbiology.net.
The
cell body or soma contains the machinery for a neuron’s
life processes. It receives and integrates EPSPs and IPSPs, small graded positive and negative changes in
membrane potential generated by axons. The cell body of a typical neuron is
20 μm in diameter, and its spherical nucleus,
which contains chromosomes comprised of DNA, is 5-10
μm across. The cell body is the only location where neurons can manufacture proteins (like enzymes,
receptors, and ion channels) and peptides (neurotransmitters like oxytocin) since this requires ribosomes. Check out the Khan Academy YouTube video, Anatomy of a Neuron.
Dendrites are branches designed to receive messages from other neurons via axodendritic synapses (junctions between axons and dendrites). Dendrites send messages to other neurons via dendrodendritic synapses (junctions between the dendrites of two neurons). Dendrites receive thousands of synaptic contacts and have specialized proteins called receptors for neurotransmitters released into the synaptic cleft (Bear, Connors, & Paradiso, 2016).
A neuron's dendrites are called a dendritic tree, and each branch is called a dendritic branch. Graphic by Bruce Blaus from Wikipedia article Neuron.
An axon is a cylindrical structure only found in neurons. Axons are specialized for distributing information within the central and peripheral nervous systems. Axons range from 1 to 25 µm in diameter and 0.1 mm to more than a meter in
length. Over 90% of neurons are interneurons whose axons and dendrites are very short and do not
extend beyond their cell cluster. Axons usually branch repeatedly. Each branch is called an axon collateral.
Axons transmit
action potentials
toward a neuron's terminal buttons. An axon also bidirectionally transports molecules between the cell body and terminal buttons using microtubules.
An axon hillock is a swelling of the cell body where the axon
begins. The middle of an axon is the axon proper, and the end is the axon terminal (Bear, Connors, & Paradiso, 2016). Graphic by M.alijar3i from the Wikipedia article Axon Hillock.
The axon hillock sums EPSPs and IPSPs for milliseconds to generate an action potential.
Axon terminals are buds located on the ends of axon branches
that form synapses and release neurochemicals to other neurons. Axon terminals contain vesicles that store
neurotransmitters for release when an action potential arrives. Their presynaptic membranes
possess reuptake transporters that return neurotransmitters from the synapse or extracellular space for
repackaging. The graphic of serotonin reuptake transporters below is courtesy of NIDA.
Excitatory and Inhibitory Postsynaptic Potentials
Graded positive and negative changes in membrane potential are called excitatory postsynaptic potentials
and inhibitory postsynaptic potentials. These potentials are essential to the EEG and communication among neurons.
An excitatory postsynaptic potential (EPSP) is a subthreshold depolarization that makes the membrane
potential less negative and pushes the neuron towards its excitation threshold. EPSPs are produced when
neurotransmitters bind to receptors and cause positive sodium ions to enter the cell. A postsynaptic membrane may have tens to thousands of transmitter-gated ion channels at a single synapse. The amount of transmitter released determines how many of these channels will be activated. The size of an EPSP will be a multiple of the number of vesicles, each containing several thousand transmitter molecules.
An inhibitory postsynaptic potential (IPSP) is a hyperpolarization that makes the membrane potential
more negative and pushes the neuron away from its excitation threshold. At most inhibitory synapses, IPSPs are produced when
neurotransmitters like GABA or glycine bind to receptors and cause negative chloride ions to
enter the cell. When an inhibitory synapse is closer to the soma than an excitatory synapse, it can counteract positive current flow and decrease the size of the EPSP. This mechanism is called shunting inhibition (Bear, Connors, & Paradiso, 2016).
Integrating Postsynaptic Potentials
Integration is the summation of EPSPs and IPSPs at the unmyelinated axon
hillock.
The axon hillock of a postsynaptic neuron uses two methods to sum EPSPs and IPSPs: spatial and temporal
summation.
In spatial summation, the axon hillock adds postsynaptic potentials (PSPs) from thousands of synapses on dendrites. In temporal summation, the axon hillock combines the PSPs from presynaptic neurons that repeatedly fire within a 1-15-ms time window.
Local activity is a composite of local and network influences. Network communication systems and local cortical functions show different characteristics across the cortex and produce unique and specific EEG patterns in other regions.
We can follow the progression from stimulus to behavior response with the EEG. This allows us to determine the correct function at each step and identify causal factors in dysfunctional outcomes or reactions.
Source of the Scalp EEG
The scalp EEG results from summating large areas of gray matter activity. Areas are polarized synchronously due to the input of oscillatory or transient evoked activity. These areas comprise thousands of cortical columns consisting
of large pyramidal cells aligned perpendicularly to the cortical surface.
Pyramidal neurons are found in all cortical layers, except layer 1, and represent the cerebral cortex's primary type of output neuron.
Cortical columns are synchronously polarized (made more negative) and depolarized (made less negative) due to the input of oscillatory or transient evoked activity.
Local Field Potentials
Firing by interconnected pyramidal neurons within cortical columns produces the local field potential (LFP). Glial cells also contribute to the LFP by modulating the cortical electrical gradient.
Revised caption from Wikipedia's article on Neural Oscillation. Simulation of neural oscillations at 10 Hz. The upper panel shows spiking of individual neurons (with each dot representing an individual action potential within the population of neurons). In the lower panel, the LFP reflects their summed activity. This figure illustrates how synchronized action potentials may result in macroscopic oscillations that can be measured outside the scalp.
Do not confuse the "spiking" of individual neurons with epileptogenic spikes in the scalp EEG.
Scalp Electrical Potentials
Scalp electrical potentials represent the sum of all available electrical fields. Fields of opposite polarity (+/-) cancel each other out, so that scalp potentials are greater when large aggregates of neurons polarize and depolarize synchronously. The scalp EEG represents a weighted sum of all active currents with the brain that generate open fields, including non-cortical sources.
Action potentials reflect neuronal output. They are seen in extracellular recordings as fast (~300 Hz)
activity that exceeds 90 mV and lasting less than 2 ms. Action potentials play a minor role in scalp surface EEG. They fall below 60 V outside of a 50-μm radius. Scalp electrodes are several centimeters from cortical neurons and are generally aligned away from the scalp. Therefore, action potentials are unlikely to contribute significant voltages to the scalp EEG.
Local Field Potentials Regulate Neuron Excitability and Firing
Neurons are most likely to fire during the depolarizing phase of the local field potential. Neurons are more excitable when they are "in phase" with the LFP and are inhibited when they are out of phase. Thus, at any instant, EEG amplitude and frequency are regulated by the LFP, which in turn, is influenced by oscillatory mechanisms such as SCPs.
The EEG is a moment-to-moment measure of the excitability of action potential firing, like gates opening and closing on the half cycle.
The synchronous activity of large pyramidal neurons networked in cortical columns creates the EEG.
The Composition of the EEG
The EEG is composed of electrical potentials varying in frequency and amplitude.
Sources of IPSP and EPSP Inputs
Many sources contribute input that results in IPSP and EPSP activity within cortical neurons. These sources primarily contribute influences such as oscillatory generator input or ascending event-related evoked input.
EEG Sources
Generators like the thalamus produce oscillatory activity among many interconnected neurons, including EEG patterns like the alpha rhythm.
Generators like the thalamus produce oscillatory activity among many interconnected neurons, including EEG patterns like the alpha rhythm.
The
eyes open again at 14’31”, and alpha attenuates (alpha-blocking). This movie shows the posterior dominant rhythm (generally known as alpha) appearing in the eyes-closed condition when visual sensory input is stopped, and the attenuation or blocking of this rhythm as sensory input returns in the eyes-open condition.
The diagram below, which shows bidirectional connections between the thalamus and cortex, was modified from the original on www.lib.mcg.edu.
Caption by W. D. Jackson, PhD, and S. D. Stoney, PhD (2006): Thalamocortical cells are subject to excitatory drive from their system afferents, from monosynaptic corticothalamic fibers, and from the brainstem reticular formation (ascending reticular activating system, ARAS). They receive inhibitory drive from local interneurons and neurons in the reticular nucleus of the thalamus (RNT). Note that the RNT neurons are excited by activity in thalamocortical cells and by corticothalamic cells. The connections are precisely organized. For example, each column in a primary cortical area sends corticothalamic fibers back to the same part of its specific thalamic nucleus that sends its thalamocortical fibers to that cortical column. The corticothalamic fibers also synapse on the RNT cells receiving input from that part of the thalamic nucleus. Each cortical receiving area is said to be "reciprocally connected" with its specific thalamic nucleus. Like the thalamocortical cells, RNT cells and cortical neurons also receive excitatory drive from the ARAS.
The EEG is generated by thalamocortical (alpha) and cortical-cortical
(beta) sources.
In the ascending reticular activating system, neurons produce event-related potentials in response to diverse stimuli like a flashing light or sound. Event-related potentials (ERPs) are the brain's response to externally applied stimuli, events, or cognitive/motor tasks. They are time-locked measures of brain electrical activity.
Dipole Generators
Large cortical pyramidal neurons organized in macrocolumns are oriented with an apical dendrite projecting toward the scalp and an axon descending in the opposite direction. An "Equivalent Dipole Generator usually represents the sum of all multipolar current sources." Summed generators are modeled as dipoles to aid the conceptual understanding of the electrical fields involved.
EEG Signals (Brainwaves)
The EEG represents changes in the brain area's electrical activity (potential) compared to a "neutral" site or another brain area. The EEG is displayed as oscillations or voltage fluctuations, which show a "wave" pattern when plotted on a graph.
"These oscillations are generated spontaneously in several areas of the cerebral cortex as neuronal networks transiently form assemblies of synchronously firing cells." Klaus Linkenkaer-Hansen.
The EEG is composed of electrical potentials that vary along the dimensions of amplitude and frequency.
EEG Amplitude
The "amount" or amplitude and the "pattern" or morphology of any EEG frequency band reflect the number of neurons discharging simultaneously at that frequency. Lower neuron firing rates correspond to lower signal amplitude.
Amplitude measures signal energy and is usually expressed in microvolts.
Greater synchrony in firing among neurons results in higher amplitude, as shown with alpha in the graphic below.
Greater firing synchrony produces larger EEG potentials measured from the scalp surface.
EEG Frequencies
The raw EEG contains all EEG frequencies, as white light contains all light frequencies. Digital filters separate the EEG frequencies just as a prism separates individual colors.
EEG frequency is measured in cycles per second or Hz. Count the number of peaks or count the number of zero (0.0) crossings divided by 2.
The slower the waves, the lower the EEG frequency.
The longer the wavelength, the slower the frequency.
While frequency band labels are helpful descriptors, they can also be misleading. Classification of an EEG rhythm is based on context (measurement conditions and EEG activity during the specific epoch), frequency, and waveform morphology.
The process of up-training or down-training signal amplitude in one or more of the EEG bands using an EEG is called EEG biofeedback or neurofeedback. Minimum EEG voltages of 20-30 μV are seen in children and adults (Kraus et al., 2011).
Brainwaves Reflect Behavior
The ratio of slow (theta) to faster (beta)
brainwaves is the theta/beta ratio. It shows client alertness.
The next section will examine delta, theta, rhythmic slow-wave, alpha, mu, synchronous "alpha," SMR, beta, high or fast beta, and gamma activity.
Local Versus Global Decision-making
The short time windows of fast oscillators facilitate local integration and decision-making. The long time windows of slow oscillators can involve many neurons in large or distant brain areas and favor complex, global decisions.
Delta (0.5-3 Hz)
There are two delta rhythms, slow oscillation under 1 Hz and traditional 1-4 Hz oscillations. The slow 0.3-0.4 Hz oscillation originates in the neocortex and persists when the thalamus is removed. Thalamo-cortical neurons generate the 1-4 Hz oscillations observed during human stage-3 sleep. Slow neocortical oscillations may synchronize the thalamic delta rhythm (Steriade, 2005).
Delta activity is generated by cortical neurons when other connections do not activate them. Delta is predominantly found in frontal areas. Delta is associated with sleep and infancy. During stage-3 sleep, delta allows astrocytes to rebuild their stores of glycogen. Clinicians observe delta in clients diagnosed with ADHD, brain tumors, learning disorders, and traumatic brain injury (TBI). Rhythmic high-amplitude delta is associated with TBI, especially if localized. Diffuse delta may be found in ADHD and learning disorders.
Normal Values
Delta should not be present in significant amounts in the awake adult EEG. "Apparent" delta is usually eye-movement artifact. Some delta activity probably occurs in the waking adult EEG.
The mechanisms that generate the theta rhythm are poorly understood. Theta differs depending on location and source. Amzica and Lopes da Silva (2011) consider the classic septal/diagonal band pacemaker model incomplete. Hippocampal interneurons, which innervate the hypothetical medial septum pacemaker, exercise top-down control. The hypothalamic supramammillary nucleus is extensively connected to the brainstem, diencephalon, and medial septum. This nucleus may also pace and modulate hippocampal theta. Further, a noncholinergic theta source has been found within the entorhinal cortex of the hippocampus.
Theta is associated with creativity, global synchronization, memory formation, and recall. Increased theta amplitudes indicate hypoperfusion and decreased glucose metabolism.
Excessive frontal theta is linked with depression, daydreaming, distractibility, and inattention. A theta/beta (T/B) ratio of 3.0 may indicate ADHD depending on age, as T/B ratios are developmentally-mediated (Monastra et al., 1999).
Normal Values
Theta voltage is age-related in the awake EEG. Voltage diminishes from age 8-30 with minimal amounts over age 30. A typical 6-7 Hz rhythm in the frontal midline (FCz) is associated with mental activity such as problem-solving and a wide variety of other functions. This rhythm appears to be limbic in origin. It is higher in amplitude and more synchronous when processing the feedback that an error has occurred. The 4 Hz rhythm is associated with pleasurable childhood experiences and memory searches in adults.
Rhythmic Slow Wave (RSW or Theta)
Inhibit theta to remediate symptoms. Reward posterior RSW in alpha/theta training for addictions, global synchronization, optimal performance, and PTSD. RSW is generally not increased frontally. Clinicians may train for increases or decreases in phase synchrony. RSW is mainly seen in the frontal-midline (FCz) when awake with eyes open. The limbic system and thalamus generate RSW. Depending on location, RSW may be slowed-alpha as thalamic output slows.
The 8-13-Hz alpha rhythm differs from spindle waves in both its source and the activity during which it is
observed. Alpha 1 (low alpha) ranges from 8-10 Hz and alpha 2 (high alpha) from 10-13 Hz (Thompson & Thompson, 2016). Alpha rhythms depend on the interaction between rhythmic burst firing by a subset of thalamocortical (TC) neurons linked by gap junctions and rhythmic inhibition by widely distributed reticular nucleus neurons (Hughes & Crunelli, 2005). The alpha rhythm is maintained and propagated by cortical networks (Amzica & Lopes da Silva, 2018). Graphic of thalamocortical architecture courtesy of the Laboratory of Neuro Imaging and Martinos Center for Biomedical Imaging, Consortium of the Human Connectome Project.
Researchers have correlated the alpha rhythm with
relaxed wakefulness. There are age- and function-related differences. Spindle waves, in contrast, originate in the thalamus and occur during
unconsciousness and stage 2 sleep (Steriade, 2005).
Alpha is the dominant rhythm in adults when their eyes are closed and is located posteriorly. The 8-10 Hz range is associated with ADHD, daydreaming, fogginess, OCD, and TBI. Frontal asymmetry is associated with depression. The 10-12 Hz range is seen with inner calm (calm and alert) and meditation. Clinicians train alpha amplitude and phase synchrony up or down for remediation of symptoms, depending on location.
Posterior Dominant Rhythm (PDR)
The posterior alpha rhythm is visible at about 4 months with a frequency of around 4 Hz. Between 3-5 years, this rhythm is approximately 8 Hz with amplitudes as high as 100 μV. From 6-15 years, this rhythm is 9 Hz by age 7 and 10 Hz by ages 10-15 with a mean amplitude of 50-60 μV. Girls show a statistically faster acceleration of posterior alpha frequency than boys. From 13-21 years, the mean alpha frequency is 10 Hz, and amplitudes decline throughout this period. Faster alpha frequencies are associated with higher IQ and better memory performance.
The typical adult alpha frequency ranges from 9.5-10.5 Hz. Alpha below 8 Hz is considered abnormal.
Mu Rhythm (7-11 Hz)
While the 7-11-Hz mu rhythm usually overlaps with the alpha range, its morphology deviates from the alpha waveform as one end is pointed. The mu rhythm can be recorded at C3 and C4 in a minority of subjects and may represent suppression of hand movement or imagining hand movement (Thompson & Thompson, 2016). Mu rhythms appear to regulate motor cortex activities via prefrontal cortical mirror neurons. These circuits may play a critical role in imitation learning and our ability to understand the actions of others. Mu rhythms facilitate the conversion of visual and auditory input into integrated skill-building functions. Attenuation of the mu rhythm appears to be associated with the activation of this function (Pineda, n.d.). The mu rhythm is highlighted below.
This second example of the mu rhythm shows a classic 10-11 Hz and
19-20 Hz "Owl Eye" presentation.
Synchronous "Alpha"
Auditory, somatosensory, and visual systems produce localized and semi-independent "alpha" activity. Synchronous, distributed alpha integrates perception and facilitates action. Synchronous "alpha" appears to block the localized alpha-like patterns such as mu and the posterior rhythm in favor of more broadly distributed network integration.
Sensorimotor Rhythm (13-15 Hz)
The sensorimotor rhythm (SMR) is beta 1 located on the sensorimotor strip (C3, Cz, C4). SMR amplitude increases when the motor circuitry is idle. SMR increases with stillness and decreases with movement. Deficient SMR may be observed in movement spectrum complaints like hyperactivity and tics. SMR appears as sleep spindles during stage-2 sleep. SMR is associated with neutral blood perfusion of the brain and resting levels of glucose metabolism. Clinicians typically reward increased SMR amplitude to calm hyperactivity and during theta/beta ratio training.
Beta consists of rhythmic activity between 13-38 Hz. There are four beta ranges: beta 1 (12-15 Hz), beta 2 (15-18 Hz), beta 3 (18-25 Hz), and beta 4 (25-38 Hz). Beta is located mainly in the frontal lobes. Beta is associated with focus, analysis, and relaxed thinking (Thompson & Thompson, 2016).
Excessive beta is observed in anxiety, depression (asymmetry), insomnia, OCD, and sleep disorders. Deficient beta is seen in ADHD, cognitive decline, and learning disorders.
Since beta overlaps with the EMG range, clinicians must be careful when up-training this rhythm and use an EMG inhibit. Beta is primarily seen in the frontal lobes. Beta is generated by the brainstem and cortex and is associated with hyper-perfusion and increased glucose metabolism.
Fast 20-35-Hz oscillations are generated by the mesencephalic reticular formation activation. Thalamocortical, rostral thalamic intralaminar, and cortical neurons spontaneously oscillate in this range. This activity is mainly seen in the frontal lobes and is associated with hyper-perfusion and increased glucose metabolism. Persistent excessive activity can lead to metabolic exhaustion.
This activity may be associated with peak performance and cognitive processing and related to specificity and precision in information processing. Excessive high beta is associated with alcoholism, anxiety, OCD, rumination, and worry.
Clinicians often inhibit high beta activity but rarely reward it.
Amzica and Lopes da Silva (2011)
concluded that gamma oscillations may speed information distribution and processing. Gamma bursts occur during problem-solving, and the absence of gamma is associated with cognitive deficits and learning disorders. Gamma synchrony is related to cognitive processing and is vital in coding by contributing specificity and precision to information processing. Gamma is a "binding rhythm" that integrates sensory inputs into perception and consciousness.
Neural networks are systems of interconnected ensembles of neurons that collaborate to achieve a goal (Thompson & Thompson, 2016).
Networks communicate and perform functions via hub- or node-based communication systems. Connectome graphic from van den Heuvel and Sporns (2011).
Networks like the Affect, Attention, Default, Executive, and Salience systems synchronize cortical and subcortical regional activity to perform functions. Connectivity is the degree of synchrony between the oscillations of specialized brain regions (nodes) within a network (Bastos & Schoffelen, 2016). Strongly connected brain regions are called hubs. Hubs can be
primarily connected to nodes (vertices) within their local modules (sets of interconnected nodes) or nodes in more distant modules
(Bullmore & Sporns, 2009).
Neurofeedback training can increase or decrease connectivity using a normative database. For example, BrainMaster's BrainAvatar software allows clinicians to train specific networks, like the Default Mode Network (DMN).
Glossary
40-Hz rhythm: gamma rhythm hypothesized to be associated with feature binding
(linking an apple's color to its shape) and attributed to the neocortex and thalamocortical neurons.
action potential: a propagated electrical signal that usually starts at a neuron’s
axon hillock and travels to presynaptic axon terminals.
afferent: a neuron that transmits sensory information towards the central
nervous system or from one region to another.
all-or-none law: once an action potential is triggered in an axon, it is
propagated, without decrement, to the end of the axon. The action potential amplitude is unrelated to the intensity of the stimulus that triggers it.
alpha blocking: arousal and specific forms of cognitive activity may reduce
alpha amplitude or eliminate it entirely while increasing EEG power in the beta range.
alpha rhythm: 8-12-Hz activity that depends on the interaction between rhythmic burst firing by a subset of thalamocortical (TC) neurons linked by gap junctions and rhythmic inhibition by widely distributed reticular nucleus neurons. Researchers have correlated the alpha rhythm with "relaxed wakefulness." Alpha is the dominant rhythm in adults and is located posteriorly. The alpha rhythm may be divided into alpha 1 (8-10 Hz) and alpha 2 (10-12 Hz).
alpha spindles: regular bursts of alpha activity.
amplitude: the energy or power contained within the EEG signal measured in
microvolts or picowatts.
amygdala: a limbic system structure that participates in evaluating whether
stimuli are threatening, establishing unconscious emotional memories, learning conditioned emotional responses,
and producing anxiety and fear responses.
anion: a negative ion, for example, chloride (Cl-).
anterior cingulate cortex (ACC): a division of the prefrontal cortex that plays a vital
role in attention and is activated during working memory. The ACC mediates emotional and physical pain and has
cognitive and affective conflict-monitoring
components.
apical dendrite: a dendrite that arises from the top of the pyramid and extends
vertically to layer 1 of the neocortex.
arousal: a process that combines alertness and wakefulness, produced by at least
five neurotransmitters, including acetylcholine, histamine, hypocretin, norepinephrine, and serotonin.
astrocytes: star-shaped glial cells that communicate with and support neurons
and help determine whether synapses will form.
asynchronous waves: an EEG structure produced when neurons depolarize and hyperpolarize independently.
axon: long, cylindrical structures that convey information from the soma to
the terminal buttons. An axon also transports molecules in both directions along the outer surface of protein
bundles called microtubules.
axon hillock: a swelling in the cell body where a neuron integrates the
messages it has received from other neurons and decides whether to fire an action potential.
basal dendrite: a dendrite that horizontally branches out from the 30-micrometer
base of the pyramid through the layer where the neuron resides.
basal ganglia: these forebrain structures consist of an egg-shaped nucleus
that contains the putamen and globus pallidus and a tail-shaped structure called the caudate,
which together are responsible for the production of movement. The basal ganglia have also been implicated in
obsessive-compulsive disorder, Parkinson’s disease, and Huntington’s chorea.
beta rhythm: 12-38-Hz activity associated with arousal and attention generated by brainstem mesencephalic reticular stimulation that depolarizes neurons in both the thalamus and cortex. The beta rhythm can be divided into multiple ranges: beta 1 (12-15 Hz), beta 2 (15-18 Hz), beta 3 (18-25 Hz), and beta 4 (25-38 Hz).
bilateral synchronous slow waves: a pathological sign observed in drowsy
children. When detected in alert adults, intermittent bursts of high amplitude slow waves may signify gray
matter lesions in deep midline structures.
cation: a positive ion, for example, sodium (Na+).
cell body or soma: a structure that contains machinery for cell life processes and receives and
integrates EPSPs and IPSPs from axons generated by axosomatic synapses (junctions between axons and
somas). The cell body of a typical neuron is 20 micrometers in diameter, and its spherical nucleus, which contains
chromosomes comprised of DNA, is 5-10 micrometers across.
cerebral cortex: the layer of gray matter that covers the cerebral
hemispheres. The cerebral cortex consists of gray matter and white matter.
classical routes for EEG activation: specific sensory pathways like the visual, auditory, and somatosensory systems. Increased information transmission through these
pathways desynchronizes EEG activity in targeted cortical regions.
commissures: axon tracts. The left and right hemispheres communicate using the
corpus callosum, anterior commissure, and posterior commissure.
contingent negative variation (CNV): a steady, negative potential shift (15
microvolts in young adults) detected at the vertex. This SCP may reflect expectancy,
motivation, intention to act, or attention. The CNV appears 200-400 milliseconds after a warning signal (S1), peaks within
400-900 milliseconds, and sharply declines after a second stimulus that requires the performance of a response (S2).
corpus callosum: the largest commissure that connects the left and right
frontal, parietal, and occipital lobes.
corticothalamic network: a unified network that generates diverse brain
rhythms grouped by the cortical slow oscillations.
delta rhythm: 0.05-3 Hz oscillationsgenerated by thalamocortical neurons during stage 3 sleep.
dendrite: a branched structure designed to receive messages from other neurons via axodendritic synapses (junctions between axons and dendrites), determining whether the axon hillock will initiate an action potential.
dendritic spines: protrusions on the dendrite shaft where axons typically form
axodendritic synapses.
dendrodendritic synapses: junctions between dendrites that communicate
chemically across synapses and electrically across gap junctions.
depolarize: to make the membrane potential less negative by making the inside
of the neuron more positive with respect to its outside.
dipole: the electrical field generated between the sink (where current enters
the neuron) and the source (place at the other end of the neuron where current leaves), which may be located
anywhere along the dendrite.
dominant frequency: EEG frequency with the greatest amplitude.
dorsolateral prefrontal cortex: the left dorsolateral prefrontal cortex is
concerned with approach behavior and positive affect. It helps us select positive goals and organizes and
implements behavior to achieve these goals. The right dorsolateral prefrontal cortex organizes withdrawal-related
behavior and negative affect and mediates threat-related vigilance. It plays a role in working memory for object
location.
EEG activity: a single wave or successive waves.
EEG power: signal energy in the EEG spectrum. Most EEG power falls
within the 0-20 Hz frequency range. EEG power is measured in microvolts or picowatts.
efferent: a motoneuron that transmits information towards the periphery.
electroencephalogram (EEG): the voltage difference between at least two
electrodes, where at least one electrode is located on the scalp or inside the brain. The EEG is a recording of
both EPSPs and IPSPs that occur primarily in dendrites in pyramidal cells located in macrocolumns, several millimeters in
diameter, in the upper cortical layers
entorhinal cortex: a structure located in the caudal region of the temporal lobe. The entorhinal cortex receives pre-processed sensory information from all modalities and reports on cognitive operations. It furnishes the primary input to the hippocampus and is involved in memory consolidation, spatial
localization. Finally, it communicates with the septohippocampal system to generate the 4-7 Hz theta rhythm.
evoked potential: an event-related potential (ERP) elicited by external sensory
stimuli (auditory, olfactory, somatosensory, and visual). An evoked potential has a negative peak at 80-90 milliseconds and
a positive peak at about 170 milliseconds following stimulus onset. The orienting response ("What is it?") is a sensory
ERP. The N1-P2 complex in the auditory cortex of the temporal cortex reveals whether an uncommunicative person can
hear a stimulus.
excitatory postsynaptic potential (EPSP): a brief positive shift in a
postsynaptic neuron's potential produced when neurotransmitters bind to receptors and cause positive
sodium ions to enter the cell. An EPSP pushes the neuron towards the threshold of excitation when it can initiate
an action potential.
fast cortical potentials: EEG rhythms that range from 0.5 Hz-100 Hz. The main
frequency ranges include delta, theta, alpha, the sensorimotor rhythm, and beta.
feature binding: the process of linking information to perceptual objects
(linking an apple's color to its shape) that may involve the 40-Hz rhythm.
frequency: the number of cycles completed each second expressed in hertz (Hz).
frontal lobes: the most anterior cortical lobes of the brain divided
into the motor cortex, premotor cortex, and prefrontal cortex.
gamma rhythm: EEG activity frequencies above 30 or 35 Hz. Frequencies from 25-70 Hz are called low gamma, while those above 70 Hz represent high gamma.
glial cells: nonneural cells that guide, insulate, and repair neurons, and
structural, nutritional, and information-processing support. Glial cells generate slow cortical
potentials (SCPs). Glial cells include astrocytes, microglia, oligodendrocytes, radial glial cells, and Schwann
cells.
gray matter: brain tissue that looks grayish brown and comprises cell
bodies, dendrites, unmyelinated axons, glial cells, and capillaries.
gyrus: a ridge of cortex demarcated by sulci or fissures, for example, the
precentral gyrus.
hertz (Hz): a unit of frequency, an abbreviation for cycles per second.
hippocampus: a limbic structure located in the medial temporal lobe
involved in 4-7 Hz theta activity, control of the endocrine system’s response to stressors, formation of
explicit memories, and navigation. Cortisol binding to this structure disrupts these functions, interferes with creating new neurons, and harms and kills hippocampal neurons.
hubs: highly centralized nodes through which other node pairs communicate;
hubs allow efficient communication.
hyperpolarize: a negative shift in membrane potential (the inside becomes more
negative with respect to the outside) due to the loss of positive ions or gain of negative ions.
inhibitory postsynaptic potential (IPSP): a brief negative shift in a postsynaptic neuron's potential produced when cations (K+) leave or anions (Cl-) enter a neuron, hyperpolarizing the cell.
integration: the addition of EPSPs and IPSPs at the axon hillock. Neurons sum
EPSPs and IPSPs over their surface in spatial integration and over ms of time in temporal
integration to raise the membrane from its resting potential to the excitation threshold. EPSPs and IPSPs
last from 15-200 ms, while action potentials occur in 1-2 milliseconds.
interneurons: neurons that receive input from and distribute output to other
neurons. They have short processes and are confined to the CNS. They provide the integration
required for decisions, learning and memory, perception, planning, and movement.
ion: a charged atom or molecule with a positive or negative charge. Positive
ions are called cations, and negative ions are called anions.
left dorsolateral prefrontal cortex: division of the prefrontal cortex
concerned with approach behavior and positive affect. It helps us select positive goals and organizes and
implements behavior to achieve these goals.
locus coeruleus: the noradrenergic branch of the ascending reticular activating
system responsible for vigilance. Subnormal norepinephrine transmission may contribute to ADHD.
macrocolumns: circuits of cortical pyramidal neurons several mm in diameter
that create extracellular dipole layers parallel to the surface of the cortex that send opposite charges towards
the surface and the deepest of the 5-7 layers of cortical neurons. Since the pyramidal neurons are all
aligned with the cortical surface, the postsynaptic potentials at cells within the same macrocolumn add together
because they have the same positive or negative charge. The macrocolumns fire synchronously.
medial prefrontal cortex: a division of the prefrontal cortex that integrates
cognitive-affective information and helps control the hypothalamic–pituitary–adrenal (HPA) axis during
emotional stress.
membrane potential: a neuron’s electrical charge created by a difference
in ion distribution within and outside the neuron. A typical resting potential is about -70 millivolts
(thousandths of a volt).
module: a set of interconnected nodes in a neural network.
motor cortex: subdivision of the frontal lobe located in the
precentral gyrus and guides fine motor coordination (like writing).
motor neurons: efferent neurons that convey commands to glands, muscles, and
other neurons.
mu rhythm: 7-11-Hz waves resemble wickets and appear as several-second trains over central or centroparietal sites (C3 and C4).
neuron: a nerve cell that is the basic anatomical unit of the nervous
system.
node: the vertex within a neural network.
nodes of Ranvier: gaps between myelinated axon segments where the axon
membrane is exposed to extracellular fluid and action potentials are regenerated by sodium ion entry.
nucleus accumbens: a limbic structure targeted by dopamine released by
the mesolimbic pathway. The nucleus accumbens plays a critical role in reinforcing diverse activities,
including ingesting drugs like CNS stimulants.
occipital lobes: cortical lobes that are posterior to the parietal lobes. They
process visual information from the eyes in collaboration with the frontal, parietal, and temporal
lobes.
orbitofrontal cortex: a frontal lobe subdivision that is concerned with
affective evaluation. It decodes the punishment and reward value of stimuli and helps inhibit inappropriate
behavior. Phineas Gage's profound personality changes were produced by damage to this region.
parietal lobes: the cortical lobes posterior to the frontal lobes are divided
into the primary somatosensory cortex (postcentral gyrus) and secondary somatosensory cortex. Their primary function
is to process somatosensory information like pain and touch. The right posterior parietal lobe helps guide
movements, locate objects in three-dimensional space, and create body boundaries.
phase: the degree to which the peaks and valleys of EEG waveforms
coincide.
prefrontal cortex: the most anterior frontal lobe division subdivided
into dorsolateral, medial, orbitofrontal, and anterior cingulate regions responsible for executive
functions like attention and planning.
rate law: the principle that neurons represent the intensity of a stimulus by
variation in the rate of axon firing.
readiness potential: slow-rising, negative potential (10-15 microvolts)
detected at the vertex before voluntary and spontaneous movement. This slow cortical potential precedes voluntary
movement by 0.5 to 1 second and peaks when the subject responds.
resting potential: membrane potential of a neuron when it is not influenced by
messages from other neurons.
reticular formation: a network of 90 nuclei within the central brainstem from
the lower medulla to the upper midbrain. The reticular formation sends axons to the spinal cord, thalamus, and cortex, contributing to diverse functions like neurological reflexes, muscle tone and movement, attention, arousal, and sleep.
right dorsolateral prefrontal cortex: division of the prefrontal cortex that
organizes withdrawal-related behavior and negative affect and mediates threat-related vigilance. It plays a role
in working memory for object location.
saltatory conduction: action potential conduction in myelinated axons. Myelination allows
action potentials to jump from node to node, increasing speed 200 times.
secondary somatosensory cortex (S2): a parietal lobe region that receives
somatosensory information from the primary somatosensory cortex (S1).
sensorimotor rhythm (SMR): 13-15 Hz (beta 1) rhythm that is located
over the sensorimotor strip (C3, Cz, C4). The waves are synchronous. SMR increases with stillness and decreases with movement. Deficient SMR may be observed in movement spectrum complaints like hyperactivity and tics.
sensory event-related potentials (ERPs): event-related potentials elicited by external sensory stimuli (auditory, olfactory, somatosensory, and visual). These evoked potentials or exogenous ERPs have a negative peak at 80-90 ms and a positive peak at about 170 ms following stimulus onset. These changes in brain activity in response to specific stimuli. ERPs can be detected throughout the cortex. Investigators detect ERPs by placing electrodes at locations like the midline (Fz, Cz, and Pz). A computer analyzes a participant's EEG responses to the same stimulus or task over many trials to subtract random EEG activity. ERPs always have the same waveform morphology. Their negative and positive peaks occur at regular intervals following the stimulus.
sensory neurons: neurons specialized for sensory intake. They are called
afferent because they transmit sensory information towards the CNS (brain, retina, and spinal cord).
septohippocampal system: a subcortical circuit from the septum to hippocampus
that contributes to 4-7 Hz theta activity.
septum: a limbic structure that contains several nuclei involved in emotion and
addiction and control of aggressive behavior.
slow cortical potentials (SCPs): the gradual changes in the membrane potentials of
cortical dendrites that last from 300 ms to several seconds. These potentials include the contingent negative
variation (CNV), readiness potential, movement-related potentials (MRPs), and P300 and N400 potentials. SCPs
modulate the firing rate of cortical pyramidal neurons by exciting or inhibiting their apical dendrites. They
group the classical EEG rhythms using these synchronizing mechanisms.
soma or cell body: a part of a neuron that
contains machinery for cell life processes and receives and integrates EPSPs and IPSPs from axons
generated by axosomatic synapses (junctions between axons and somas). The cell body of a typical neuron is 20
micrometers in diameter, and its spherical nucleus, which contains chromosomes comprised of DNA, is 5-10 micrometers
across.
source: the place at the neuron's end opposite the sink where current leaves. The source is represented by +ve. The extracellular area surrounding the source becomes electrically
positive.
spatial integration: the addition of EPSPs and IPSPs over a neuron’s
surface.
sulcus: a shallow groove in the surface of the cerebral hemisphere, for
example, the central sulcus.
synchronous: an adverb meaning that groups of neurons depolarize and
hyperpolarize simultaneously.
synchrony: the coordinated firing of pools of neurons. EEG signals can display
local synchrony, frequency synchrony, and phase synchrony.
telencephalon: the frontal subdivision of the forebrain, including the
cerebral cortex, basal ganglia, and limbic system.
temporal integration: the addition of EPSPs and IPSPs over time. Summation is
more effective when postsynaptic potentials are generated more closely in time.
temporal lobes: the lobes separated from the rest of the cortical lobes by the
Sylvian fissure. The temporal lobes process hearing, smell, and taste information and help us understand spoken
language and recognize visual objects and faces. The amygdala and hippocampus, which lie beneath the temporal
cortex, play crucial roles in emotion, declarative, emotional, and working memory, and navigation.
terminal buttons: the buds located on the ends of axon branches that form synapses
and release neurochemicals to other neurons. They contain vesicles that store neurotransmitters for release when
an action potential arrives. A terminal button’s presynaptic membrane often possesses reuptake transporters that
return neurotransmitters from the synapse or extracellular space for repackaging.
thalamus: a forebrain structure above the hypothalamus that receives, filters,
and distributes most sensory information. The thalamus contains neurons that can block or relay ascending sensory
information. When these thalamic neurons rhythmically fire, this blocks the transmission of information to the cortex.
When they depolarize in response to sensory information, this integrates and transmits this information to the
cortex. Inputs to the thalamus determine whether these neurons block or relay sensory information.
theta rhythm: 4-8-Hz rhythms are generated by a cholinergic septohippocampal system
that receives input from the ascending reticular formation and a noncholinergic system that originates in the entorhinal cortex.
threshold of excitation: the membrane potential at which an axon initiates an
action potential, nominally -40 millivolts.
waveform: the shape and form of an EEG signal.
white matter: the layer beneath the cortex that mainly comprises myelinated
axons.
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Assignment
Now that you have completed this unit, how would you explain the relationship between local field potentials and the EEG? How does anatomy explain why the EEG is comprised of EPSPs and IPSPs instead of action potentials?
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