An event-related potential (ERP) is the measured
brain
A brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as Visual perception, vision. I ...
response that is the direct result of a specific
sensory,
cognitive
Cognition refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses all aspects of intellectual functions and processes such as: perception, attention, thought ...
, or
motor event.
More formally, it is any stereotyped
electrophysiological response to a stimulus. The study of the brain in this way provides a
noninvasive means of evaluating brain functioning.
ERPs are measured by means of
electroencephalography
Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocorte ...
(EEG). The
magnetoencephalography (MEG) equivalent of ERP is the ERF, or event-related field.
Evoked potentials and
induced potentials are subtypes of ERPs.
History
With the discovery of the
electroencephalogram (EEG) in 1924,
Hans Berger revealed that one could measure the electrical activity of the human brain by placing
electrodes on the scalp and amplifying the signal. Changes in voltage can then be plotted over a period of time. He observed that the voltages could be influenced by external events that stimulated the senses. The EEG proved to be a useful source in recording brain activity over the ensuing decades. However, it tended to be very difficult to assess the highly specific neural process that are the focus of
cognitive neuroscience because using pure EEG data made it difficult to isolate individual
neurocognitive processes. Event-related potentials (ERPs) offered a more sophisticated method of extracting more specific sensory, cognitive, and motor events by using simple averaging techniques.
In 1935–1936, Pauline and
Hallowell Davis recorded the first known ERPs on awake humans and their findings were published a few years later, in 1939. Due to
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
not much research was conducted in the 1940s, but research focusing on sensory issues picked back up again in the 1950s. In 1964, research by
Grey Walter and colleagues began the modern era of ERP component discoveries when they reported the first cognitive ERP component, called the
contingent negative variation (CNV). Sutton, Braren, and Zubin (1965) made another advancement with the discovery of the P3 component. Over the next fifteen years, ERP component research became increasingly popular. The 1980s, with the introduction of inexpensive computers, opened up a new door for cognitive neuroscience research. Currently, ERP is one of the most widely used methods in
cognitive neuroscience research to study the
physiological
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemica ...
correlates of
sensory,
perceptual
Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous syste ...
and
cognitive
Cognition refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses all aspects of intellectual functions and processes such as: perception, attention, thought ...
activity associated with processing information.
Calculation
ERPs can be
reliably measured using
electroencephalography (EEG), a procedure that measures
electrical activity of the brain over time using
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...
s placed on the
scalp. The EEG reflects thousands of simultaneously
ongoing brain processes. This means that the brain response to a single stimulus or event of interest is not usually visible in the EEG recording of a single trial. To see the brain's response to a stimulus, the experimenter must conduct many trials and average the results together, causing random brain activity to be averaged out and the relevant waveform to remain, called the ERP.
The random (
background) brain activity together with other bio-signals (e.g.,
EOG
Electrooculography (EOG) is a technique for measuring the corneo-retinal standing potential that exists between the front and the back of the human eye. The resulting signal is called the electrooculogram. Primary applications are in ophthalmo ...
,
EMG,
EKG) and electromagnetic interference (e.g.,
line noise, fluorescent lamps) constitute the noise contribution to the recorded ERP. This noise obscures the signal of interest, which is the sequence of underlying ERPs under study.
From an engineering point of view it is possible to define the
signal-to-noise ratio (SNR) of the recorded ERPs. Averaging increases the SNR of the recorded ERPs making them discernible and allowing for their interpretation. This has a simple mathematical explanation provided that some simplifying assumptions are made. These assumptions are:
# The signal of interest is made of a sequence of event-locked ERPs with invariable latency and shape
# The noise can be approximated by a zero-mean
Gaussian random process of variance
which is uncorrelated between trials and not time-locked to the event (this assumption can be easily violated, for example in the case of a subject doing little tongue movements while mentally counting the targets in an experiment).
Having defined
, the trial number, and
, the time elapsed after the
th event, each recorded trial can be written as
where
is the signal and
is the noise (Note that, under the assumptions above, the signal does not depend on the specific trial while the noise does).
The average of
trials is
:
.
The
expected value
In probability theory, the expected value (also called expectation, expectancy, mathematical expectation, mean, average, or first moment) is a generalization of the weighted average. Informally, the expected value is the arithmetic mean of a ...
of
is (as hoped) the signal itself,
.
Its
variance
In probability theory and statistics, variance is the expectation of the squared deviation of a random variable from its population mean or sample mean. Variance is a measure of dispersion, meaning it is a measure of how far a set of numbe ...
is
:
.
For this reason the noise amplitude of the average of
trials is expected to deviate from the mean (which is
) by less or equal than
in 68% of the cases. In particular, the deviation wherein 68% of the noise amplitudes lie is
times that of a single trial. A larger deviation of
can already be expected to encompass 95% of all noise amplitudes.
Wide amplitude noise (such as eye blinks or movement
artifacts) are often several orders of magnitude larger than the underlying ERPs. Therefore, trials containing such artifacts should be removed before averaging. Artifact rejection can be performed manually by visual inspection or using an automated procedure based on predefined fixed thresholds (limiting the maximum EEG amplitude or slope) or on time-varying thresholds derived from the statistics of the set of trials.
Nomenclature
ERP waveforms consist of a series of positive and negative voltage deflections, which are related to a set of underlying components. Though some ERP components are referred to with acronyms (e.g.,
contingent negative variation – CNV,
error-related negativity – ERN), most components are referred to by a letter (N/P) indicating polarity (negative/positive), followed by a number indicating either the latency in milliseconds or the component's
ordinal position in the waveform. For instance, a negative-going peak that is the first substantial peak in the waveform and often occurs about 100 milliseconds after a stimulus is presented is often called the
N100 (indicating its latency is 100 ms after the stimulus and that it is negative) or N1 (indicating that it is the first peak and is negative); it is often followed by a positive peak, usually called the
P200 or P2. The stated latencies for ERP components are often quite variable, particularly so for the later components that are related to the cognitive processing of the stimulus. For example, the
P300 component may exhibit a peak anywhere between 250 ms – 700 ms.
Advantages and disadvantages
Relative to behavioral measures
Compared with behavioral procedures, ERPs provide a continuous measure of processing between a stimulus and a response, making it possible to determine which stage(s) are being affected by a specific experimental manipulation. Another advantage over behavioral measures is that they can provide a measure of processing of stimuli even when there is no behavioral change. However, because of the significantly small size of an ERP, it usually takes a large number of trials to accurately measure it correctly.
Relative to other neurophysiological measures
Invasiveness
Unlike microelectrodes, which require an electrode to be inserted into the brain, and
PET scans that expose humans to radiation, ERPs use EEG, a non-invasive procedure.
Spatial and temporal resolution
ERPs provide excellent
temporal resolution—as the speed of ERP recording is only constrained by the sampling rate that the recording equipment can feasibly support, whereas
hemodynamic
Hemodynamics or haemodynamics are the dynamics of blood flow. The circulatory system is controlled by homeostatic mechanisms of autoregulation, just as hydraulic circuits are controlled by control systems. The hemodynamic response continuously mo ...
measures (such as
fMRI
Functional magnetic resonance imaging or functional MRI (fMRI) measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area ...
,
PET, and
fNIRS) are inherently limited by the slow speed of the
BOLD
In typography, emphasis is the strengthening of words in a text with a font in a different style from the rest of the text, to highlight them. It is the equivalent of prosody stress in speech.
Methods and use
The most common methods in W ...
response. The
spatial resolution of an ERP, however, is much poorer than that of hemodynamic methods—in fact, the location of ERP sources is an
inverse problem that cannot be exactly solved, only estimated. Thus, ERPs are well suited to research questions about the speed of neural activity, and are less well suited to research questions about the location of such activity.
Cost
ERP research is much cheaper to do than other imaging techniques such as
fMRI
Functional magnetic resonance imaging or functional MRI (fMRI) measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area ...
,
PET, and
MEG
Meg is a feminine given name, often a short form of Megatron, Megan, Megumi (Japanese), etc. It may refer to:
People
* Meg (singer), a Japanese singer
*Meg Cabot (born 1967), American author of romantic and paranormal fiction
* Meg Burton Cahill ...
. This is because purchasing and maintaining an EEG system is less expensive than the other systems.
Clinical
Physician
A physician (American English), medical practitioner (Commonwealth English), medical doctor, or simply doctor, is a health professional who practices medicine, which is concerned with promoting, maintaining or restoring health through th ...
s and
neurologists will sometimes use a flashing
visual
The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight ...
checkerboard stimulus to test for any damage or trauma in the visual system. In a healthy person, this stimulus will elicit a strong response over the primary
visual cortex
The visual cortex of the brain is the area of the cerebral cortex that processes visual information. It is located in the occipital lobe. Sensory input originating from the eyes travels through the lateral geniculate nucleus in the thalamus ...
located in the
occipital lobe, in the back of the brain.
ERP component abnormalities in clinical research have been shown in neurological conditions such as:
*
AD/HD
*
Dementia
Dementia is a disorder which manifests as a set of related symptoms, which usually surfaces when the brain is damaged by injury or disease. The symptoms involve progressive impairments in memory, thinking, and behavior, which negatively affe ...
*
Parkinson's disease
Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms beco ...
*
Multiple sclerosis
Multiple (cerebral) sclerosis (MS), also known as encephalomyelitis disseminata or disseminated sclerosis, is the most common demyelinating disease, in which the insulating covers of nerve cells in the brain and spinal cord are damaged. This ...
* Head injuries
* Stroke
*
Obsessive-compulsive disorder
*
Schizophrenia
Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social w ...
*
Depression
*
Autism Spectrum Disorder
Research
ERPs are used extensively in
neuroscience
Neuroscience is the science, scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions and disorders. It is a Multidisciplinary approach, multidisciplinary science that combines physiology, an ...
,
cognitive psychology
Cognitive psychology is the scientific study of mental processes such as attention, language use, memory, perception, problem solving, creativity, and reasoning.
Cognitive psychology originated in the 1960s in a break from behaviorism, which ...
,
cognitive science, and
psycho-physiological research.
Experimental psychologists and
neuroscientists have discovered many different stimuli that elicit reliable ERPs from participants. The timing of these responses is thought to provide a measure of the timing of the brain's communication or timing of information processing. For example, in the checkerboard paradigm described above, healthy participants' first response of the visual cortex is around 50–70 ms. This would seem to indicate that this is the amount of time it takes for the
transduced visual stimulus to reach the
cortex after
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
first enters the
eye. Alternatively, the
P300 response occurs at around 300ms in the
oddball paradigm, for example, regardless of the type of stimulus presented:
visual
The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight ...
,
tactile
Tactile may refer to:
* Tactile, related to the sense of touch
* Haptics (disambiguation)
* Tactile (device), a text-to-braille translation device
See also
* Tangibility, in law
* Somatosensory system
In physiology, the somatosensory system ...
,
auditory,
olfactory
The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste.
In humans, ...
,
gustatory, etc. Because of this general invariance with regard to stimulus type, the P300 component is understood to reflect a higher cognitive response to unexpected and/or cognitively
salient stimuli. The P300 response has also been studied in the context of information and memory detection. In addition, there are studies on abnormalities of P300 in depression. Depressed patients tend to have a reduced P200 and P300 amplitude and a prolonged P300 latency.
Due to the consistency of the P300 response to novel stimuli, a
brain–computer interface can be constructed which relies on it. By arranging many signals in a grid, randomly flashing the rows of the grid as in the previous paradigm, and observing the P300 responses of a subject staring at the grid, the subject may communicate which stimulus he is looking at, and thus slowly "type" words.
Another area of research in the field of ERP lies in the
efference copy. This predictive mechanism plays a central role in for example human verbalization. Efference copies, however, do not only occur with spoken words, but also with inner language - i.e. the quiet production of words - which has also been proven by event-related potentials.
Other ERPs used frequently in research, especially
neurolinguistics research, include the
ELAN, the
N400, and the
P600/SPS. The analysis of ERP data is also increasingly supported by machine learning algorithms.
Number of Trials
A common issue in ERP studies is whether the observed data have a sufficient number of trials to support statistical analysis. The background noise in any ERP for any individual can vary. Therefore simply characterizing the number of ERP trials needed for a robust component response is inadequate. Therefore, ERP researchers can use metrics like the standardized measurement error (SME) to justify the examination of between-condition or between-group differences or estimates of internal consistency to justify the examination of individual differences.
See also
*
Bereitschaftspotential
*
C1 and P1
*
Contingent negative variation
*
Difference due to memory
*
Early left anterior negativity
*
Erich Schröger
*
Error-related negativity
*
Evoked potential
*
Induced activity
*
Lateralized readiness potential
*
Mismatch negativity
* Negativity:
N100 •
Visual N1 •
N170 •
N200 •
N2pc •
N400
* Positivity:
P200 •
P300 •
P3a •
P3b
The P3b is a subcomponent of the P300, an event-related potential (ERP) component that can be observed in human scalp recordings of brain electrical activity. The P3b is a positive-going amplitude (usually relative to a reference behind the ear ...
•
Late positive component •
P600
*
Somatosensory evoked potential
References
Further reading
*
*
*
*
*
*
External links
– ERP Summer School 2017 was held in The School of Psychology, Bangor University from 25–30 June 2017
EEGLAB Toolbox– A freely available, open-source, Matlab toolbox for processing and analyzing EEG data
ERPLAB Toolbox– A freely available, open-source, Matlab toolbox for processing and analyzing ERP data
The ERP Boot Camp – A series of training workshops for ERP researchers led by Steve Luck and Emily Kappenman
Virtual ERP Boot Camp– A blog with information, announcements, and tips about ERP methodology
{{DEFAULTSORT:Event-Related Potential
Evoked potentials
Electroencephalography
Neurophysiology
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