Event-related potentials

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As event-related potentials , or event-related potentials (ERP, English : event-related potentials, ERP ) are waveforms in the electroencephalogram designated (EEG), related to an observable event. Such an event can be a sensory stimulus that acts on the test person or a movement of the test person. Therefore, one can differentiate between stimulus-related and movement-related EKPs. In order to recognize the connection between event and potential, the event must be repeated many times and the EEG must be measured with the same time reference to the event with each repetition.

If the event is always the same stimulus without further instruction to the test person, then their EKPs only have perception-related components and one speaks of evoked potentials . However, one can use different events and only allow some of them to be observed or to react to only some, so that the processing of these events can be differentiated in processes of attention , discovery of irregularities, awareness of perception , decision , expectation, preparation for movement. The EKPs can be used to measure neurophysiological correlates of these cognitive processes. Research into EKPs is therefore part of cognitive neuroscience .

EKPs provide precise information about when areas of the cerebral cortex are activated, but only imprecise information about which areas these are. In this respect, they are complementary to functional magnetic resonance imaging , which can measure the activation of brain areas very well spatially, but only imprecisely over time. Advantages of the EKP method are the relatively low costs and the non-invasiveness of the measurement, as only measuring electrodes are stuck to the scalp.

Schematic representation of the course of event-related potentials when receiving and processing a visual or auditory stimulus (see Birbaumer & Schmidt, 2006, p. 481)


In order to make the EKPs, which are often small relative to the spontaneous EEG, visible at all, the event must be repeated many times and the EEG measured with each repetition with the same time reference to the event. The EEG is then usually averaged over these repetitions . The event-independent parts of the EEG (the spontaneous EEG, noise) are averaged out and the event-dependent waveform is shown. In addition, more complex methods of signal analysis are being developed. B. Changes in the vibration strengths ( time-frequency analysis , wavelet analysis ), in the synchronization or in the coherence can be demonstrated via the individual measurements.

EKPs are usually measured simultaneously with many electrodes, often 32 or 64, which are placed over the scalp. For physiological and physical reasons, almost only the activity of the cerebral cortex can be measured directly, only in exceptional cases from deeper centers such as the thalamus , hippocampus , basal ganglia or from the cerebellum . This measurable part from the cerebral cortex is also weakened and spatially blurred on its transmission path to the outer scalp. Electric voltage can only be measured between two points; therefore the measurement from the scalp electrodes needs a reference point. This is often formed by electrodes on the ears or on the nose, or is redefined at each point in time as the mean value of all electrodes at this point in time. The voltages measured on the scalp in this way are a few microvolts and must therefore always be checked for disturbing voltages that have not developed in the brain; the most important source of such interference voltages in otherwise calmly sitting test participants are blinking and eye movements. Restriction to activity of the cerebral cortex, spatial fuzziness, dependence on the choice of reference and, in general, measurements from outside (instead of invasive direct from the brain, as possible in animal experiments or in patients with electrodes implanted for medical reasons) are the methodological reasons why EKPs give precise information about when areas of the cerebral cortex are activated, but only imprecise information about which areas these are.

EKPs can be obtained not only via the EEG, but also via magnetoencephalography (MEG) (since, according to the rule of thumb, every electrical field has a magnetic field); the measurement is much more complex, but does not have the problem of the spatial imprecision of the transmission from the cerebral cortex to the scalp.


Applications of EKPs can be found in psychophysics and in cognitive sciences . In the public discussion, in particular, the importance of the readiness potential in the Libet experiment has reached the question of whether our brain determines our free will. In psycholinguistics one examines EKPs that are accompanied by difficulties in understanding sentences: For example, the N400 (a voltage fluctuation of negative polarity 0.4 seconds after a critical word) occurs in semantic processing problems , e.g. B. when you hear or read the word “concrete” in the sentence “Hanna drinks a glass of concrete”. The P600 is a positivity in the EEG that occurs 0.6 seconds after a critical word and testifies to syntactic processing difficulties, such as those found in B. the sentence "Hans believes that the discoverer told of America" ​​evokes when we equate "the discoverer of America" ​​with "Columbus" and expect his story.

In clinical psychology , psychiatry and neurology , research with EKPs is used to understand disease-related malfunctions. For example, in the case of people with schizophrenia, a deviation from the N400 was found, which supports the hypothesis of easier facilitation in the semantic network in people with schizophrenia.

EKP components

EKPs consist of several components, describable by their polarity (negative or positive voltage), the location of their maximum amplitude on the scalp and the time of this peak amplitude relative to the event (either in milliseconds or in chronological order). For example, occipital N130 denotes a negative peak with a maximum at the back of the head at 130 ms after the start of the stimulus. This is the expected first negative component of a visual stimulus (the visual cortex is on the back of the head) and is therefore also called N1 or visual N1 (as the “first negative” component) .


There are various competing approaches to explain the origin of EEG components. In the phase reset model , it is assumed that an EKP can be measured when the ongoing neural oscillations are desynchronized due to external stimulation. Accordingly, the phase of the oscillation is reset at the time of stimulation . The reorganization of the existing vibration pattern is made visible as an EKP. The additive power model opposes this concept . This is based on neural activity patterns, which are caused by external stimulation independent of the ongoing EEG. The resulting signal is superimposed on the background EEG and is visible as a component in the EKP. It seems likely that the generation of EKPs is a combination of both mechanisms. Early EKPs (<300 ms) are more likely to be attributed to a phase reset and later components reflect processes that are independent of the background EEG.


Unless otherwise stated, the time window values ​​in milliseconds relate to the time at which the stimulus was presented.

Surname Time window (ms) Maximum on the scalp description
Positive polarity
P50 40-75 central Occurs in the perception of acoustic stimuli. Represents sensory gating.
Vertex Positive Potential (VPP) 130-200 central Occurs specifically when looking at faces. It is very likely that this is the positive pole of the N170 generated in the fusiform gyrus and other temporal gyri .
P200 / P2 150-250 centro-frontal and parieto-occipital Occurs when processing visual stimuli (modulated by attention)
P3a 250-400 fronto-central middle Is triggered by unexpected, new, attention-grabbing stimuli and could be related to the orientation response.
P300 / P3b / P3 300-600 centro-parietal middle Occurs with task-relevant stimuli - the larger, the less frequently the stimuli occur.
P600 400-800 centro-parietal middle Language-related component when reading or listening to words that are grammatically incorrect or otherwise difficult to understand.
Error positivity / Pe 200–500 ms after movement centro-parietal middle Occurs after consciously noticed wrong actions.
Negative polarity
N100 / N130 / N1 80-150 auditory: central and temporal; visual: occipital; somatosensory: central The N1 occurs with clearly delimited stimuli. Their time of occurrence and scalp distribution is specific to the sensory modality. She is z. B. modulated by the strength and the interstimulus interval of a stimulus.
N170 130-200 temporo-occipital right Occurs when processing faces.
Early Left Anterior Negativity (ELAN) <200 frontal left Occurs when processing a fraction in the sentence structure or word category.
N200 / N2 / MMN 100-350 ia fronto-central center Expression of the perception of a deviating stimulus and u. U. of behavioral inhibition. A special sub-component is the mismatch negativity.
N2pc 200-300 temporo-occipital contralateral to the stimulus Shows the time and extent of the selection of one of several visual stimuli (pc = posterior contralateral).
N250 250-500 inferio-temporal Processing the identity of a face.
N400 250-500 fronto-central center or centro-parietal center Is triggered by meaningful stimuli (such as words, symbols, etc.). Represents a measure of semantic integration in previously perceived contextual information.
Heartbeat Evoked Potential (HEP) 250-400 fronto-central middle Occurs after the R wave in the electrocardiogram regardless of the heartbeat artifact. Represents the interoception of one's own heartbeat.
Reorienting Negativity (RON) 400-600 fronto-central middle Reflects the return of attention to the task after distraction in an oddball experiment.
Contingent Negative Variation (CNV) slowly increasing central center, with emphasis on the contralateral hand Occurs in anticipation of a target stimulus after a cue stimulus.
Stimulus-Preceding Negativity (SPN) slowly increasing fronto-central center, emphasized right hemisphere Occurs in anticipation of an information-containing stimulus.
Readiness potential slowly increasing,> 1 s before movement central center, with emphasis on the contralateral hand Occurs in preparation for an arbitrary action.
Lateralized Readiness Potential (LRP) from 200 ms before movement centrally contralateral to the hand In tasks where you want to react to one stimulus on the right and the other on the left, shows the time of the decision for the right or left hand. The LRP has the motor asymmetry in common with the readiness potential, but not its predominant slow component.
Error Negativity / ERN / Ne 0–100 ms after movement fronto-central middle Occurs in case of wrongdoing and reflects the conflict between action and action plan.
Contralateral Delay Activity (CDA) while maintaining a memory content in the visual working memory parieto-occipital (PO7 / PO8) A negative shift in the EEG while content is kept active in the visual working memory. Depending on the presentation side of the stimulus, the CDA is displayed as the difference between the signals from the contralateral minus ipsilateral side.


The so-called Mismatch Negativity (MMN) occurs approx. 100–350 ms after an acoustic stimulus that deviates from the rule of the previous series of stimuli. In the simplest case, such a stimulus is a tone that differs in frequency, duration, location or intensity from previously similar stimuli. B. be a tone repetition after multiple alternations or a wrong tone in a well-known melody. The derivation of the MMN does not require active participation by the test subjects; it is therefore an expression of an automatic reaction. They were first described by Näätänen and colleagues in 1978. An MMN on visual stimuli has also been demonstrated.

For the auditory modality, the MMN occurs most prominently on the fronto-central electrodes and shows a slight right-hemispheric dominance. Dipole analyzes and converging research results using imaging methods show a supratemporal generator and suggest a second, prefrontal source.

Näätänen initially assumed that the MMN is the expression of a pre-attentive process that continuously captures the invariant acoustic environment and, in the event of deviating stimuli, makes resources available so that attention could be paid to the relevant stimulus. An alternative and supplementary hypothesis states that the MMN is an expression of the updating of the rule that has developed in the hearing system due to the previous regularity. This hypothesis resulted in cross-connections to Friston's assumption of "predictive coding", according to which the brain is a predictive organ and EKPs reflect the comparison of these predictions with the external stimulation.

The presence of MMN in patients in a coma is a favorable sign - of course only within the bounds of probability - that these patients will wake up from the coma again. In healthy people, the MMN can also be used to measure the current level of language competence; z. B. Japanese people have reduced MMN to the distinction between the English L and R, and English-speaking Americans have reduced MMN to the differences in pitch change important in Mandarin .


As soon as the presented stimuli are linked to a task and thus become relevant, they trigger a P300 ; For example, in the first two descriptions of the P300 1965: if the stimulus had to be guessed which of two stimuli would come (Sutton et al.), or if one of two stimuli that came in random order had to be reacted to (Desmedt et al.). The P300 increases with decreasing frequency of the rare stimulus (so-called oddball effect ).

The P300 generally consists of two different components that can overlap: the P3a and the P3b. The P3a , with the greatest amplitude fronto-central at the apex, can be triggered by rare stimuli that should actually be ignored; presumably this is an expression of an orientation reaction . Similar P3a-like potentials are triggered by novel stimuli that are undefined in the task ( novelty P3 ) and by rare stimuli to which one should expressly not react ( no-go P3 ). The "real" P300 is the P3b , with the greatest amplitude in the centro-parietal region at the apex; she is sensitive to the task relevance of the stimuli. The name P300 is a bit inaccurate, as the peak of the P300 is usually later than 300 ms for visual stimuli - for simple tasks in young adults around 350 ms, otherwise even later. Therefore, advocates in the literature for instead of the designation P300 simply P3 as the third positive deflection to use after stimulus presentation.

The P3b measured on the scalp is mainly produced by activities in the parietal and temporal cortex, with a key role for the area of ​​the temporo-parietal junction. Interestingly, there is also P3b-like activity in the hippocampus, which is important for memory (in EEG recordings directly from the brain in patients with severe epilepsy , to decide whether to have an operation), but rather later than the P3b that can be measured on the scalp, therefore apparently not generating these.

There are different opinions about which psychological process the P3b reflects (more recent overview :). According to Donchin, these different hypotheses can be divided into strategic and tactical hypotheses. Tactical here means that the P3b process is there to organize the reaction to the current stimulus; Strategic hypotheses, on the other hand, assume that the P3b process stands for a reaction-independent function. In this strategic sense, Donchin's influential context updating hypothesis considers the P3b to be the expression of a re-evaluation of the situation based on new evidence. Other strategic hypotheses are Closure , according to which the P3b expresses the end of a cognitive epoch, and Dehaene's global workspace hypothesis, according to which the P3b is an expression of the awareness of the event by connecting cortical areas. Tactical hypotheses of the P3b have gained a bit of influence in recent years, as it has been shown that the P3b is at least as closely linked to the reaction as to the triggering stimulus, i.e. the underlying process could exercise a kind of mediator function and therefore reflect decision-making processes (among others).

Applications of the P300 result from its property as a relatively large and therefore relatively easily measurable EKP component that reflects the degree of relevance of an event for the respective person. If a person is shown a series of words, some of which are related to a wrongdoing that the person knows more precisely, this knowledge of the perpetrator should be reflected in the size of the P300 that is specifically triggered by these words. Due to this logic were in the US P300 lie detectors developed. The same logic is used by brain-computer interfaces for immobile patients who otherwise can no longer communicate with the outside world; Here the patients use the P300 to choose from a series of letters who they would next type into a keyboard if they could move, and if successful, they can write SMS-like messages to their caregivers.


  • Jan Seifert: Event-related EEG activity. Pabst, Lengerich 2005, ISBN 3-89967-236-4 .
  • Steven J. Luck: An Introduction to the Event-Related Potential Technique. The MIT Press, Cambridge, Mass. 2005, ISBN 0-262-62196-7 .
  • Todd C. Handy: Event-Related Potentials: A Methods Handbook. The MIT Press (B&T), Cambridge, Mass. 2004, ISBN 0-262-08333-7 .

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