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m (Dr Joe Kiff moved page Event-related potential to Event-related potentials: plural)
 
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{{Main|Evoked potentials}}
 
{{Main|Evoked potentials}}
An '''event-related potential''' (ERP) is any stereotyped [[electrophysiology|electrophysiological]] response to an internal or external stimulus. More simply, it is any measured [[brain]] response that is directly the result of a [[thought]] or [[perception]].
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[[File:ComponentsofERP.svg|right|thumb|A waveform showing several ERP components, including the [[N100 (neuroscience)|N100]] and [[P300 (neuroscience)|P300]]. Note that the ERP is plotted with negative voltages upward, a common, but not universal, practice in ERP research]]
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An '''event-related potential''' ('''ERP''') is the measured [[brain]] response that is the direct result of a specific [[sense|sensory]], [[cognition|cognitive]], or [[motor system|motor]] event.<ref name="Luck">{{Cite book | last = Luck | first = Steven J. | title = An Introduction to the Event-Related Potential Technique | publisher = The MIT Press | year = 2005 | isbn =0-262-12277-4 }}</ref> More formally, it is any stereotyped [[electrophysiology|electrophysiological]] response to a stimulus. The study of the brain in this way provides a [[Invasiveness of surgical procedures|noninvasive]] means of evaluating brain functioning in patients with cognitive diseases.
   
==Measurement==
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ERPs are measured with [[electroencephalography]] (EEG). The [[magnetoencephalography]] (MEG) equivalent of ERP is the ERF, or event-related field.<ref>{{cite book | editor=Colin M. Brown and Peter Hagoort | last=Brown | first=Colin M | coauthors=Peter Hagoort | chapter=The cognitive neuroscience of language | title=The Neurocognition of Language | year=1999 | publisher=[[Oxford University Press]] | location=New York | page=6}}</ref>
   
ERPs can be reliably measured using [[electroencephalograph]]y (EEG), a procedure that measures [[electricity|electrical]] activity of the brain through the [[skull]] and [[scalp]]. As the EEG reflects thousands of simultaneously [[Ongoing brain activity|ongoing brain processes]], the brain response to a certain stimulus or event of interest is usually not visible in the EEG. One of the most robust features of the ERP response is a response to unpredictable stimuli. This response-known as the [[P300]] (or simply "P3")-manifests as a positive deflection in [[volt|voltage]] approximately 300 [[second|milliseconds]] after the stimulus is presented.
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==History==
   
In actual recording situations, it is difficult to see an ERP after the presentation of a single stimulus. Rather the most robust ERPs are seen after many dozens or hundreds of individual presentations are [[average]]d together. This technique cancels out [[noise (electronic)|noise]] in the data allowing only the voltage response to the stimulus to stand out clearly.
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With the discovery of the [[electroencephalogram]] (EEG) in 1929, [[Hans Berger]] revealed that one could measure the electrical activity of the human brain by placing electrodes on the scalp and intensifying 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.
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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]] 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).<ref>Walter, W.G; Cooper, R.; Aldridge, V.J.; McCallum, W.C.; Winter, A.L. (1964). "Contingent Negative Variation: an electric sign of sensorimotor association and expectancy in the human brain". Nature 203 (4943): 380–384.</ref> Sutton, Braren, and Zubin (1965) made another advancement with the discovery of the P3 component.<ref>Sutton, S., Braren, M., Zubin, J., & John, E.R. (1965). Evoked-Potential Correlates of Stimulus Uncertainty. Science, 150, 1187-1188</ref> 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]] correlates of [[sensory]], [[perceptual]] and [[cognitive]] activity associated with processing information.<ref>Handy, T. C. (2005). Event Related Potentials: A Methods Handbook. Cambridge, MA: Bradford/MIT Press.</ref>
   
While [[evoked potential]]s reflect the processing of the physical stimulus, event-related potentials are caused by the "higher" processes, that might involve [[memory]], expectation, [[attention]], changes in the mental state etc.
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==Calculation==
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ERPs can be [[Reliability (statistics)|reliably]] measured using [[electroencephalograph]]y (EEG), a procedure that measures [[electricity|electrical]] activity of the brain over time using [[electrode]]s placed on the [[scalp]]. The EEG reflects thousands of simultaneously [[Ongoing brain activity|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 (100 or more: reference?) and average the results together, causing random brain activity to be averaged out and the relevant waveform to remain, called the ERP.<ref>{{cite book | url=http://l3d.cs.colorado.edu/~ctg/classes/lib/cogsci/Rugg-ColesChp1.pdf | chapter=Event-related brain potentials: an introduction | year=1996 | title=Electrophysiology of Mind | pages=1&ndash;27 | publisher=Oxford Scholarship Online Monographs | last=Coles | coauthors=[[Michael D. Rugg]] | first=Michael G.H.}}</ref>
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The random ([[Neural_oscillation#Ongoing_activity|background]]) brain activity together with other bio-signals (e.g., [[Electrooculography|EOG]], [[Electromyography|EMG]], [[Electrocardiography|EKG]]) and electromagnetic interference (e.g., [[Noise (electronics)|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.
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From an engineering point of view it is possible to define the signal-to-noise ratio ([[Signal-to-noise ratio|SNR]]) of the recorded ERPs. The reason that averaging increases the SNR of the recorded ERPs (making them discernible and allowing for their interpretation) has a simple mathematical explanation provided that some simplifying assumptions are made. These assumptions are:
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# The signal of interest is made of a sequence of event-locked ERPs with invariable latency and shape
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# The noise can be approximated by a zero-mean [[Gaussian process|Gaussian random process]] of variance <math>\sigma^2</math> 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 [[oddball paradigm]]).
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Having defined <math>k</math>, the trial number, and <math>t</math>, the time elapsed after the <math>k</math><sup>th</sup> event, each recorded trial can be written as <math>x(t,k)=s(t)+n(t,k)</math> where <math>s(t)</math> is the signal and <math>n(t,k)</math> 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 <math>N</math> trials is
  +
:<math>\bar x(t) = \frac{1}{N} \sum_{k=1}^N x(t,k) = s(t) + \frac{1}{N} \sum_{k=1}^N n(t,k)</math> .
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The [[expected value]] of <math>\bar x(t)</math> is (as hoped) the signal itself, <math>\operatorname{E}[\bar x(t)] = s(t)</math>.
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Its [[variance]] is
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:<math>\operatorname{Var}[\bar x(t)] = \operatorname{E}\left[\left(\bar x(t) - \operatorname{E}[\bar x(t)]\right)^2\right] = \frac{1}{N^2} \operatorname{E}\left[\left(\sum_{k=1}^N n(t,k)\right)^2\right] = \frac{1}{N^2} \sum_{k=1}^N \operatorname{E}\left[n(t,k)^2\right] = \frac{\sigma^2}{N}</math>.
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For this reason the noise amplitude of the average of <math>N</math> trials is <math>1/{\sqrt{N}}</math> times that of a single trial.
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Wide amplitude noise (such as eye blinks or movement [[Artifact (error)|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.<ref>{{cite web |url=http://www.mathworks.com/matlabcentral/fileexchange/33207 |title=ERP_REJECT, rejection of outlier trials from ERP studies |author= |publisher=Matlab File Exchange |accessdate=December 30, 2011}}</ref>
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==ERP Components Nomenclature==
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ERP waveforms consist of a series of positive and negative voltage deflections, which are related to a set of underlying '''components'''.<ref>{{cite book|title=The Oxford Handbook of Event-Related Potential Components|year=2012|publisher=Oxford University Press|isbn=9780195374148|pages=664|url=http://www.oup.com/us/catalog/general/subject/Psychology/CognitivePsychology/?view=usa&ci=9780195374148|editor=Luck, S.J., and Kappenman, E.S.}}</ref> Though some ERP components are referred to with acronyms (e.g., [[contingent negative variation]] - CNV, [[error-related negativity]] - ERN, [[early left anterior negativity]] - ELAN, [[closure positive shift]] - CPS), 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 number|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 (neuroscience)|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. For example, the [[P300 (neuroscience)|P300]] component may exhibit a peak anywhere between 250ms - 700ms.<ref>For discussion of ERP component naming conventions see Luck, Steven (2005), ''An Introduction to the Event-Related Potential Technique'', MIT Press, pp. 10-11.</ref>
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While [[evoked potential]]s reflect the processing of the physical stimulus, event-related potentials are caused by the "higher" processes, that might involve [[memory]], [[Expectation (epistemic)|expectation]], [[attention]], or changes in the mental state, among others.
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==Relative Advantages & Disadvantages==
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===ERP vs Behavioral Measures===
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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 sample size to accurately measure it correctly.<ref>Luck, Steven (2005). An Introduction to the Event-Related Potential Technique. MIT Press, pp. 21-23.</ref>
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===ERP vs Other Physiological Measures===
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====Invasiveness====
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Unlike microelectrodes which require an electrode to be inserted into the brain and [[Positron emission tomography|PET]] scans that expose humans to radiation, ERPs use EEG, a non-invasive procedure.
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====Spatial and Temporal Resolution====
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ERPs provide excellent [[temporal resolution]] of 1 ms or better. The [[spatial resolution]] of an ERP, however, is currently undefined making it impossible to definitively localize ERPs. This provides the ERP with a major disadvantage over [[hemodynamic]] measures ([[fMRI]] and [[Positron emission tomography|PET]]) which have a spatial resolution in the millimeter range. The fact that ERPs cannot easily be localized makes it extremely difficult to isolate a single ERP component from the overall ERP component.<ref>{{cite book|last=Luck|first=Steven|title=An Introduction to the Event-Related Potential Technique|year=2005|publisher=MIT Press|location=Massachusetts Institute of Technology|isbn=0-262-62196-7|pages=25–26}}</ref>
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===Cost===
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ERP research is much cheaper to do than other imaging techniques such as [[fMRI]] and [[Positron emission tomography|PET]]. This is because purchasing and maintaining an EEG system is less expensive than the other systems.
   
 
==Clinical ERP==
 
==Clinical ERP==
[[Physician]]s and [[neurology|neurologists]] will sometimes use a flashing [[vision|visual]] 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]] located in the [[occipital lobe]] in the back of the brain.
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[[Physician]]s and [[neurology|neurologists]] will sometimes use a flashing [[Visual perception|visual]] 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]] located in the [[occipital lobe]], in the back of the brain.
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ERP component abnormalities in clinical research have been shown in neurological conditions such as:
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*dementia<ref>Boutros, N., et al. (1995). Evoked potentials in subjects at risk for Alzheimer's disease. Psychiatry Res, 57, (1), 57-63.</ref>
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*Parkinson's disease<ref>Prabhakar, S., Syal, P. and Srivastava, T. (2000). P300 in newly diagnosed nondementing
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Parkinson's disease: effect of dopaminergic drugs. Neurol India, 48, (3), 239-
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242.</ref>
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*multiple sclerosis<ref>Boose, M. A. and Cranford, J. L. (1996). Auditory event-related potentials in multiple sclerosis. Am J Otol, 17, (1), 165-170.</ref>
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*head injuries<ref>Duncan, C. C., Kosmidis, M. H. and Mirsky, A. F. (2003). Event-related potential assessment of information processing after closed head injury. Psychophysiology, 40, (1), 45-59.</ref>
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*stroke<ref>D'Arcy, R. C., et al. (2003). Electrophysiological assessment of language function following stroke. Clin Neurophysiol, 114, (4), 662-672.</ref>
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*Obsessive-Compulsive Disorder<ref>Hanna, G.L., Carrasco, M., Harbin, S.M., Nienhuis, J.K., LaRosa, C.E., Chen, P., Fitzgerald, K.D., Gehring, W.J. (2012). Error-Related Negativity and Tic History in Pediatric Obsessive-Compulsive Disorder. Child Adolescent Psychiatry, 51, (9), 902-910.</ref>
   
 
==Research ERP==
 
==Research ERP==
[[Experimental psychology|Experimental psychologists]] and [[neuroscience|neuroscientists]] have discovered many different stimuli, such as erotica (in a Washington University study), to elicit reliable EEG ERPs from participants. The timing of these responses is thought to provide a measure of the timing of the brain's communication or time of information processing. For example, in the checkerboard paradigm described above, in healthy participants the response of the visual cortex is around 150-200ms. This would seem to indicate that this is the amount of time it takes for the transduced visual stimulus to reach the [[telencephalon|cortex]] after [[light]] first enters the [[eye]]. Alternatively, the P300 response occurs at around 300ms regardless of the stimulus presented: visual, [[tactition|tactile]], [[sound|auditory]], etc. Because of this general invariance in regard to stimulus type, this ERP likely reflects a higher cognitive response to new stimuli.
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ERPs are used extensively in [[neuroscience]], [[cognitive psychology]], [[cognitive science]], and [[Psychophysiology|psycho-physiological]] research. [[Experimental psychology|Experimental psychologists]] and [[neuroscience|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 [[Transduction (physiology)|transduced]] visual stimulus to reach the [[telencephalon|cortex]] after [[light]] first enters the [[eye]]. Alternatively, the [[P300 (neuroscience)|P300]] response occurs at around 300ms in the [[oddball paradigm]], for example, regardless of the type of stimulus presented: [[Visual system|visual]], [[tactition|tactile]], [[sound|auditory]], [[olfaction|olfactory]], [[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 [[Salience (neuroscience)|salient]] stimuli.
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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.<ref>{{cite journal|last=Farwell|first=L.A.|coauthors=Donchin E.|title=Talking off the top of your head: toward a mental prosthesis utilizing event-related brain potentials|journal=Electroencephalogr Clin Neurophysiol.|year=1988|volume=70|issue=6|pages=510–23|pmid=2461285|url=http://www.ncbi.nlm.nih.gov/pubmed/2461285|accessdate=5 December 2011|doi=10.1016/0013-4694(88)90149-6}}</ref>
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Other ERPs used frequently in research, especially [[neurolinguistics|neurolinguistics research]], include the [[ELAN (neurolinguistics)|ELAN]], the [[N400 (neuroscience)|N400]], and the [[P600 (neuroscience)|P600/SPS]].
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==ERP Software and Training Resources==
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* [http://sccn.ucsd.edu/eeglab/ EEGLAB Toolbox]- A freely available, open source, Matlab toolbox for processing and analyzing EEG data
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* [http://erpinfo.org/erplab ERPLAB Toolbox]- A freely available, open source, Matlab toolbox for processing and analyzing ERP data
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* [http://erpinfo.org/the-erp-bootcamp/bootcamp The ERP Boot Camp]- A series of training workshops for ERP researchers
   
 
==See also==
 
==See also==
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* [[Erich Schröger]]
 
* [[Evoked potential]]
 
* [[Evoked potential]]
 
* [[Induced activity]]
 
* [[Induced activity]]
* [[P300]]
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* [[Somatosensory evoked potential]]
* [[N400]]
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* [[C1 & P1 (Neuroscience)|C1 and P1]]
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* [[Mismatch negativity]]
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* Negativity: [[N100 (neuroscience)|N100]] - [[Visual N1]] - [[N170]] - [[N200 (neuroscience)|N200]] - [[N2pc]] - [[N400 (neuroscience)|N400]]
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* Positivity: [[P200]] - [[P300 (neuroscience)|P300]] - [[P3a]] - [[P3b]] - [[Late Positive Component]] - [[P600 (neuroscience)|P600]]
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* [[Difference due to Memory]]
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* [[Contingent negative variation]]
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* [[Error-related negativity]]
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* [[Bereitschaftspotential]]
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* [[Lateralized readiness potential]]
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* [[Early left anterior negativity]]
   
The P300 only peaks in the vicinity of 300 msec for very simple decisions. More generally, its latency appears to reflect the amount of time necessary to come to a decision about the stimulus. The harder the decision, the longer it takes for the P300 to appear. The leading theory, the context updating hypothesis (Donchin and Coles, 1988), is that it reflects an updating of expectancies about how probable events are in the current context. Because this updating cannot be conducted until the stimulus has been categorized, its latency is dependent on how long it took to come to the decision. One of its useful properties is that, unlike measure of physical responses like button pressing, the P300 appears to reflect only this stimulus evaluation time and not the time required to translate the decision into the physical response (such as which finger to use).
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==Further reading==
   
The P300 also has useful properties of being larger to rare stimuli, especially if they are targets. The amplitude of the P300 therefore gives information about how the person is categorizing the stimuli and how rare they are considered to be subjectively. The P300 is only seen when the person is actively keeping track of the stimulus so it also gives information about what they are paying attention to, which makes it useful for BCI applications. It also has proven to be quite sensitive to a wide variety of pathologies, usually being diminished in amplitude.
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* Steven J. Luck: ''An Introduction to the Event-Related Potential Technique''. Cambridge, Mass.: The MIT Press, 2005. ISBN 0-262-62196-7
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* Todd C. Handy: ''Event-Related Potentials : A Methods Handbook''. Cambridge, Mass.: The MIT Press (B&T), 2004. ISBN 0-262-08333-7
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* Luck, S.J., and Kappenman, E.S., ed. (2012). [http://www.oup.com/us/catalog/general/subject/Psychology/CognitivePsychology/?view=usa&ci=9780195374148 The Oxford Handbook of Event-Related Potential Components]. Oxford University Press. pp.&nbsp;664. ISBN 9780195374148.
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* Monica Fabiani, Gabriele Gratton, and Kara D. Federmeier: "Event-Related Brain Potentials: Methods, Theory, and Applications". In ''Handbook of Psychophysiology'', ed. by John T. Cacioppo, Louis G. Tassinary, and Gary G. Berntson. 3rd. ed. Cambridge: Cambridge University Press, 2007. ISBN 978-0-521-84471-0. pp.&nbsp;85–119
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* John Polich and Jody Corey-Bloom, ''Alzheimer's Disease and P300: Review and evaluation of Task and Modality''. '''Current Alzheimer Research''', 2005, 2, 515-525
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* Zani A. & Proverbio A.M. (2003) ''Cognitive Electrophysiology of Mind and Brain''. Academic Press/Elsvier.
   
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==References==
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{{reflist}}
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{{EEG}}
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{{DEFAULTSORT:Event-Related Potential}}
 
[[Category:Evoked potentials]]
 
[[Category:Evoked potentials]]
 
[[Category:Electroencephalography]]
 
[[Category:Electroencephalography]]
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[[Category:Neurophysiology]]
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Main article: Evoked potentials
File:ComponentsofERP.svg

An event-related potential (ERP) is the measured brain response that is the direct result of a specific sensory, cognitive, or motor event.[1] 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 in patients with cognitive diseases.

ERPs are measured with electroencephalography (EEG). The magnetoencephalography (MEG) equivalent of ERP is the ERF, or event-related field.[2]

HistoryEdit

With the discovery of the electroencephalogram (EEG) in 1929, Hans Berger revealed that one could measure the electrical activity of the human brain by placing electrodes on the scalp and intensifying 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 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).[3] Sutton, Braren, and Zubin (1965) made another advancement with the discovery of the P3 component.[4] 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 correlates of sensory, perceptual and cognitive activity associated with processing information.[5]

CalculationEdit

ERPs can be reliably measured using electroencephalography (EEG), a procedure that measures electrical activity of the brain over time using electrodes 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 (100 or more: reference?) and average the results together, causing random brain activity to be averaged out and the relevant waveform to remain, called the ERP.[6]

The random (background) brain activity together with other bio-signals (e.g., EOG, 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. The reason that averaging increases the SNR of the recorded ERPs (making them discernible and allowing for their interpretation) has a simple mathematical explanation provided that some simplifying assumptions are made. These assumptions are:

  1. The signal of interest is made of a sequence of event-locked ERPs with invariable latency and shape
  2. The noise can be approximated by a zero-mean Gaussian random process of variance \sigma^2 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 oddball paradigm).

Having defined k, the trial number, and t, the time elapsed after the kth event, each recorded trial can be written as x(t,k)=s(t)+n(t,k) where s(t) is the signal and n(t,k) 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 N trials is

\bar x(t) = \frac{1}{N} \sum_{k=1}^N x(t,k) = s(t) + \frac{1}{N} \sum_{k=1}^N n(t,k) .

The expected value of \bar x(t) is (as hoped) the signal itself, \operatorname{E}[\bar x(t)] = s(t).

Its variance is

\operatorname{Var}[\bar x(t)] = \operatorname{E}\left[\left(\bar x(t) - \operatorname{E}[\bar x(t)]\right)^2\right] = \frac{1}{N^2} \operatorname{E}\left[\left(\sum_{k=1}^N n(t,k)\right)^2\right] = \frac{1}{N^2} \sum_{k=1}^N \operatorname{E}\left[n(t,k)^2\right] = \frac{\sigma^2}{N}.

For this reason the noise amplitude of the average of N trials is 1/{\sqrt{N}} times that of a single trial.

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.[7]

ERP Components NomenclatureEdit

ERP waveforms consist of a series of positive and negative voltage deflections, which are related to a set of underlying components.[8] Though some ERP components are referred to with acronyms (e.g., contingent negative variation - CNV, error-related negativity - ERN, early left anterior negativity - ELAN, closure positive shift - CPS), 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. For example, the P300 component may exhibit a peak anywhere between 250ms - 700ms.[9]

While evoked potentials reflect the processing of the physical stimulus, event-related potentials are caused by the "higher" processes, that might involve memory, expectation, attention, or changes in the mental state, among others.

Relative Advantages & DisadvantagesEdit

ERP vs Behavioral MeasuresEdit

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 sample size to accurately measure it correctly.[10]

ERP vs Other Physiological MeasuresEdit

InvasivenessEdit

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 ResolutionEdit

ERPs provide excellent temporal resolution of 1 ms or better. The spatial resolution of an ERP, however, is currently undefined making it impossible to definitively localize ERPs. This provides the ERP with a major disadvantage over hemodynamic measures (fMRI and PET) which have a spatial resolution in the millimeter range. The fact that ERPs cannot easily be localized makes it extremely difficult to isolate a single ERP component from the overall ERP component.[11]

CostEdit

ERP research is much cheaper to do than other imaging techniques such as fMRI and PET. This is because purchasing and maintaining an EEG system is less expensive than the other systems.

Clinical ERPEdit

Physicians and neurologists will sometimes use a flashing visual 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 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:

  • dementia[12]
  • Parkinson's disease[13]
  • multiple sclerosis[14]
  • head injuries[15]
  • stroke[16]
  • Obsessive-Compulsive Disorder[17]

Research ERPEdit

ERPs are used extensively in neuroscience, cognitive psychology, 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 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, tactile, auditory, olfactory, 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.

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.[18]

Other ERPs used frequently in research, especially neurolinguistics research, include the ELAN, the N400, and the P600/SPS.

ERP Software and Training ResourcesEdit

  • 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

See alsoEdit

Further readingEdit

  • Steven J. Luck: An Introduction to the Event-Related Potential Technique. Cambridge, Mass.: The MIT Press, 2005. ISBN 0-262-62196-7
  • Todd C. Handy: Event-Related Potentials : A Methods Handbook. Cambridge, Mass.: The MIT Press (B&T), 2004. ISBN 0-262-08333-7
  • Luck, S.J., and Kappenman, E.S., ed. (2012). The Oxford Handbook of Event-Related Potential Components. Oxford University Press. pp. 664. ISBN 9780195374148.
  • Monica Fabiani, Gabriele Gratton, and Kara D. Federmeier: "Event-Related Brain Potentials: Methods, Theory, and Applications". In Handbook of Psychophysiology, ed. by John T. Cacioppo, Louis G. Tassinary, and Gary G. Berntson. 3rd. ed. Cambridge: Cambridge University Press, 2007. ISBN 978-0-521-84471-0. pp. 85–119
  • John Polich and Jody Corey-Bloom, Alzheimer's Disease and P300: Review and evaluation of Task and Modality. Current Alzheimer Research, 2005, 2, 515-525
  • Zani A. & Proverbio A.M. (2003) Cognitive Electrophysiology of Mind and Brain. Academic Press/Elsvier.

ReferencesEdit

  1. Luck, Steven J. (2005). An Introduction to the Event-Related Potential Technique, The MIT Press.
  2. Brown, Colin M; Peter Hagoort (1999). "The cognitive neuroscience of language" Colin M. Brown and Peter Hagoort The Neurocognition of Language, New York: Oxford University Press.
  3. Walter, W.G; Cooper, R.; Aldridge, V.J.; McCallum, W.C.; Winter, A.L. (1964). "Contingent Negative Variation: an electric sign of sensorimotor association and expectancy in the human brain". Nature 203 (4943): 380–384.
  4. Sutton, S., Braren, M., Zubin, J., & John, E.R. (1965). Evoked-Potential Correlates of Stimulus Uncertainty. Science, 150, 1187-1188
  5. Handy, T. C. (2005). Event Related Potentials: A Methods Handbook. Cambridge, MA: Bradford/MIT Press.
  6. Coles, Michael G.H.; Michael D. Rugg (1996). "Event-related brain potentials: an introduction" Electrophysiology of Mind, 1–27, Oxford Scholarship Online Monographs.
  7. ERP_REJECT, rejection of outlier trials from ERP studies. Matlab File Exchange. URL accessed on December 30, 2011.
  8. (2012) Luck, S.J., and Kappenman, E.S. The Oxford Handbook of Event-Related Potential Components, 664, Oxford University Press.
  9. For discussion of ERP component naming conventions see Luck, Steven (2005), An Introduction to the Event-Related Potential Technique, MIT Press, pp. 10-11.
  10. Luck, Steven (2005). An Introduction to the Event-Related Potential Technique. MIT Press, pp. 21-23.
  11. Luck, Steven (2005). An Introduction to the Event-Related Potential Technique, 25–26, Massachusetts Institute of Technology: MIT Press.
  12. Boutros, N., et al. (1995). Evoked potentials in subjects at risk for Alzheimer's disease. Psychiatry Res, 57, (1), 57-63.
  13. Prabhakar, S., Syal, P. and Srivastava, T. (2000). P300 in newly diagnosed nondementing Parkinson's disease: effect of dopaminergic drugs. Neurol India, 48, (3), 239- 242.
  14. Boose, M. A. and Cranford, J. L. (1996). Auditory event-related potentials in multiple sclerosis. Am J Otol, 17, (1), 165-170.
  15. Duncan, C. C., Kosmidis, M. H. and Mirsky, A. F. (2003). Event-related potential assessment of information processing after closed head injury. Psychophysiology, 40, (1), 45-59.
  16. D'Arcy, R. C., et al. (2003). Electrophysiological assessment of language function following stroke. Clin Neurophysiol, 114, (4), 662-672.
  17. Hanna, G.L., Carrasco, M., Harbin, S.M., Nienhuis, J.K., LaRosa, C.E., Chen, P., Fitzgerald, K.D., Gehring, W.J. (2012). Error-Related Negativity and Tic History in Pediatric Obsessive-Compulsive Disorder. Child Adolescent Psychiatry, 51, (9), 902-910.
  18. Farwell, L.A., Donchin E. (1988). Talking off the top of your head: toward a mental prosthesis utilizing event-related brain potentials. Electroencephalogr Clin Neurophysiol. 70 (6): 510–23.




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