Understanding the Brain

Event-related potential

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A wave showing several ERP components, including the N100 and P300

An event-related potential (ERP) is any measured brain response that is directly the result of a thought or perception. More formally, it is any stereotyped electrophysiological response to an internal or external stimulus.

ERPs are measured with electroencephalography (EEG). The magnetoencephalography (MEG) counterpart of ERP is the ERF, or event-related field.[1]

Measurement

ERPs can be reliably measured using electroencephalography (EEG), a procedure that measures electrical activity of the brain through the skull and scalp. As the EEG reflects thousands of simultaneously ongoing brain processes, 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 response to the stimulus, the experimenter must conduct many trials (100 or more) and average the results together, causing random brain activity to be averaged out and the relevant ERP to remain.[2]

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.

Nomenclature

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 indicating polarity, followed by a number indicating either the latency in milliseconds or the component's ordinal position in the waveform. Thus, 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) 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.[3]

Clinical ERP

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.

Research ERP

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 time of information processing. For example, in the checkerboard paradigm described above, in healthy participants the first response of the visual cortex is around 50-70 msec. 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 stimulus presented: visual, tactile, auditory, olfactory, gustatory, etc. Because of this general invariance in regard to stimulus type, this ERP 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.[citation needed]

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

See also

Further reading

  • Steven J. Luck: An Introduction to the Event-Related Potential Technique. Cambridge, Mass.: The MIT Press, 2005. ISBN 0262621967
  • Todd C. Handy: Event-Related Potentials : A Methods Handbook. Cambridge, Mass.: The MIT Press (B&T), 2004. ISBN 0262083337
  • 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 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.

Notes

  1. Brown, Colin M; Peter Hagoort (1999). "The cognitive neuroscience of language". In Colin M. Brown and Peter Hagoort. The Neurocognition of Language. New York: Oxford University Press. p. 6. 
  2. Coles, Michael G.H.; Michael D. Rugg (1996). "Event-related brain potentials: an introduction". Electrophysiology of Mind. Oxford Scholarship Online Monographs. pp. 1–27. http://www.google.com/url?sa=U&start=4&q=http://l3d.cs.colorado.edu/~ctg/classes/lib/cogsci/Rugg-ColesChp1.pdf&ei=fWk7SduzEJqDtgfhyJDsBg&sig2=ir4zxeCKt--_nLiitd8KKQ&usg=AFQjCNGd2BBtf23IGTl4JVLkbhxViORx_g. 
  3. For discussion of ERP component naming conventions see Luck, Steven (2005), An Introduction to the Event-Related Potential Technique, MIT Press, pp. 10-11.
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