Event-related potential

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.

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.

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 ERPs. This noise is superimposed to 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 why 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: a) the signal of interest is made of a sequence of event-locked ERPs with invariable latency and shape; b) the noise can be approximated by a zero-mean Gaussian random process of variance σ² 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 k^th 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 bigger 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

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.

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.

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.

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

See also

• Erich Schröger
• Evoked potential
• Induced activity
• Somatosensory evoked potential
• C1 and P1
• Visual N1
• Mismatch negativity
• N100
• N200
• N2pc
• N170
• P200
• N400
• P300
• P3a
• P3b
• P600
• Late Positive Component
• Difference due to Memory
• Contingent negative variation
• Error-related negativity
• Bereitschaftspotential
• Lateralized readiness potential
• Early left anterior negativity

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.