Event-related potential
An event-related potential (ERP) is the measured brain response that is the direct result of a specific sensory, cognitive, or motor event.
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.
The EEG proved to be a useful source in recording brain activity over the ensuing decades.
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.
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.
Wide amplitude noise (such as eye blinks or movement artifacts) are often several orders of magnitude larger than the underlying ERPs.
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.
[citation needed] ERP waveforms consist of a series of positive and negative voltage deflections, which are related to a set of underlying components.
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.
However, because of the significantly small size of an ERP, it usually takes a large number of trials to accurately measure it correctly.
[9] 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.
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 measures (such as fMRI, PET, and fNIRS) are inherently limited by the slow speed of the BOLD response.
[1] ERP research is much cheaper to do than other imaging techniques such as fMRI, PET, and MEG.
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.
Experimental psychologists and neuroscientists have discovered many different stimuli that elicit reliable ERPs from participants.
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.
The analysis of ERP data is also increasingly supported by machine learning algorithms.
[28][29] A common issue in ERP studies is whether the observed data have a sufficient number of trials to support statistical analysis.
Therefore simply characterizing the number of ERP trials needed for a robust component response is inadequate.
ERP researchers can use metrics like the standardized measurement error (SME) to justify the examination of between-condition or between-group differences[31] or estimates of internal consistency to justify the examination of individual differences.
