ERPS - General

Event-related Potentials (ERPs)

ERPs show the brains electrical response to a stimulus or cognitive event.

An ERP is obtained by repeatedly presenting a stimulus to an individual while EEG is being recorded. Segments of EEG data (called epochs) that are time-locked to the presentation of these stimuli are then averaged to derive the brains averaged response to that stimulus - this is the ERP.

Time-locking is acheived by stimulus tags, shown below in blue (e.g. 100, 3, 40). These allow millisecond accurate locking of the EEG data and the stimulus presentation.

The next slides show how averaging results in an ERP that is relatively free on non-stimulus-related brain activity (called "noise") and contains activity related to stimulus processing, called the signal.

Click on the numbers below to view ERP Samples.

Waveform specificity

The waveform structure of an ERP is dependent on the sensory modality that the stimulus was presented in, and the processing of the stimulus that was required.

The processing of the stimulus is governed by the requirements of the particular experimental task.

The following example is from a Go-Nogo task.

In this simple task the subject is required to press a response button when one auditory tone is presented (the GO stimulus) and withhold from pressing when a different tone is presented (the NOGO, or "inhibit", stimulus).

There are typically more GO than NOGO stimuli, so that button-pressing is the dominant repsonse, making it difficult to stop or inhibit the response when a NOGO stimulus is presented.