This tool will allow you to calculate event-related potentials (ERPs). ERPs are derived from brain electrical activity recorded via electroencephalography (EEG). An ERP represents the brain's averaged response to a processed stimulus (e.g. a sound or an image). EEG epochs (i.e. small segments of EEG) are averaged together to calculate the ERP. ERPs contains characteristic peaks and trough (referred to as components) that correspond to the early sensory and later cognitive/motor processing of the stimulus.
The EEG epochs you will see here were recorded during a Go-Nogo task, which is used to examine response activation and response inhibition processing. In this task the participant is required to press a response button when a frequently presented "Go" stimuli is presented, but withhold a response when an infrequent "Nogo" stimulus is presented. Data are presented from an EEG electrode in the frontal brain area and another in the posterior brain area.
Task: EEG epochs in the 'Selected epochs' graph are averaged to calculate the ERP which is shown in the 'ERP to selected epochs' graph. Select different groups of epochs to average, and see how the ERP differs.
Notes: Each EEG epoch is made up of signal (i.e. the brain's actual response to the stimulus) and "noise" (i.e. brain activity not related to processing the stimulus). Averaging reveals what is consistent about the epochs, while the noise is averaged away. The resultant ERP shows the brain's averaged response to the stimulus.
Task: For a particular stimulus in a particular region (e.g. Stimulus Nogo in the Frontal region) compare the ERP to 'Clean epochs plus blinks' with the ERP to '20 clean epochs'. Also, compare the ERP to 'Clean epochs plus muscle artefacts' with the ERP to '20 clean epochs'.
Notes: The EEG recording process will also record non-brain electrical activity, known as artefacts. These include blinks and other eye movements, as well as other facial and neck muscle activity, and gross movements of the head (which affect the EEG recording electrodes). Artefacts are an unwanted but inevitable part of EEG recording and have substantial effect on ERP calculation. There are several techniques that can be used when recording the data, or in the post-processing of the data, that can be used to reduce and/or remove artefacts.
Task: For a particular stimulus in a particular region (e.g. Stimulus Nogo in the Frontal region) compare 5 clean epochs, 10 clean epochs, 15 clean epochs, and 20 clean epochs.
Notes: Here you can see that the ERP morphology (i.e. shape of the waveform) changes as more epochs are added to the average. The signal-to-noise ratio is improved by increasing the number of epochs that are averaged.
Task: For a particular region (e.g. Posterior) compare the '20 clean epochs' ERP for Nogo stimuli with the '20 clean epochs' ERP to Go stimuli.
Notes: Here you can see that the ERPs in that particular region are different as Nogo and Go stimuli are processed differently. Generally, the ERP differences are likely to be large in the later part of the waveform (say, 200-800 ms) as the later "cognitive" processing of the stimuli would be different. The ERP differences are likely to be smaller in the early part of the waveforms (say, 0-200 ms) as the early sensory processing of the stimuli would be similar for the different stimuli.
Task: For a particular stimulus (e.g. Nogo) compare the '20 clean epochs' ERP in the Frontal region with the '20 clean epochs' ERP in the Posterior region.
Notes: Here you can see that the ERPs to a particular stimulus are different in the Frontal and Posterior regions. This is because these brain regions are anatomically and functionally distinct and are involved in processing different aspects of the stimulus; accordingly the ERPs reflect that difference.