Computing Averaged ERPs
Computing averaged ERPs from an epoched dataset is quite simple. There is only one slight complication, which is that it is possible to average across multiple sets of EEG data at one time. Imagine, for example, that you recorded half of the data for a given subject on one day and the other half on a different day, resulting in two separate EEG files (stored in separate datasets). Or imagine that you have 6 conditions, and each was recorded in a separate file. You could average each of the datasets separately and then combine the averages. However, ERPLAB makes it possible to combine them during the averaging process, which is usually easier. This is accomplished by simply specifying multiple datasets when you average. All the datasets are then treated as if they are one big dataset.
To compute averaged ERPs, you must first load one or more epoched datasets in EEGLAB. You can then select ERPLAB > Compute Averaged ERPs. In the window that appears, enter the number of the dataset that contains the data you would like to average (or multiple datasets). For example, if you want to average the epochs in dataset #2, you would simply type ‘2’ into the window. If you wanted to average the epochs in datasets 2, 3, and 5, you would type ‘2 3 5’ into the window. The current dataset is the default. If you aren’t sure what the number of your dataset is, just look in the Datasets menu. Click RUN to compute the averaged ERPs.
When the averaged ERPs have been computed, a window will appear allowing you to name and save the ERPset containing the new ERP structure (see screenshot below). This same window appears whenever you create a new ERPset. Here’s how it works:
- You can choose to either overwrite the current ERPset or create a new ERPset. ERPsets don’t usually take up much RAM, so you will almost always want to create a new ERPset.
- You need to choose a name for the ERPset. This will be the name shown in the ERPsets menu.
- You can optionally save the ERPset as a file. If you click the same as erpname button, it will give the file the same name as the ERPset (recommended in most cases), but with a .erp extension. By default, it will be saved in the current folder. However, you can use the Browse button to browse through your file system to find a place for it (or you can include the path explicitly along with the filename).
Ordinarily, you will perform artifact detection prior to averaging, which marks epochs containing artifacts by means of the artifact flags in the EVENTLIST structure. To exclude these epochs during averaging, make sure that Exclude epochs marked during artifact detection is set. You can also choose to include only the epochs with artifacts by selecting the Include ONLY epochs marked with artifacts option (this is useful for seeing how the artifacts might contribute to the averaged ERPs). To include all epochs, irrespective of the artifact flags, simply de-select both of these options.
Important note: Although ERPLAB keeps track of artifacts using the artifact flags in the EEG.EVENTLIST structure, EEGLAB instead uses the EEG.reject structure. When ERPLAB detects an artifact, it updates both EEG.EVENTLIST and EEG.reject. When EEGLAB’s artifact detection routines are used, only EEG.reject is updated. Consequently, ERPLAB consults both EEG.EVENTLIST and EEG.reject to determine which epochs to include and exclude during averaging. You can also synchronize the information in EEG.EVENTLIST and EEG.reject with the ERPLAB > Artifact Detection > Synchronize Artifact Info in EEG and EVENTLIST command.
Because of Matlab’s memory limitations, it is often useful to split a given subject’s data into multiple datasets. All the EEG processing is then done on the individual datasets, and they are effectively combined during the averaging process. This often means that you need to open multiple dataset files at a given time so that they can all be averaged together. To facilitate this, we have added a function that allows you to open multiple dataset files in a single command. This command is not found in the Tools>ERPLAB Toolbox menu, but is instead found in File>Import data>Load multiple ERPLAB datasets. This will pop up a file selection window, and you can control-click or command-click on multiple files from this window. Note: EEGLAB has recently implemented this feature as well. For instance, in EEGLAB 7.x you could use EEGLAB>File>Load existing dataset to load several dataset in one operation.