In this exercise, we will take a look at two kinds of processing operations that are commonly applied to ERPs in which either the bins or the channels are mathematically recombined. For example, we could average together two bins or two channels. These kinds of operations are carried out with ERP Bin Operations and ERP Channel Operations, respectively.
Let’s start by looking at ERP channel operations. In the previous exercise, I noted that it is often better to average across channels rather than to include channels as a factor in the statistical analysis. In the present exercise, we’ll average across the nine channels that we used in the previous exercise (F3, Fz, F4, C3, Cz, C4, P3, Pz, and P4). Ordinarily, you would perform this averaging process on the single-subject waveforms, then obtain the N400 amplitude scores, and then conduct the statistical analysis on these scores. To make this exercise quick, however, we will instead average across channels in the grand average ERP waveform. This will allow us to visualize the results of averaging but not perform a statistical analysis on the averaged data.
Before we look at how the averaging process is implemented in ERPLAB, let’s think for a minute about how you would compute the average of these nine channels by hand. The average of nine values is just the sum of those nine values divided by nine. So, to compute the average, you would use this equation:
This is how you perform channel operations in ERPLAB. That is, you specify an equation that describes exactly what you want to compute, and ERPLAB computes it for you. The only difference is that you need to use the channel numbers rather than the channel names.
Let’s give it a try. If necessary, load the grand average you made earlier (Grand_N400) into ERPLAB and make it the active ERPset. Now select EEGLAB > ERPLAB > ERP Operations > ERP Channel operations.
You’ll see a window like the one shown in Screenshot 3.10. The panel on the right side gives you a list of the available channels and the channel numbers. The panel on the left is a text window that allows you to write one or more equations. You can see that we currently have 30 channels, so the new channel we will create will be channel 31. We’ll call this new channel Cluster. To make this happen, we use the equation:
It’s just like the previous equation, except that we use channel numbers instead of channel names, and we add label Cluster to the end of the equation to indicate the name of the new channel. Type this equation into the text box. Make sure that all the other parts of the window match Screenshot 3.10, especially Modify existing ERPset, and then click RUN.
Because we’re modifying an existing ERPset rather than creating a new ERPset, you won’t see a window for saving the ERPset. However, if you look at the ERPsets menu, you’ll see that the name of the current ERPset has been changed from Grand_N400 to Grand_N400_chop to indicate that Channel Operations (abbreviated as “chop”) has been performed. If you want to save the changed ERPset, you can select EEGLAB > ERPLAB > Save Current ERPset as.
Screenshot 3.10
Now let’s take a look at the result of this operation. Select EEGLAB > ERPLAB > Plot ERP > Plot ERP waveforms and plot Bins 3 and 4. At the bottom of the plot, you’ll see the new channel, labeled Cluster. And you’ll see that it looks like what you’d expect for the average of the 9 individual channels. If you performed this operation on the single-subject data, you could use the ERP Measurement Tool to score the N400 amplitude from this new channel. Note that there is also an EEG Channel Operations routine in ERPLAB that works in the same way except that it operates on EEG data (continuous or epoched) rather than on averaged ERP data.
