ERP Measurement Tool
ERPLAB > ERP Measurement Tool is used to measure features of the ERP waveforms, such as amplitudes and latencies.
The basic idea behind this tool is that you specify a set of ERPsets, a set of bins, a set of channels, and a specific measurement to be performed (e.g., peak latency between 300 and 600 ms), and then the tool performs all of the measurements and saves them in a text file. You can either specify ERPsets that have been loaded into ERPLAB (and are therefore visible in the ERPsets menu) or ERPsets that have been stored in files on disk. To make it easy to repeatedly measure the same set of files, you can save a list of the files and then load it again later. The GUI is shown in the screenshot below.
Here are the different types of values that can be measured:
a) Instantaneous amplitude- This measures the amplitude at one latency (a single latency is specified in the latency text box).
b) Peak amplitude or latency- This finds the most positive or most negative amplitude within a specified time range (the starting and ending latencies of the time range are specified in the latency text box). The amplitude and/or latency of this peak is provided in the output file. If the number of points for the local peak option is nonzero, a peak is defined as the most positive or most negative value that is also (a) larger than the voltages in the samples on either side of the peak, and (b) larger than the average of the N sample points on either side (where N is the value you specify in the GUI). If there is no point that meets this definition, then a warning message is printed in the command window and the simple peak (the most positive or negative point) or NaN is printed (depending on what is specified in the GUI). Note that this requires that the N sample points actually exist. If, for example, you ask for the local peak amplitude between 300 and 900 ms, with N set at 10, then the epoch length must be at least 10 sample points longer than 900 ms.
c) Mean amplitude- This finds the mean voltage within a specified time range (the starting and ending latencies of the time range are specified in the latency text box).
d) Area amplitude- This finds the area under the curve within a specified time range (the starting and ending latencies of the time range are specified in the latency text box). Area and mean are identical, except that area is multiplied by the duration of the time range.
The measured values are stored in a text file, and optionally in Microsoft Excel (Only Windows machine), which you can import into the statistical analysis package of your choice. There are two output formats. In the first, each line contains the data from every requested bin and channel from a given ERPset. Ordinarily, each ERPset will contain the data from one subject, so the result contains one line for each subject, as shown here:
ERPset Frequent_FZ Frequent_CZ Frequent _PZ Rare_FZ Rare_CZ Rare_PZ
S1 2.265 4.173 4.352 4.300 10.754 9.814
S2 1.776 4.027 4.175 3.914 10.029 9.268
S3 2.265 4.173 4.352 4.300 10.754 9.814
In this example, peak amplitude was measured from two bins (“Frequent” and “Rare”) and three channels (Fz, Cz, and Pz). The data from each subject (S1, S2, and S3) appears on a separate line, and each column represents the measurement from a particular channel in a particular bin. This format is useful for most statistics programs, such as SPSS.
The second output format provides one measurement per line, along with various pieces of information that can be used for sorting. This format is ideal for using the pivot table feature of Excel. Here are the same data as in the previous example but shown in this alternative format:
ERPset bin channel value
S1 1 1 2.265
S1 1 2 4.173
S1 1 3 4.352
S1 2 1 4.300
S1 2 2 10.754
S1 2 3 9.814
S2 1 1 1.776
S2 1 2 4.027
S2 1 3 4.175
S2 2 1 3.914
S2 2 2 10.029
S2 2 3 9.268
S3 1 1 2.265
S3 1 2 4.173
S3 1 3 4.352
S3 2 1 4.300
S3 2 2 10.754
S3 2 3 9.814
The values can also be stored in the Matlab workspace in the variable ERP_MEASURES. You can see them by typing “ERP_MEASURES” at the Matlab command line, yielding an output that looks like this:
2.2648 4.1730 4.3524
4.2995 10.7537 9.8138
1.7761 4.0267 4.1753
3.9139 10.0288 9.2685
2.2648 4.1730 4.3524
4.2995 10.7537 9.8138