Abstract
Global field power is a valuable summary of multi-channel electroencephalography data. However, global field power is biased by the noise typical of electroencephalography experiments, so comparisons of global field power on data with unequal noise are invalid. Here, we demonstrate the relationship between the number of trials that contribute to a global field power measure and the expected value of that global field power measure. We also introduce a statistical testing procedure that can be used for multi-subject, repeated-measures (also called within-subjects) comparisons of global field power when the number of trials per condition is unequal across conditions. Simulations demonstrate the effect of unequal trial numbers on global field power comparisons and show the validity of the proposed test in contrast to conventional approaches. Finally, the proposed test and two alternative tests are applied to data collected in a rapid serial visual presentation target detection experiment. The results show that the proposed test finds global field power differences in the classical P3 range; the other tests find differences in that range but also at other times including at times before stimulus onset. These results are interpreted as showing that the proposed test is valid and sensitive to real within-subject differences in global field power in multi-subject unbalanced data.
Highlights
Global field power (GFP) is the spatial standard deviation of a montage of average-referenced electrode voltages (Lehmann and Skrandies 1980; Skrandies 1990) and is used in analysis of electroencephalography (EEG) data
We introduce a statistical testing procedure that can be used for multi-subject, repeated-measures comparisons of global field power when the number of trials per condition is unequal across conditions
Experiment 3 uses the three statistical tests to analyze an experimental data set. This simulation tested the validity of three statistical methods for detecting GFP difference when they were applied to unbalanced datasets
Summary
Global field power (GFP) is the spatial standard deviation of a montage of average-referenced electrode voltages (Lehmann and Skrandies 1980; Skrandies 1990) and is used in analysis of electroencephalography (EEG) data. The first approach is to compute the GFP of single-trial data and average the resulting single-trial GFP measurements. The other approach is to average several trials of EEG data and compute the GFP of the resulting average. Both approaches are in use (for an example of analyses involving single-trial GFP see Wagner et al 2014), but these two approaches estimate quantities that must be interpreted differently. The first approach computes the average GFP of a single trial. This includes the GFP of both the stimulus-locked neural activity
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