MethodologyIssues and assumptions on the road from raw signals to metrics of frontal EEG asymmetry in emotion
Section snippets
From raw signals to handy metrics
Investigators who examine frontal EEG asymmetry use a set of relatively specialized signal processing routines, which will be reviewed anon. This review is not intended so serve as a primer for basic signal processing, but rather is designed to highlight the data reduction trail typically followed in this specific research domain. For a basic primer, many sources are available, including easily accessible chapters by Gratton (2000) and by Reilly (1987), and a more in depth treatment by Glaser
Comparing left and right activity
Because asymmetrical activity is of interest, investigators often use a difference score (ln(Right)−ln(Left) alpha power) to conveniently summarize the relative activity at homologous right and left leads.
How much raw data should be acquired?
Sufficient data are required to ensure that reliable estimates of EEG activity are derived. Although the power spectrum derived from any single epoch via the FFT will reflect both frequencies that are common across epochs as well as those idiosyncratic to any given epoch, averaging together multiple spectra can allow those frequencies to emerge that are present in a reasonably large proportion of epochs (see Fig. 1, Panel E), while mitigating the influence of infrequent or irregular signals (
Alpha and activity
A guiding assumption underlying the interpretation of findings involving frontal EEG alpha asymmetry is that greater alpha power is indicative of less cortical activity in broad underlying regions (cf. Davidson, 1988). Although there is good evidence to support this assumption, one might question whether this relationship is ubiquitous.
It has been well documented that sensory input shows modality-specific blocking of alpha activity at cortical regions involved in processing such input. With
Synopsis
Research on frontal EEG asymmetry and emotion now represents a substantial body of literature. There are numerous methodological issues to which the field may have paid insufficient attention, while at the same time paying potentially too much attention to other factors. The field may have been too concerned with recording at least 8 min of data to obtain reliable estimates of asymmetry, overly concerned about the impact of blink artifacts, and overly concerned with closely matching impedances
Acknowledgements
This research was supported, in part, by a grant from the National Alliance for Research on Schizophrenia and Depression (NARSAD) and an Exploratory/Development Grant from the National Institutes of Health (1 R21 RR09492) to J.J.B.A., and by a Graduate Research Fellowship from the National Science Foundation to J.A.C.
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