5: Referencing and Other Channel Operations
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Learning Objectives
In this chapter, you will learn to:
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Avoid the incorrect assumption that an ERP waveform primarily reflects signals from the active electrode site and instead recognize that the waveform equally reflects signals from the active and reference sites
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Compute the absolute voltage at the scalp electrodes given the internal generator voltage and the propagation weights between the generator and the scalp sites
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Compute the voltage between each electrode site and the ground electrode given the absolute voltages at each site
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Compute the referenced voltage at each site given the voltage between each site and the ground electrode
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Re-reference the voltage to another site
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Use simple algebra to determine the correct equations for re-referencing your data
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Use two alternatives to referencing your data, namely current density and global field power
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Implement several different reference schemes and alternatives to referencing in ERPLAB using ERP Channel Operations and EEG Channel Operations
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Implement common referencing scenarios using a script
This chapter dives deeper into channel operations, especially with respect to how the reference electrode works and how you can re-reference your data. Once you know what you’re doing, re-referencing the data will take you just a few seconds, and many researchers don’t give it much thought. However,
the reference has an enormous impact on your ERP waveforms
, so you really need to understand what you’re doing when you reference or re-reference your data.
The exercises in this chapter are designed to give you greater insight into what the reference site does, why we need to reference the data, and how re-referencing the data can clarify or obfuscate the results depending on what new reference you choose. We’ll also discuss the mechanics of re-referencing in ERPLAB so that you can have confidence that you’re doing it correctly.
We’ll start with simulated data so that you can see how the original and referenced data on the scalp are related to the underlying neural generator. The simulations use Excel rather than ERPLAB, which makes it easier to see exactly what’s going on. You can use Google Sheets instead of Excel, if you prefer. I’m assuming that you know the basics of spreadsheets, including how an equation in one cell can compute a value on the basis of other cells.