Importance of Clean Data

By Grace Lim for Neurotech@Davis

Having clean data is one of the most important considerations for your EEG experiment, second only to participant safety (but we wont get into that now). The reasoning for the important of clean data is to save you time and money. By having clean data in the first place, you can reduce the amount of recording sessions you need to do; whether this is the amount of trials or number of participants being run. By have less sessions, you save lots of time and resources (electrode gel isn’t cheap). Besides these points, it is also just good practice as this is the standard for labs that publish papers. If you’re trying to impress your PI, you want to spend some time learning why and how to have clean data.

You may be tempted to just increase the amount of trials used in your experiment to circumvent the problem of bad data, but this is actually counterintuitive as “increasing the number of trials eventually has diminishing returns because the effect of averaging on noise is not linearly proportional to the number of trials” and “The noise decreases as a function of the square root of the number of trials in the average” (Luck, 2014, Chapter 5).

Now that you know why having clean data is important I will describe how to have clean data while you are recording, in addition to the preparation stage. A good amount of noise in the EEG recording can be attributed to skin potentials, which describes “the tonic voltage between the inside and outside of the skin. In addition, magnitude of this voltage changes as the impedance changes, thus if the electrode impedance goes up and down over time, this will lead to voltages that go up and down in EEG recording” (Luck, 2014, Chapter 5). A few steps to decrease noise due to skin potentials are to: use Nuprep, a type of abrasive gel used to prep the skin, having your participant wash their hair prior, having a cool room to reduce the amount of sweat produced, and removing any makeup on the skin where the electrode will be placed. Now to describe ways to reduce noise during recording: use medical tape to hold down electrodes (usually on the mastoids if referencing there), reducing any movements, ensure that the reference and ground electrodes are not being interfered, and giving the participant breaks to help them not get sleepy.