Overview of EEG Experimental Design

Made by Sofia Fischel and Grace Lim for Neurotech@Davis

General Structure of EEG Experiments

The general structure of EEG experiments consists of four main components: stimuli, trials, participants, and responses. Stimuli is a physical item or event that the participant is reacting to. It can take any form, but usually the type of stimuli informs the design of the experiment. For example, some common stimuli presentations are the Posner cueing task, Stroop test, and detection of stimuli (using luminance). The type of stimuli used must be specific to purposely not elicit any unwanted behavior. These are known as sensory confounds, which will be covered later in this article. A trial defines how long a participant is presented with a stimulus (stimulus onset) and their response. More importantly, the researcher needs to think about how many trials will be done by the participant. Typically, a set number of trials is defined by a block. For example, you may have 50 trials within a block and have 10 blocks total. The reasoning for splitting it up like this is to organize your data, especially if you plan to manipulate some stimuli within blocks or simply to have a time to give the participant a break. There is no commonly set amount of trials, so it’s best to refer to previous research done on a similar topic to figure out this aspect of design. Another component of the experiment is the participants. The researcher must consider how many participants they will have complete the experiment as well as the demographic. If you are not interested in a specific subset of the population, it is best to diversify the participant pool (age, sex, location, etc…). Another thing to consider is that participants may choose to drop out of your study at any time, as well as not be able to meet some benchmark, causing you to exclude them from the final results. The last aspect of the experiment is the response, which is defined by how the participant is responding to the stimuli. This can be intentional through a keyboard click or less intentional by passively viewing their EEG results while they view the stimuli. In order to calculate things like reaction time or accuracy, you will need to have an intentional response.

This is a very general outline of what is needed for your experiment, so we will go more into detail about each topic later on in this article.

Overview

A well-designed experiment (for our consideration) will include the following characteristics:

“As simple as possible to operate for the experimenter and easy to reproduce for later researchers”$^{1}$.

Simplicity can be summarized as ease of collection. To average out the noise, many trials per participant are required. The more complex the trial, the more difficult it is to collect enough usable data. A little on the nose, I’ll keep this explanation as you should keep your design and trials: short and to the point!

Reproducibility is important in two regards: during successive repetitions conducted by your team and, as stated above, for those outside your team as well. You should be able to replicate similar results across numerous data collection periods; whether that be across different participants running the same trials, or the same participant across numerous days, there is no use in a procedure that yields inconsistent results.

Additionally, your experimental design is not the place to cut corners; though your group may understand exactly what you mean when you write something like “participants will look at the stimulus for 30 seconds,” others with no experience viewing your exact procedure will be left confused and bewildered. Research and projects are meant to be seen by others! A good, standard rule of thumb is to imagine that you have never seen your project before and, with only the experimental design as a guide, you must replicate its results fully.

In practice, components of your experimental design may look something like this

  1. Paradigm or particular wave of interest
  2. Applicable brain regions involved
    1. Key electrode regions outlined (and a brief explanation for their inclusion)
  3. Layout of blocks/trials
    1. Includes stimuli [specifically for exogenous BCI designs, or designs that utilizes brain activity that is generated in response to external stimuli)] + participant response [determine if passive (viewing a screen, attending to a noise, etc.)] or active (verbal response, button press, etc.)]
    2. Determine length (SOA, ITI, etc.) + number of trials
      1. Includes practice blocks (if necessary to accommodate to any experiment specific tasks + practice timing of any responses)
      2. Includes break/rest periods

Participants

In an attempt to ensure replicability beyond the confines of the experiment, an increased sample size is ideal to better replicate the general population as a whole. There is a trade off between open-field application of a project (increasing sample size) and ease of collection (decreasing sample size). Though of course it is easy to idealize the perfect list of candidates for an experiment, it is best to consider participant time constraints, as trials may be rerecorded numerous times given data usability.

Ethical Issues

Please ensure a basic level of respect and awareness for all participants. There should be no undue mental strain nor physical harm applied, and a fundamental principle from standard procedures will hold: “Each participant has the right to withdraw from the experiment without any reason or penalty”.