Multimodal fNIRS-EEG measurements — Experimental Design
In the previous blog post, we discussed integrated fNIRS-EEG placement on the headcap. Another important step in running a combined fNIRS-EEG study is deciding on the experimental design. In this blog post, we will briefly introduce commonly used experimental designs and how to adopt them in a multimodal study.
The experimental design refers to the time sequence in which different stimuli are presented to the subject. A good study design should capture changes in the signal based on the temporal resolution of the technique at hand, as well as the hypotheses of the study.
Block Design
With fNIRS, we measure concentration changes in oxy- and deoxy-hemoglobin. These changes are relative, meaning that the values we obtain are always in reference to a baseline. Besides that, the hemodynamic response is by nature a slow response: for a single stimulus it takes 2-3 seconds to show, about 6 seconds to peak, and it slowly goes back to baseline.
The block design is the most common experimental design to use in fNIRS experiments since it meets the prerequisites for capturing the hemodynamic response by continuous wave NIRS.
A block design, as the name already indicates, consists of task blocks, which are alternated with rest. A block usually is a fixed period of time during which the participant performs a specific task, such as finger tapping or watching stimuli on a screen, and that is expected to activate the brain regions of interest. The rest period should be as similar as possible to the blocks, except for the presence of the task. The blocks usually consist of several trials (e.g., one instance of the stimulus or one finger tap) that follow each other.
In a fNIRS experiment with a block design, each trial is assumed to trigger a hemodynamic response. If the interval between trials is short, given that the initiated hemodynamic response is slow, these responses will add up, resulting in a strong hemodynamic response. Eventually, the response will reach a plateau that only starts to return s to baseline once rest begins.
Figure 1: Block design illustration in a multimodal fNIRS-EEG experiment
Event-related design
Although with a block design it is easy to achieve a strong response, it makes it impossible to isolate the response to each individual trial. Therefore, if you are interested in the hemodynamic response for individual trials difference between trials, another experimental design should be adopted.
EEG measures the sum of post-synaptic potentials (PSPs) which are short-lasting and instantaneous. Therefore, in contrast to fNIRS, the high temporal resolution of EEG allows us to look at faster changes in brain activity. This makes the event-related design a popular choice for EEG studies.
In an event-related study design, trials can be presented at any time. Trial duration can vary from milliseconds to seconds, and the interval between consecutive trials can be short or long. As EEG is faster, the response to individual trials can be isolated. However, this single-trial activation is not strong compared to the response to a block of trials. Therefore, it’s also common practice to average over trials of the same condition to get a stronger response.
Figure 2: Event-related design illustration in a multimodal fNIRS-EEG experiment
Further considerations when choosing a study design
Even though each of the aforementioned study designs might be more suitable for one of fNIRS and EEG techniques, they can be used with either technique as well. The choice depends on the aim of the study and the nature of the changes we are looking at. Therefore, for combined fNIRS-EEG measurements, the researcher can freely decide on the study design with some considerations in mind:
If responses to individual trials are of interest, then the onset of these trials should be marked.
If the occurrence of trials cannot be predicted, such as stuttering or freeze of gait, then event-related design is the better choice.
Mind wandering could occur in longer blocks, so lengthy blocks do not necessarily always result in a stronger response.
Adding jitter to block or trial intervals can minimize confounds such as anticipation and habituation and event-related systemic physiology response.
Although not as common as the experimental designs mentioned above, the multimodal fNIRS-EEG technique can also be used to study resting states. In this case, no explicit task is performed and communication between different brain regions is the focus of interest.
Finally, we always recommend having a look at similar previous publications to ensure an appropriate study design for your planned research. Please have a look at our publication website.
If you have any questions or doubts regarding experimental design in multimodal set-ups, please contact us at askforinfo@artinis.com.
When simultaneously measuring fNIRS and EEG, placement of both devices should ideally ensure proper coverage of the desired measurement location, minimize interference and take into account (technical) characteristics and basic of both techniques. Read this blog post to learn more about relevance of these points and further recommendations for integrating fNIRS and EEG on one head.