fNIRS measures fluctuations in oxygenated and deoxygenated hemoglobin. It is therefore only an indirect measure of brain activity and more a marker of metabolism. In order to interpret these fluctuations in a meaningful way, we have to compare blocks of time with other blocks of time of these signals. The first step is to compare brain metabolism during a task with brain metabolism during rest. We have several options to acquire resting signals:
A baseline that the software kindly calculates and saves for us at the beginning of the recording.
Our 5-minute rest period.
The time before the onset of each task event (valsalva, breath hold, inspiratory loading, sigh, hand grip).
With our current setup, we acquire 100 samples as a baseline at the beginning of the recording.
Note that a baseline ideally is close to the onset of each task, as the beginning of the experiment might not be representative of the person’s state of mind an hour or two later. For example, over the course of the experiment, participants get more and more tired, or potentially more and more agitated. This will again be different before vs. after the valsalva and before vs. after the meditation task. Therefore, we can manually determine a baseline that will serve to measure task-related signal changes.
There can be different reasons for choosing one option over the other. However, it is important to think about and understand the meaning of each choice and whether it helps you answer your desired research question.