EEG-NIRS co-registration: challenges and solutions
Judit Gervain and Hellmuth Obrig
Duration: 90 min
Synopsis: There is an increasing interest in co-registering NIRS with electroencephalography (EEG), as the two modalities provide complementary information about brain activity, with NIRS offering accurate localization, EEG high temporal resolution. Yet, NIRS-EEG co-recording is not without its challenges. Finding the optimal headgear configuration, choosing an appropriate experimental design respecting the different time-scales of the two techniques (fast neural response for EEG, slow hemodynamic response for NIRS), analyzing the data and interpreting the results, especially in the case of non-convergence are all issues researchers and clinicians need to face to achieve successful co-recording. This mini-course, taught by two researchers with an established record of successful NIRS-EEG co-recording studies, covers the physiological, methodological and theoretical challenges of co-recording and offers possible solutions. The instructors will share their experience, describing successful, published studies, but also unsuccessful or only partially successful and thus unpublished attempts at co-recording from their laboratories. Using a highly interactive format, with ample time for questions and discussion, the course will offer methodological know-how that often remains implicit in publications.
Rationale: Interest in NIRS-EEG co-registration is increasing, as the two technologies provide complementary information about brain activity. NIRS, measuring the hemodynamic response, has accurate spatial localization, while EEG, directly reflecting neural activity, offers high temporal resolution. Both techniques are non-invasive, relatively easy to apply and well tolerated by participants, portable and increasingly wearable. Their co-recording thus has great potential for research and clinical applications in laboratory, bed-side and global settings. Despite these advantages, NIRS-EEG co-recording poses considerable challenges. The two techniques do not record the same neurophysiological response and, accordingly, operate at different time-scales, with NIRS measuring a response in the second-range, while EEG in the millisecond-range. This raises issues for stimulus presentation and experimental design: how best to reconcile EEG’s need for the fast/short presentation of a large number of stimuli with the relatively longer and fewer stimulation blocks required by NIRS? Relatedly, since different aspects of brain activity are queried by the two techniques, results may not always converge: how to best analyze the NIRS and EEG datasets to allow for a meaningful match and how to interpret the findings when the two modalities do not point in the same direction (e.g. different responses between two conditions in one modality, but not in the other)? Also, while inserting the NIRS optodes and the EEG electrodes into the same headgear is technically not too difficult, finding the optimal configuration may be challenging and raises questions about whether and how the data from the two techniques may be co-localized. The mini-course aims to provide answers and solutions to help users overcome these challenges. Taught by two researchers with considerable experience in NIRS-EEG co-recording (Telkemeyer et al. 2009, 2011, Obrig et al. 2017, Benavides & Gervain 2017, Cabrera & Gervain, forthcoming), the course will cover the basic physiological principles of neuro-vascular coupling, offering a better understanding of what the two modalities measure, and most importantly, how the two relate to one another. This will be followed by detailed presentations and discussion of successful and less successful attempts at NIRS-EEG co-registration from the instructors’ laboratories. Through these examples, the course will tackle issues of headgear configuration, experimental design, data analysis and interpretation. Equal emphasis will be given to the discussion of successful and unsuccessful studies, as the latter are often not published, resulting in each new laboratory going through a highly inefficient process of repeating previous mistakes. This mini-course will be an excellent opportunity for the instructors as well as for participants to share their implicit know-how and thus increase the openness and transparency of scientific practices. The format of the mini-course will combine a brief frontal presentation of the basic principles, questions and issues by the instructors with ample interactive question-answer sessions and discussion between the instructors and the participants. Participants will be encouraged to share their own experience, ask questions related to their own practices and reach out to the instructors and the other participants for feedback and input.
Learning objectives: basic physiological principles of neuro-vascular coupling, offering a better understanding of what the EEG and fNIRS modalities measure, and most importantly, how the two relate to one another