How Can Near-Infrared Spectroscopy (NIRS) Help Understanding of the Psychotherapy Process? A Scoping Review of Therapeutic Interactions

In recent years, near-infrared spectroscopy (NIRS) has garnered significant attention for its potential clinical applications as a non-invasive and straightforward method for measuring brain functions. While NIRS has become useful in psychiatric contexts by aiding in the differential diagnosis of depression, its application in clinical psychology is still developing. Due to its ecological validity, which allows for measurements in natural settings, NIRS shows promise in revealing the dynamic elements of the psychotherapy process. In this study, we conducted a scoping literature review of the use of NIRS in psychotherapeutic interactions, exploring its potential to unveil the complexities of psychotherapy settings involving interpersonal interactions. Employing a scoping review approach, we systematically searched the PubMed, PsycINFO, and Web of Science databases for articles published through October 19, 2023 about studies using NIRS as a measurement instrument: This yielded various therapeutic interactions. Of the 155 retrieved results that remained after the initial screening, seven met the inclusion criteria. Of these, six studies examined the synchronization of brain functions through the simultaneous brain activity measurements (hyperscanning) of two participants. The review revealed that, despite the anticipated potential of NIRS, it has become evident there is a dearth of research focusing on interactions during psychotherapy. Furthermore, the few available studies have not explored changes in line with the progression of psychotherapeutic interactions. Consequently, future studies should address whether NIRS can effectively assess alterations in brain function resulting from micro-level events, such as the therapeutic interaction between therapists and clients during psychotherapy.