Functional brain mapping using whole-head very high-density diffuse optical tomography.

Naturalistic neuroimaging tasks, such as watching movies, are becoming increasingly popular due to being more engaging than resting-state paradigms and more ecologically valid than isolated block-design tasks. As these tasks push the boundaries of naturalistic paradigms, the need for an equally naturalistic imaging device increases. Optical imaging with functional near-infrared spectroscopy (fNIRS) offers a wearable, non-invasive neuroimaging approach. Advancements in high-density diffuse optical tomography (HD-DOT) use a dense array of optical elements to provide overlapping multi-distance fNIRS light measurements for fidelity comparable with functional magnetic resonance imaging (fMRI). Here, to further improve image quality, we increased the density of the imaging grid to 9.75 mm, first nearest neighbor spacing between sources and detectors, leading to a 4-fold increase in measurement density. This very high-density DOT (VHD-DOT) system uses 255 sources and 252 detectors to improve image quality while expanding the field of view. From simulations, the increased density led to improved image resolution across multiple metrics compared with HD-DOT. In vivo group-averaged functional localizer maps are in strong agreement with those collected in MRI on the same cohort of adult participants, indicating that VHD-DOT can be used as a surrogate for fMRI in task-based studies. For a naturalistic movie-viewing task, feature regressor analysis was employed to map audiovisual features from the clip, which also revealed excellent agreement between VHD-DOT and fMRI. Template-based decoding of task and movie-viewing data demonstrates that VHD-DOT signals are repeatable and discriminable, which is necessary for more advanced naturalistic task analyses. This work builds upon previously reported HD-DOT designs to improve the image quality and resolution for whole-head optical imaging. This system is promising for future studies using complex stimuli and analysis protocols, such as decoding, and future work developing wireless VHD-DOT systems.