Functional near infrared spectroscopy (fNIRS) delivers a flexible, portable, and wearable technique for monitoring brain function in situations where fMRI is not feasible, not suitable, or inaccessible. However, variations in optode locations and head shapes and sizes throughout development lead to considerable challenges in group-based and longitudinal studies that generally use either channel-focused analyses or image reconstruction techniques that require strong participant-atlas correspondence. We present a scalp-based parcellation technique that compensates for variation in optode array placement and general head morphology and accounts for fNIRS spatial sampling with minimal assumptions about the underlying head and brain structure to support robust statistical analyses.
This study uses representational similarity-based neural decoding to test whether semantic information elicited by words and pictures is encoded in functional near-infrared spectroscopy (fNIRS) data. In experiment 1, subjects passively viewed eight audiovisual word and picture stimuli for 15 min. Blood oxygen levels were measured using the Hitachi ETG-4000 fNIRS system with a posterior array over the occipital lobe and a left lateral array over the temporal lobe. Each participant’s response patterns were abstracted to representational similarity space and compared to the group average (excluding that subject, i.e., leave-one-out cross-validation) and to a distributional model of semantic representation. Mean accuracy for both decoding tasks significantly exceeded chance. In experiment 2, we compared three group-level models by averaging the similarity structures from sets of eight participants in each group. In these models, the posterior array was accurately decoded by the semantic model, while the lateral array was accurately decoded in the between-groups comparison. Our findings indicate that semantic representations are encoded in the fNIRS data, preserved across subjects, and decodable by an extrinsic representational model. These results are the first attempt to link the functional response pattern measured by fNIRS to higher-level representations of how words are related to each other.
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