Development of a fiber-less fNIRS system and its application to hair-covered head

While most commercially available functional near-infrared spectroscopy (fNIRS) systems employ optical fibers for both the measurement optode and the transmission cable for optical signals, their material inflexibility presents some problems in stable optode fixation to the head surface and adequate cable lining to the main system. In practice, mechanical fluctuations of optical fibers in fNIRS measurement often lead to motion artifacts in the signals. A few fiberless fNIRS systems are available and equipped with light sources and detectors that directly adhere to the scalp surface. However, their shapes and detection sensitivities are not suitable for usage on a hair-covered head. Based on the commercial fiber-less fNIRS system OEG-16 (Spectratech Inc., Japan), we developed a new source-detector unit that was designed with LEDs for enhanced illumination, avalanche photodiodes instead of photodiodes, and a new holder system. The electrical circuits of the system were modified after the design. By simultaneous implementation of multidistance fNIRS measurement and hemodynamic modality separation on conventional fNIRS data at the bilateral parietal area during single-sided motor tasks, significant functional signals were observed only at the position contralateral to the side of movement. This is the first report describing a fiber-less fNIRS system that can detect functional signals on a hair-covered head. We believe this fiber-less system will improve the utility of fNIRS, particularly in less restraining conditions.