One of the common symptoms of autism,^{1, 2} Asperger syndrome,^{1, 3} schizophrenia, and depression is a lack of social skill. To alleviate these mental disorders on the basis of a neuroscience approach, it is important to investigate the relationship between social skills and brain function.⁴ Social skills, in general, include adaptability of one's own behavior to a social context, reading the atmosphere or others’ intentions, communication skills, and ethical sense.^{5, 6} Most of these social skills are acquired through interactions with others. To study the social-brain function, it is therefore meaningful to investigate interacting brains of multiple participants.

Optical topography/functional near-infrared spectroscopy (OT/fNIRS) is a functional imaging technique that noninvasively measures cerebral hemoglobin concentration changes caused by neural activities. The fNIRS method has been extensively implemented to understand the brain activity in many applications, such as neurodisorder diagnosis and treatment, cognitive psychology, and psychiatric status evaluation. To assist users in analyzing fNIRS data with various application purposes, we developed a software called platform for optical topography analysis tools (POTATo). We explain how to handle and analyze fNIRS data in the POTATo package and systematically describe domain preparation, temporal preprocessing, functional signal extraction, statistical analysis, and data/result visualization for a practical example of working memory tasks. This example is expected to give clear insight in analyzing data using POTATo. The results specifically show the activated dorsolateral prefrontal cortex is consistent with previous studies. This emphasizes analysis robustness, which is required for validating decent preprocessing and functional signal interpretation. POTATo also provides a self-developed plug-in feature allowing users to create their own functions and incorporate them with established POTATo functions. With this feature, we continuously encourage users to improve fNIRS analysis methods. We also address the complications and resolving opportunities in signal analysis.

Optical topography (OT) based on near infrared spectroscopy is effective for measuring changes in the concentrations of oxygenated hemoglobin (oxy-Hb) and deoxygenated hemoglobin (deoxy-Hb) in the brain. It can be used to investigate brain functions of subjects of all ages because it is noninvasive and less constraining for subjects. Conventional OT systems use optical fibers to irradiate the scalp and detect light transmitted through the tissue in the human head, but optical fibers limit the subject’s head position, so some small systems have been developed without using optical fibers. These systems, however, have a small number of measurement channels. We developed a prototype of a small, light, and wearable OT system that covers the entire forehead. We measured changes in the concentrations of oxy-Hb and deoxy-Hb in the prefrontal cortex while a subject performed a word fluency task. The results show typical changes in oxy-Hb and deoxy-Hb during the task and suggest that the prototype of our system can be used to investigate functions in the prefrontal cortex.

Around the turn of the century, near-infrared spectroscopy (NIRS) was used for monitoring cortical activation noninvasively to measure changes in hemoglobin concentration.^{1, 2, 3, 4} This technique may provide change in the product of the concentration $\left(C\right)$ and effective optical path length $\left(L\right)$ for oxygenated hemoglobin (oxy-Hb) and deoxygenated hemoglobin (deoxy-Hb) in the cerebral cortex: $\Delta (C_{\mathrm{oxy}}\cdot L)$ and $\Delta (C_{\mathrm{deoxy}}\cdot L)$ , respectively. These changes [ $\Delta (C_{\mathrm{oxy}}\cdot L)$ and $\Delta (C_{\mathrm{deoxy}}\cdot L)$ ] are referred to as oxy-Hb signals $\left(\Delta C_{\mathrm{oxy}}^{\prime }\right)$ and deoxy-Hb signals $\left(\Delta C_{\mathrm{deoxy}}^{\prime }\right)$ , respectively. Optical topography (OT) based on an NIRS technique has been developed as a noninvasive technique for mapping $\Delta C_{\mathrm{oxy}}^{\prime }$ and $\Delta C_{\mathrm{deoxy}}^{\prime }$ with multiple measurement points.^{5, 6, 7, 8} In this technique, $\Delta C_{\mathrm{oxy}}^{\prime }$ and $\Delta C_{\mathrm{deoxy}}^{\prime }$ in the cerebral cortex are measured at multiple points by using alternating optical fiber bundles for irradiation and detection with light at two different wavelengths. OT systems have been used for assessing the cognitive functions of infants^{9, 10, 11, 12} and for clinical applications^{13, 14} because they are noninvasive and less constraining for subjects than other functional imaging techniques, such as functional magnetic resonance imaging and positron emission tomography. OT systems have been used in studies on brain functions related to human gait^{15, 16, 17} and balance control,¹⁸ as they do not constrain a subject’s posture. The studies on human gait have revealed that walking and running on a treadmill are associated with activations in the medial sensorimotor cortices and the supplementary motor area,¹⁵ the prefrontal cortex,^{16, 17} and the premotor cortex,¹⁶ and a study with an apparatus for perturbing human balance revealed the role of the prefrontal cortex in human balance control.¹⁸ These studies effectively changed the parameters and conditions in their experiments on human walking and balance control on a treadmill or apparatus. However, conventional OT systems have required that a subject’s head position does not move beyond the length of the optical fiber bundles in such OT studies. In recent studies, wearable NIRS or OT systems have been reported and were successful in freeing up the subject’s head position.^{19, 20, 21} However, they had two to eight measurement points covering a region that was much less than or comparable to half of the forehead.