## 1 Introduction  
Clinical demand is growing for a new neurorehabilitation strategy in which post-stroke patients with insufficient or no remaining hand motor function can participate. Currently, these patients cannot benefit from Active Movement Therapy (AMT) [1], one of the widely applied rehabilitation therapies, because the level of remaining motor output in their affected hands is insufficient. Although passive movement therapy is an alternative option, it can only bring minimal motor function recovery [2]. Therefore, roughly one-third of post-stroke patients with motor impairments [3] cannot expect much functional recovery from current rehabilitation strategies. Incorporating a brain-computer interface (BCI) [4] into neurorehabilitation has been recognized and investigated as a promising solution for post-stroke patients with complete or almost complete paralysis [5]. A BCI, integrated with a motion-assistive device, enables patients to execute motor intention-induced movements that resemble active movements. Thus, patients can participate in effective AMTs and expect motor function recovery. Indeed, several clinical studies have reported that BCI-combined neurorehabilitation improves stroke-impaired motor function [6]. Near-infrared spectroscopy (NIRS) is a new non-invasive neuroimaging technique [7] that relies on the hemodynamic response to local neuronal activities. NIRS measures real-time changes in oxyhemoglobin (HbO) and deoxyhemoglobin (HbR) concentrations ([HbO] and [HbR] respectively) in local cortical areas. The background principle of NIRS is based on neurovascular coupling [8], which is the interaction between local neuronal activity and local changes in Cerebral Blood Flow (CBF); local CBF increases to meet the metabolic demand of local neuronal activity. The absorption or reflection of near-infrared light by hemoglobin depends on the amount of hemoglobin combined with oxygen in that local area [9]. Accordingly, NIRS can monitor the hemodynamic response. This resultant hemodynamic response is slower than its causing neuronal activity and takes place with a delay on the order, of seconds. A NIRS-Brain-Computer Interface (NIRS-BCI) is a hemodynamic response-based BCI using NIRS as a neuroimaging modality. Hemodynamic responses captured by NIRS are relatively easier to detect and analyze, and more robust against noise than electrical responses captured by electroencephalography (EEG), which is currently the most popular BCI modality. Thus, several studies have successfully introduced NIRS into BCI systems for motor tasks, resulting in classification accuracies ranging from 65% to 95.5% [10]. The drawback of NIRS-BCI is a longer detection latency of motor intention on the order, of seconds, which results from the slow nature of hemodynamic response. Compared with EEG-BCI having a detection latency of a few hundred milliseconds, communication and control using NIRS-BCI can be limited [11]. However, for neurorehabilitation purposes, the longer detection latency of NIRS-BCI might be acceptable. For example, a study showed that real-time NIRS-controlled neurofeedback enhanced motor imagery (MI) performance [12]. Thus, we can infer that a NIRS-BCI system with a longer detection latency evokes a rehabilitative effect, although further assessment is required. Aside from its classification accuracy and slower but potentially acceptable detection latency, NIRS-BCI has certain advantages over EEG-BCI. NIRS offers comparatively better spatial resolution than EEG. The temporal resolution of NIRS is sufficient for capturing task-induced hemodynamic responses of 0.1 Hz [10]. NIRS is less affected by motion artifacts and eye movements than EEG [7]. Positioning NIRS optodes (optical probe) is easier than positioning EEG electrodes, and NIRS optodes do not require the use of gel between the scalp and optodes, which might be preferable by both therapists and patients.