Abstract
“Locked-in” patients lose their ability to communicate naturally due to motor system dysfunction. Brain-computer interfacing offers a solution for their inability to communicate by enabling motor-independent communication. Straightforward and convenient in-session communication is essential in clinical environments. The present study introduces a functional near-infrared spectroscopy (fNIRS)-based binary communication paradigm that requires limited preparation time and merely nine optodes. Eighteen healthy participants performed two mental imagery tasks, mental drawing and spatial navigation, to answer yes/no questions during one of two auditorily cued time windows. Each of the six questions was answered five times, resulting in five trials per answer. This communication paradigm thus combines both spatial (two different mental imagery tasks, here mental drawing for “yes” and spatial navigation for “no”) and temporal (distinct time windows for encoding a “yes” and “no” answer) fNIRS signal features for information encoding. Participants’ answers were decoded in simulated real-time using general linear model analysis. Joint analysis of all five encoding trials resulted in an average accuracy of 66.67 and 58.33% using the oxygenated (HbO) and deoxygenated (HbR) hemoglobin signal respectively. For half of the participants, an accuracy of 83.33% or higher was reached using either the HbO signal or the HbR signal. For four participants, effective communication with 100% accuracy was achieved using either the HbO or HbR signal. An explorative analysis investigated the differentiability of the two mental tasks based solely on spatial fNIRS signal features. Using multivariate pattern analysis (MVPA) group single-trial accuracies of 58.33% (using 20 training trials per task) and 60.56% (using 40 training trials per task) could be obtained. Combining the five trials per run using a majority voting approach heightened these MVPA accuracies to 62.04 and 75%. Additionally, an fNIRS suitability questionnaire capturing participants’ physical features was administered to explore its predictive value for evaluating general data quality. Obtained questionnaire scores correlated significantly (r = -0.499) with the signal-to-noise of the raw light intensities. While more work is needed to further increase decoding accuracy, this study shows the potential of answer encoding using spatiotemporal fNIRS signal features or spatial fNIRS signal features only.
Highlights
Active human communication depends fully on the functional integrity of the motor system
In the oxygenated hemoglobin (HbO) selection, the same channel was selected for mental drawing and spatial navigation imagery in three participants, i.e., participant 6, 7, and 12
We presented a novel binary communication paradigm that aimed to exploit spatiotemporal characteristics of functional near-infrared spectroscopy (fNIRS) signals evoked by differently timed mental imagery tasks
Summary
Active human communication depends fully on the functional integrity of the motor system. Severe motor paralysis most often occurs through infarction of the pons (Patterson and Grabois, 1986) or in late stages of diseases such as amyotrophic lateral sclerosis (ALS) and multiple sclerosis. In some cases this leads to a state of being fully awake and aware (Laureys, 2005; Monti et al, 2009) but without any ability to communicate in a natural way, commonly referred to as the “locked-in” syndrome (LIS; see Plum and Posner, 1982; Laureys, 2005; Monti et al, 2009). In progressive motor-neuron disorders such as ALS, control of the eye muscles is lost in late stages of the disease, resulting in a “complete” or “total” LIS (Bauer et al, 1979)
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