Table 1.

Combination of the brain and non-brain signals modalities.

References	Modalities combination	Signal fusion	Benefits
Li et al. (2016) and Hong and Khan (2017)	EEG + EOG	Electrophysiological + Ocular Movements	1. To enhance accuracy and number of control commands. 2. To detect the motion artifacts due to ocular movements.
Bulea et al. (2014); Sburlea et al. (2015); Gui et al. (2017); Hong and Khan (2017), and Liu D. et al. (2017)	EEG + EMG	Electrophysiological + Electromyography	1. To increase the accuracy of the BCI system. 2. This combination could help to make comprehensive neural BCI and minimize the delay between movement detection and execution. 3. It can help to enhance the active participation of patients in gait rehabilitation. 4. EMG is usually used with EEG to detect the actual muscle movement to ensure BCI's smooth operation.
Fazli et al. (2012); Blokland et al. (2013); Khan et al. (2014); Hong and Khan (2017), and Cicalese et al. (2020)	EEG + fNIRS	Electrophysiological + Hemodynamics	1. To enhance classification accuracy. 2. To increase the number of control commands.
Tobar et al. (2018)	EEG + fMRI + EMG	Electrophysiological + Hemodynamics + Electromyography	1. fMRI is used to locate the brain activation area, EEG is used to record to cortical activity, while EMG is used to confirm motor task execution.
Zhang et al. (2010) and Liu et al. (2018)	EEG + EOG + EMG	Electrophysiological + Ocular Movements + Electromyography	1. The actual movement onset was extracted from surface EMG, and the motor intention was detected from EEG. EOG is used to detect ocular movement artifacts. 2. To increase the number of control commands.
Salazar-Varas et al. (2015); Hortal et al. (2016); Gui et al. (2017), and Elvira et al. (2019)	EEG + IMUs	Electrophysiological	1. IMUs are used along with EEG to detect actual body movements.