Table 1. The number of studies with each combination of biosignal.

Type	Input Signal		# of Studies
Single Brain Signal	EEG	EEG	44 (59%)
Multiple Physiological Signals	EEG	EOG	6 (8%)
	EEG	EMG	3 (4%)
	EEG	ECG	2 (3%)
Brain Signal with External Input	EEG	Eye Tracking	11 (15%)
	EEG	Joystick	2 (3%)
	EEG	Gyroscope	1 (1%)
Multiple Brain Signals	EEG	fNIRS	6 (8%)