Figure 7.

Experimental setups and protocols. (A) The subjects were seated in a comfortable chair in front of a 24-inch LCD monitor screen and were instructed to imagine the silent pronunciation of the given word as if they were performing real speech, without moving any articulators or making the sound. (B) Five critical main words/phrases for basic communication (“hello,” “help me,” “stop,” “thank you,” and “yes”) were selected. Seventy trials per class (70× 5 = 350 trials) are released for training (60 trials per class) and validation (10 trials per class) purposes. (C) An auditory cue was randomly presented for 2 s, followed by 0.8–1.2 s of a cross mark. The subjects were instructed to perform imagined speech of the given cue as soon as the cross mark disappeared on the screen. Four cross marks and imagined speech phases were followed in a row per random cue. After performing the imagined speech four times, 3 s of the relaxation phase was given to clear the mind.