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Biomechatronic Applications of Brain-Computer Interfaces 153

Motor-imagery-based spellers, unlike the above two designs, have the
advantage that they are not necessarily dependent on any external stimuli.
An early example of an imagery-based speller was presented by Blankertz
et al. (2007) and named “Hex-o-spell.” It is based on two imagined motions:
the right hand and the feet. On the screen, six hexagons are arranged around
a circle, and an arrow points toward the hexagons. Each hexagon contains
five letters, and the first stage of the imagery-based letter selection is to turn
the arrow so that it points toward the hexagon containing the desired letter.
Every time right-hand motion is imagined, the arrow turns one hexagon to
the right; once the arrow is pointed at the correct hexagon, the selection is
confirmed using imagined foot motion. In the second stage, the same pro-
cedure is used to select among the five letters: moving the arrow to the
desired letter one step at a time using hand imagery and confirming the selec-
tion using foot imagery. The system achieved an information transfer rate of
2–3characters/min in able-bodied users, though it was more fatiguing and
required more user training than P300- or SSVEP-based spellers (Rezeika
et al., 2018). A modified version of its graphical user interface with circles
instead of hexagons is shown in Fig. 5.

Fig. 5 A modified version of the Hex-o-spell (Blankertz et al., 2007) motor-imagery-
based speller. In the first stage of selecting a letter, the user sends motor imagery com-
mands to select one of the six circles. In the second stage, the user sends the same com-
mands to select one of the letters in the previously selected circle. (From Rezeika, A.,
Benda, M., Stawicki, P., Gembler, F., Saboor, A., Volosyak, I., 2018. Brain-computer interface
spellers: a review. Brain Sci. 8, 57, reused under the Creative Commons Attribution License.)