Chroma Speller: isotropic visual stimuli for truly gaze-independent spelling

Brain Computer Interfaces (BCIs) can be used as communication devices for people affected by neurodegenerative diseases like Amyotrophic Lateral Sclerosis (ALS). The progressive motor-control impairment caused by ALS motivates the development of BCI paradigms that do not rely on eye movement. In the present study, we investigated the feasibility of the first truly gaze-independent visual BCI, called Chroma Speller. Stimuli consist of colors displayed widescreen, which make the focus of gaze irrelevant to the BCI-spelling. The speller was tested online with 9 healthy participants. Mean online symbol-selection accuracy of 88.4% and mean online spelling speed of 1.4 char/min were achieved, using 5 repetitions of the stimuli. These results demonstrate that the abstract association between colors and symbols does not compromise the usability of the speller, leading to performances competitive with the most recent gaze-independent spellers in literature.