Impact of Speller Size on a Visual P300 Brain-Computer Interface (BCI) System under Two Conditions of Constraint for Eye Movement

Table 6

Average scores (±SE) of answers to the usability questionnaire.

Statements	Overt attention			Covert attention
	Small	Medium	Large	Small	Medium	Large
Statement 1: difficulty perceiving different characters	2.4 ± 0.8	1.3 ± 0.3	1.8 ± 0.5	5.75 ± 0.7	4 ± 0.4	5.75 ± 0.5
Statement 2: difficulty perceiving characters away from the centre	2.2 ± 0.7	1.2 ± 0.4	1.8 ± 0.5	6.5 ± 0.6	5.9 ± 0.5	7.8 ± 0.4
Statement 3: difficulty distinguishing different rows and columns	2.7 ± 0.8	1.5 ± 0.4	1.6 ± 0.5	6.8 ± 0.7	5 ± 0.7	5.8 ± 0.8