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Objective: Previous works using a visual P300-based speller have reported animprovement modifying the shape or colour of the presented stimulus. However,the effects of both blended factors have not been yet studied. Thus, the aim ofthe present work was to study both factors and assess the interaction betweenthem. Method: Fifteen na\"ive participants tested four different spellers in acalibration and online task. All spellers were similar except the employedillumination of the target stimulus: white letters, white blocks, colouredletters, and coloured blocks. Results: The block-shaped conditions offered animprovement versus the letter-shaped conditions in the calibration (accuracy)and online (accuracy and correct commands per minute) tasks. Analysis of theP300 waveform showed a larger difference between target and no target stimuluswaveforms for the block-shaped conditions versus the letter-shaped. Thehypothesis regarding the colour heterogeneity of the stimuli was not found atany level of the analysis. Conclusion: The use of block-shaped illuminationdemonstrated a better performance than the standard letter-shaped flashingstimuli in classification performance, correct commands per minute, and P300waveform.

Evaluation of flashing stimuli shape and colour heterogeneity using a P300 brain-computer interface speller