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