The effects of stimulus timing features on P300 speller performance

Objective: Despite numerous examinations of factors affecting P300 speller performance, the impact of stimulus presentation parameters remains incompletely understood. This study examines the effects of four distinct stimulus presentation parameters (stimulus-off time [ISI^*], interstimulus interval [ISI], flash duration, and flash-duration:ISI ratio) on the accuracy and efficiency of the P300 speller performance. Methods: EEG data from a 32-electrode set were recorded from six subjects using a row-column paradigm of the speller task and analyzed offline. Results: P300 speller accuracy is affected by the number of trial repetitions (F(14,354)=69.002, p<0.0001), as expected. In addition, longer ISI and ISI^* times resulted in higher accuracy and characters per minute [CPM] rates. Subsets of the entire group (i.e. good vs. poor performers) were compared to show consistency of performance trends despite great variance among subjects. Moreover, the same significant effects were observed whether using the entire 32-electrode dataset or the reduced 8-channel set described by Sharbrough et al. (1991). Conclusions: Despite variability in user performance, both ISI^* and ISI can affect P300 speller performance. Significance: P300 system optimization must consider critical stimulus timing features including ISI^* and ISI. Further characterization of the impact of these timing features in online experiments is warranted and the differential effect on accuracy and CPM should be more comprehensively explored.