Performance Analysis With Different Types of Visual Stimuli in a BCI-Based Speller Under an RSVP Paradigm

Abstract

Brain-Computer Interface (BCI) systems enable an alternative communication channel for severely-motor disabled patients to interact with their environment using no muscular movements. In recent years, the importance of research into non-gaze dependent brain-computer interface paradigms has been increasing, in contrast to the most frequently studied BCI-based speller paradigm (i.e., row-column presentation, RCP). Several visual modifications that have already been validated under the RCP paradigm for communication purposes have not been validated under the most extended non-gaze dependent rapid serial visual presentation (RSVP) paradigm. Thus, in the present study, three different sets of stimuli were assessed under RSVP, with the following communication features: white letters (WL), famous faces (FF), neutral pictures (NP). Eleven healthy subjects participated in this experiment, in which the subjects had to go through a calibration phase, an online phase and, finally, a subjective questionnaire completion phase. The results showed that the FF and NP stimuli promoted better performance in the calibration and online phases, being slightly better in the FF paradigm. Regarding the subjective questionnaires, again both FF and NP were preferred by the participants in contrast to the WL stimuli, but this time the NP stimuli scored slightly higher. These findings suggest that the use of FF and NP for RSVP-based spellers could be beneficial to increase information transfer rate in comparison to the most frequently used letter-based stimuli and could represent a promising communication system for individuals with altered ocular-motor function.

Keywords: N170; P300; brain computer-interface (BCI); electroencephalography (EEG); famous faces; neutral pictures; rapid serial visual presentation (RSVP).

Figure 1

(A) RSVP paradigm over time with the Famous Faces (FF) interface as an example; (B) Example of a stimulus representation with its equivalent and corresponding white letter (WL, “S”), famous face (FF, “Shakira”) and neutral picture (NP, “Seau”). Note that due to copyright reasons, the images presented are pixelated in this figure.

Figure 2

Accuracy (mean ± standard error) of each condition (WL, white letters; FF, familiar faces; NP, neutral pictures) as a function of the number of sequences during the calibration task.

Figure 3

Information transfer rate (ITR, mean ± standard error) of each condition (WL, white letters; FF, familiar faces; NP, neutral pictures) as a function of the number of sequences during the calibration task.

Figure 4

Grand average ERP waveform for target, non-target and amplitude difference between target and non-target stimuli signals in all used channels (Fz, Cz, Pz, Oz, P3, P4, PO7, and PO8) for the three conditions: white letters (WL), familiar faces (FF), and neutral pictures (NP). These plots were obtained from the EEG data recorded during the calibration phase.

Figure 5

Topographical scalp map of each condition (WL, white letters; FF, familiar faces; NP, neutral pictures) for the next components: P100 (60–110 ms), N170 (110–180 ms), P300 (450–520 ms), and N400 (520–570 ms). These plots were obtained from the EEG data recorded during the calibration phase.

Figure 6

Scores (mean ± standard error) of each condition (WL, white letters; FF, familiar faces; NP, neutral pictures) for the variables collected in the subjective questionnaire.