Operation of a P300-based brain-computer interface by patients with spinocerebellar ataxia

Abstract

Objective: We investigated the efficacy of a P300-based brain-computer interface (BCI) for patients with spinocerebellar ataxia (SCA), which is often accompanied by cerebellar impairment.

Methods: Eight patients with SCA and eight age- and gender-matched healthy controls were instructed to input Japanese hiragana characters using the P300-based BCI with green/blue flicker. All patients depended on some assistance in their daily lives (modified Rankin scale: mean 3.5). The chief symptom was cerebellar ataxia; no cognitive deterioration was present. A region-based, two-step P300-based BCI was used. During the P300 task, eight-channel EEG data were recorded, and a linear discriminant analysis distinguished the target from other nontarget regions of the matrix.

Results: The mean online accuracy in BCI operation was 82.9% for patients with SCA and 83.2% for controls; no significant difference was detected.

Conclusion: The P300-based BCI was operated successfully not only by healthy controls but also by individuals with SCA.

Significance: These results suggest that the P300-based BCI may be applicable for patients with SCA.

Keywords: BCI; BMI; P300; Spinocerebellar ataxia; Visual stimuli.

Fig. 1

A region-based, two-step P300-based hiragana speller. (A) This shows a region-based, two-step P300-based hiragana speller. Each visual stimulus flickered 8 times, pseudo-randomly. The interval between the stimuli was 175 ms: 100 ms of intensification and 75 ms of rest. In the first step, 6 circles contained 9 hiragana characters or symbols. In the second step, the selected circle enlarged, and the participants attempted to select the character that they wanted to input. (B) We used 10 electrodes, including eight recording electrodes (Fz, Cz, Pz, P3, P4, Oz, PO7, and PO8). EEG data were collected and used for the classification. (C) The additional averaged EEG waveforms obtained from the Po7 electrode are shown in both groups. The 700 ms waveforms at 100 ms after the onset of intensification were averaged. The solid line indicates the ERP when attending to the target, while the dashed line indicates the ERP when attending to the non-target. We found two positive peaks, which corresponded to the early and late components of P300, in both groups.

Fig. 2

Performance in BCI operation. (A) Online BCI accuracy was evaluated from the total performance on both the first and second steps. Accuracy in BCI operation by patients with SCA and controls is plotted with box plots. The central black solid line represents the median, the edges of the box are the 25th and 75th percentiles, and the whiskers extend to the most extreme data points. Each gray dot represents an actual subject’s accuracy score. We found no significant difference between the SCA and control groups (mean 82.9% and 83.2%, respectively; p = 0.68). (B) Offline evaluation of each sequence. Mean accuracies of the SCA and control groups are plotted in each sequence. Each circle represents the accuracy of each sequence in the SCA group, while each square represents that in the control group. Two-way repeated-measures ANOVA revealed that the main effect of group (SCA vs. control) and the main effect of number (1–8) of sequence were significant (p = 0.0045 and p < 0.0001, respectively).

Fig. 3

Distribution of errors in BCI operation. This figure lists the error distribution rate at the region relative to the target region in the SCA and control groups during BCI operation. The position in the figures represents the relative location, and the numbers indicate error ratios. In the SCA group, 7 of the 8 patients showed horizontal nystagmus, but horizontal-dominant error distribution was not observed in the patients with SCA.

Fig. 4

Performance on character input. (A) Performance on character input was evaluated, in which correct responses in both the first and second steps were required for correct character input. The accuracy of each SCA and control participant was plotted with box plots. We found no significant difference between the SCA and control groups (mean 71.4% and 69.1%, respectively; p = 0.87). (B) Offline evaluation for each sequence. Mean accuracies of the SCA and control groups were plotted in each sequence. The main effects of group (SCA vs. control) and number of sequences (1–8) were significant (p < 0.01, two-way repeated-measures ANOVA).