Probing command following in patients with disorders of consciousness using a brain-computer interface

Objective: In the recovery from coma, the acquisition of command following represents an important milestone, indicating emergence from the vegetative state. In some patients, recovery of consciousness may precede motor recovery. Brain-computer interfaces (BCI) might permit these patients to show non-motor dependent signs of awareness and in a next step might enable communication. This study aimed at testing to what extent an EEG-based BCI could help detecting signs of awareness and communication in disorders of consciousness.
Methods: We studied 13 patients with a minimally conscious state (MCS, 5 TBI – 8 anoxic, mean time post injury 70±109 months; mean age 42 ± 21) and 16 healthy controls (aged 45±19). Patients were evaluated using the Coma Recovery Scale Revised. 16-Channel EEG was recorded using a g.tec USBAmp amplifier. An auditory P300 four choice speller paradigm based on the BCI2000 system was used. Subjects were asked to answer yes or no to simple questions by paying attention to one out of four auditorily presented stimuli (‘yes’, ‘no’, ‘stop’, ‘go’). A trial constituted of 15 presentations of each sound the order of presentation being randomized. After a training session, patients and healthy subjects were required to answer 10 to 12 questions. A stepwise linear discriminant analysis based on the training session was used to classify the data. Offline, all training and testing trials were pooled and a leave-one-out approach was used to classify the data.

Results: Healthy subjects presented a mean correct response rate of 73% online and 93% offline. Three MCS patients had a correct response rate of ≥50% offline (10, 18, 0% online and 50, 53, 57% offline). Two of these three patients did not show any command following at the bedside. The 10 remaining MCS cases showed online and offline correct answers <50% (mean 33±9% online and 25±13% offline).

Conclusion: Our auditory P300-based BCI permitted functional interactive communication in 15/16 controls (online) and in all offline. Our data obtained in patients with disorders of consciousness demonstrate the potential clinical usefulness of the technique following coma but also show lower accuracy in patients as compared to healthy volunteers. This might be due to fluctuating attentional levels and exhaustibility in the MCS and to the suboptimal EEG recording quality due to movement, ocular and respiration artifacts in these challenging patients.