P 135 Using a BCI approach to record ERPs for detecting signs of awareness in patients with severe disorders of consciousness

Abstract

Objective In the clinical setting mainly behavioral methods are used for differentiating between patients with unresponsive wakefulness syndrome (UWS) and minimally consciousness syndrome (MCS). These may result in a high rate of false negative classifications, as some patients cannot produce clear behavioral responses, although they are conscious. To increase diagnostic objectivity additional methods to assess patients” states of consciousness are needed. FMRI methods have proven useful, but are costly and rarely available. Building up on previous ERP (event related potential ) studies in patients with severe disorders of consciousness ( Neumann and Kotchoubey, 2004 ) we tested an ERP measurement system based on brain-computer interface (BCI) technology (mindbeagle, g.tec, Schiedlberg) as patient bedside instrument to detect consciousness under exclusion of behavioral response requirements. Methods Recordings were performed in 13 patients with severe disorders of consciousness(5MCS, 8UWS), as diagnosed by the Coma Recovery Scale revised (CRSr). The BCI testing consisted of two subtests, an auditory and vibro-tactile paradigm. For both subtests patients were instructed to react to a rare target stimulus and to neglect an often occurring non-target stimulus. Accordingly the subtests combine the passive (Oddball structure) and the active (instruction to react to target stimulus) paradigm. In the auditory subtest two distinct pure tones (500 Hz and 1000 Hz) were presented on both ears. In the vibro-tactile subtest vibration via vibro-tactile trajectories was either transported to the patients left (target) or right (non-target) wrist. The accuracy level at which a non-target and target could be differentiated by the system classifier, trained on the patient’s P300 response, was used as an outcome measure. Results With exception of two patients (one in UWS and one in MCS ) all participants showed an accuracy level above chance (12.5%) in at least one of the subtests. In the auditory subtest this was true for two patients with MCS (20%, 30%) and three with UWS (100%, 30%, 80%). In the vibro-tactile subtest for three patients with MCS (40%, 20%, 20%) and four patients with UWS (50%, 20%, 60%, 60%) the target vs. non-target discrimination could be made. Conclusion Although there are still improvements necessary to use the applied system as a diagnostic instrument in the clinical setting (e.g. adaptations to the clinical population, precise manual for standardized testing), we were able to detected signs of awareness in patients that were not accessible by behavioral measurements. Additional research is needed to determine validity and significance of that finding.