"Clearly, the system still needed improvement”, says Damien Lesenfants. “That’s why, in parallel to the initial task requested (focusing on the red diodes or yellow diodes), we studied focal attention, i.e. the subject’s degree of concentration on the designated target. More precisely, this alternative method to detect voluntary responses was based on the assessment, through spectral entropy, of the subject’s level of attention when performing the requested task and, consequently, their conscious effort involved in accomplishing this task. Thanks to this hybrid system combining two assessment methods, we obtained a 95 % level of performance in healthy subjects and LIS patients.”

Complementarity

The new hybrid SSVEP/Entropy system was then tested among patients with disorders of consciousness (DOC). The results? A response to command was detected in the majority of patients who were previously diagnosed in a minimally conscious state on the basis of a behavioural assessment. The rate of false negatives was limited to 9 %, while no false positives were recorded – none of the 16 patients in a vegetative/unresponsive state were considered by the system as having responded to the command. “Our hybrid system therefore seems a better alternative than the other BCI systems previously presented in the literature relating to disorders of consciousness, with a false negative rate oscillating between 25 and 100 %”, Damien Lesenfants states.

But what about communication with LIS patients with a hybrid interface? The development of such a tool based on the modulation of attention will be the subject of Damien Lesenfants’ next piece of research. Of course, many LIS patients communicate through other channels, in particular, through vertical eye or palpebral movements. But in complete locked-in syndrome, immobility is total, including the eyes. Furthermore, if they are uncoordinated as in the case of nystagmus, it is impossible to use eye movements in eye-tracking systems. Hence the interest of a brain-computer interface system for a small group of LIS patients.

Considered as a means of detecting residual consciousness in severely brain-injured patients, the hybrid SSVEP/Entropy system isn't destined to go it alone. On the contrary, it is intended as a complement to other existing approaches – behavioural scales, BCIs, etc. “The combination of several tests aims to reduce uncertainty as much as possible in an area where there is no 'gold standard’. The future is more than ever a combination of information”, Steven Laureys concludes.