Brain-computer interfaces and communication in paralysis: extinction of goal directed thinking in completely paralysed patients?

Abstract

Objective: To investigate the relationship between physical impairment and brain-computer interface (BCI) performance.

Method: We present a meta-analysis of 29 patients with amyotrophic lateral sclerosis and six patients with other severe neurological diseases in different stages of physical impairment who were trained with a BCI. In most cases voluntary regulation of slow cortical potentials has been used as input signal for BCI-control. More recently sensorimotor rhythms and the P300 event-related brain potential were recorded.

Results: A strong correlation has been found between physical impairment and BCI performance, indicating that performance worsens as impairment increases. Seven patients were in the complete locked-in state (CLIS) with no communication possible. After removal of these patients from the analysis, the relationship between physical impairment and BCI performance disappeared. The lack of a relation between physical impairment and BCI performance was confirmed when adding BCI data of patients from other BCI research groups.

Conclusions: Basic communication (yes/no) was not restored in any of the CLIS patients with a BCI. Whether locked-in patients can transfer learned brain control to the CLIS remains an open empirical question.

Significance: Voluntary brain regulation for communication is possible in all stages of paralysis except the CLIS.

Figure 1

Three types of BCI (left) and EEG analyses averaged over several trials (right). Top: the SMR-BCI: targets are presented at the top or bottom right margin of the screen. Patients’ task is to move the cursor into the target. Cursor movement is indicated by the squares; during feedback of SMR amplitude, only one square is visible. The cursor moves steadily from left to right, vertical deflections correspond to the SMR amplitude. Top right panel shows amplitude of the EEG as a function of frequency averaged across 230 trials separated by task requirement (top vs. bottom target). Bold line indicates frequency power spectrum when the cursor had to be moved toward the top target; dashed line when the cursor had to be moved toward the bottom target. A difference in amplitude can be clearly seen around the 10 Hz SMR peak. Middle: P300-BCI: a 6x6 letter matrix is presented. Rows and columns flash in random order, indicated by the bright row. In the copy spelling mode (Kübler et al., 2001), the patients’ task is to copy the word presented in the top line (GEHIRN = German for “brain”). In each trial, patients have to count how often the target letter flashes. The target letter is presented in parenthesis at the end of the word. Selected letters are presented in the second line below the word to copy. Middle right panel depicts EEG to target letters (bold line) averaged across 43 trials comprising 430 flashes of the target letter and 2150 flashes of all rows and columns not containing the target letter. Dashed line indicates the course of the EEG to all the non-target rows and columns (for an exact description of letter selection see e.g., Sellers et al., 2006). EEG to target letters is clearly distinguishable from non-target letters. Bottom: SCP-BCI: Targets are presented at the top or bottom of the screen. Patients’ task is to move the cursor (white dot) toward the target with the highlighted frame. The cursor moves steadily from left to right and its vertical deflection corresponds to the SCP amplitude. Bottom right: Time course of the SCP amplitude averaged across 350 trials separated by task requirement. A negative SCP amplitude (dashed line) moves the cursor toward the top, positive SCP amplitude (bold line) toward the bottom target. Before each trial, a baseline is recorded indicated by the gray bar. At time point -2 seconds the task is presented, at 500ms the baseline is recorded and at 0 cursor movement starts. Positive and negative SCP amplitude shifts are clearly distinguishable indicating that the patient learned to manipulate the SCP amplitude.

Figure 2

Mean rank of level of success in each of the five categories of physical impairment. The higher the mean rank the better the performance in the group. Diamonds depict mean ranks of the Tübingen patient group; triangles depict mean ranks when patients from other BCI research groups were included. Other BCI research groups investigated no CLIS and only one LIS patient.