Figure 4. Performance of the BCI with movements suppressed.

A potential concern is that the demonstrated performance improvement for participant T6 relative to previous studies is due to her retained movement ability. Participant T6 was capable of dexterous finger movements (as opposed to participants T5 and T7, who retained no functional movements of their limbs). To control for the possibility that physical movements underlie the demonstrated improvement in neural control, we measured T6’s BCI performance during the same quantitative performance evaluation tasks, but asked her to suppress her movements as best as she could. In these sessions, decoders were calibrated based on imagined (rather than attempted) finger movements. (a) During copy typing evaluations with movements suppressed, T6’s average performance using the OPTI-II keyboard was 28.6 ± 2.0 ccpm (mean ± s.d.), and her average performance using the QWERTY keyboard was 19.9 ± 4.3 ccpm (as discussed in the main text, her performance while moving freely was 31.6 ± 8.7 ccpm and 23.9 ± 6.5 ccpm for the OPTI-II and QWERTY keyboards, respectively). (b) During grid evaluations with movements suppressed, T6’s achieved bitrate was 2.2 ± 0.17 bps (compared to 2.2 ± 0.4 bps while moving freely). We note that using the BCI while suppressing movements is a more difficult and cognitively demanding task - since the participant’s natural, intuitive attempts to move actually generate physical movements, she needed instead to imagine movements, and restrict her motor cortical activity to patterns that do not generate movement. (This is supported by the participants own comment regarding the difficulty in controlling the BCI while imagining movement without actually moving: ‘It is a learning curve for me to not move while imagining.’) Despite this additional cognitive demand, performance with movements suppressed was quite similar to performance when the participant moved freely (within 0–20%) - in all three cases, the differences in performance were not significant (p>0.2 in all cases, Student’s t test). Data are from T6’s trial days 595 and 598.

DOI: http://dx.doi.org/10.7554/eLife.18554.027

Figure 4—figure supplement 1. Participant T6’s movements are greatly reduced when movements are actively suppressed.

In the previous analysis (Figure 4), we demonstrated that T6’s performance was largely unchanged even when she actively suppressed her movements. Here we quantified the degree to which movements were suppressed during those sessions. We first analyzed the participant’s movements during decoder calibration (panels a and b) and then closed-loop BCI control (panel c). For decoder calibration, we compared freely moving sessions and sessions in which movements were suppressed (see Materials and methods: Quantifying movement suppression). Decoders were calibrated using a center-out-and-back task, with the cursor’s position tied to the measured finger position (freely moving sessions) or with the cursor’s position following pre-programmed movements (i.e., ‘open-loop’ calibration) and finger movements were imagined (movement suppressed sessions). For each condition (i.e., freely moving vs. suppressed movement), we measured finger position as a function of time (relative to the starting position for each trial), and averaged these positions across all trials for a given target direction (the position of each pair of traces denotes the target’s position relative to the center target). (a) During movement-based decoder calibration (freely moving sessions), thumb movements (red) controlled the vertical axis, while index finger movements (blue) controlled the horizontal axis. Horizontal scale bars represent 200 ms, and the vertical scale bar represents 100 units on the glove sensor scale (arbitrary units). (b) During open-loop decoder calibration (movement suppressed sessions), in which T6 was asked to simply imagine finger movements but avoid moving to the best of her abilities, finger movements were largely suppressed but minute movement was still detectable. Scale bars match the previous panel. Overall, during decoder calibration, movements were greatly reduced (p<0.01, paired Student’s t test), and the median suppression ratio was a factor of 7.2 (index finger) and 12.6 (thumb). (c) We also quantified the amount of movement during closed-loop BCI control (grid task) in sessions in which movements were suppressed. Because individual trials were highly variable (targets appeared in random locations during the grid task), we grouped trials by the target direction (i.e., the angle between the previous target and the prompted target for the current trial). The position of each pair of traces in the circle denotes the target direction. To ensure that any minute movements were captured in the analysis, the absolute value (rather than the signed value) of the finger position was taken prior to averaging across trials. Scale bars match the previous panel. As shown, movement during closed-loop BCI control was comparable to or less than movement during decoder calibration (panel b), which itself was a factor of 7.2–12.6 times less than movement during movement-based decoder calibration (panel a).

DOI: http://dx.doi.org/10.7554/eLife.18554.028