Figure 2 | Scientific Reports

Figure 2

From: Optimising non-invasive brain-computer interface systems for free communication between naïve human participants

Figure 2

The three phases of Experiment 1, which allowed BCI free communication through prompted free association. (a) Template training. Participants (N = 17) were cued to focus their attention and gaze on each flickering key in a random order (N = 20 repetitions/key). Keys were cued prior to and during flicker. Participants with low classification accuracy (< 80%), determined via QWERTY classification, underwent retraining with an increased flicker duration (3.0 vs. 1.5 s) to improve single-trial SSVEP SNR. (b) QWERTY classification. Participants freely BCI typed the complete QWERTY sequence, without guiding cues other than feedback displayed at the top of each key. Flicker-free periods allowed participants 0.75 s to redirect their attention to the next uncued key. The three previous classified letters were displayed at the top of each key, allowing participants to monitor classification while performing keystrokes. (c) BCI free communication undertaken by participants with high classification accuracy (>80%). Prompt words allowed participants to freely associate words/phrases. To assess accuracy, participants entered intended character strings using a manual keyboard before BCI typing. A new prompt was presented when participants either matched the intended character string using BCI, or had entered three times more characters than in the intended string.

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