In last four tasks, participants could choose a thought that represented a more "personalized secret." They could imagine performing a repetitive motion from a sport of their choice, like kicking a ball or swinging a baseball bat. They could think about singing a song or imagining a set of objects with a distinct color scheme. Or they could choose their own thought and focus on it for ten seconds.
By measuring the subject's unique brainwaves during this personalized cycle of tasks and cross referencing them with their baseline measurements, researchers were able to glean a unique EEG signature so that each "pass-thought" could identify individual users and distinguish them from other participants. Therefore, even if in the unlikely event that volunteers thought of the same "choice" tasks, their unique EEG signatures would prevent others from hacking their "pass-thought."
By customizing each participant's authentication threshold via the headset, researchers reduced error rates below one percent, which is comparable to more-invasive methods for measuring multi-channel EEG signals.