## 2 Materials and Methods
### Procedure  
Eligible participants who completed a phone screening scheduled a single 2.5 h experimental visit to OHSU. Upon arrival, participants provided written informed consent and completed a limited set of intake questionnaires, including a demographics questionnaire. Participants reported the number of hours they had slept the night before the visit and completed a near-field Snellen visual acuity test (binocular) to confirm that they had at least 20/30 visual acuity. All individuals confirmed that they had abstained from alcohol, marijuana, and other recreational drug use on the day of the visit. After intake, research staff helped participants to don an EEG cap before proceeding with the primary visit procedure.  
The visit procedure consisted of three repetitions of the RSVP calibration task (Figure 1), each of which was followed by two attempts at an RSVP copy phrase task (see Section 2.3). That is, participants would complete a calibration, then attempt to spell two different words in copy phrase using a model trained on the calibration data, before finally moving on to the next calibration. Each of the three repetitions corresponded with a test condition: (1) letters presented at a steady rate of 5 Hz, with no jitter applied to the SOI; (2) letters presented at 5 Hz, with a \"small\" uniform distribution jitter of \(\pm\)50 ms; and (3) letters presented at 5 Hz, with a \"large\" uniform distribution jitter of \(\pm\)100 ms. In order to control for order effects due to fatigue and/or practice, the sequence of these three conditions was pseudo-randomly balanced across the 24 participants, such that each of the six possible permutations of condition order occurred exactly four times. Participants were not informed of the condition types or order.  
Before and after each round of calibration and copy phrase, participants provided self-report estimates of their sleepiness, according to the Stanford Sleepiness Scale (SSS) [34; 35]. Sleepiness is one aspect of vigilance (or fatigue) that is known to have an impact on BCI performance [6]. Participants were also asked at these same intervals to rate their levels of both head (or headache) pain and general pain. After completing the third assigned test condition, participants responded to a brief series of user experience questions. Specifically, participants were asked (1) if they perceived a difference in various rounds of calibration and copy phrase tasks; (2) if so, what that difference was; (3) if they had a preference for or against any particular round of calibration or copy phrase; and (4) to rank the conditions from best to worst, even if they perceived no differences or had no strong feelings.  
\begin{table}
\begin{tabular}{c c} \hline \hline  & **Participants (\(n\) = 24)** \\ \hline Education & \\ Some college but no degree & 2 \\ Associate degree & 1 \\ Bachelor’s degree & 10 \\ Postgraduate degree & 11 \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._After completing the user experience questions, participants completed one final round of calibration with the letters presented at a significantly slower rate of one per second, with no jitter. This \"slow\" instance of the calibration task was recorded in order to elicit non-overlapping brain responses for the sake of visual and secondary comparisons. However, since this version of calibration generally seemed to be far more fatiguing than the test conditions, it was always presented at the end of the visit, so as not to interfere with the other presentations. Likewise, this \"slow\" version of the task was not accompanied by any copy phrase tasks, since its primary purpose was to collect non-overlapping profiles of the ERP and alpha attenuation responses for the individual participants.