 2.3.2 Visual Sustained Attention AX-CPT. Sustained attention will be assessed within the A-X version of the continuous performance task AX-CPT. Participants will be presented with a series of letters and will be instructed to respond to the letter X only when it was immediately preceded by A. In order to test sustained attention, 400 letters will be presented over a span of 11 minutes consisting of 80 cues A, 40 targets A followed by X, 40 NoGos A followed by any other letter, and 240 distractors other letters or X not preceded by A. Behavioural endpoints include the number and RTs of correct responses, and false alarms i.e., nontarget responses. Neural ERP measures of interest include the P3b to correct targets. Hypotheses: Consistent with previous reports in sustained attention, caffeine will not impact behavioural performance, but will increase P300 amplitudes, with this effect vs. placebo being seen in both participant groups. 