 2.3.3 Visual Working Memory. This paradigm, which has previously been used in psychopharmacology research, will employ four randomized conditions of the verbal N-Back task; each condition will have identical stimuli and response demands, but consist of increasing levels of working-memory load. A series of letters will be presented and participants will be required to respond as quickly as possible only when the letter on the screen matches the letter n stimuli back i.e., for the 0-back condition, the target is any letter that matches a pre-specified letter x, while in the 1-back, 2-back and 3-back conditions, a target is any letter that is identical to the letter presented one, two or three trials back, respectively. As the N-back task requires information storage, updating, and manipulation, it has become the dominant tool used in assessing WM control functions. Behavioural endpoint measures will include accuracy correct target detections and reaction time ms to targets. Additionally, for each condition and each drug treatment I will calculate signal detection sensitivity d and response bias C. Electrophysiological endpoint measures include amplitudes and latencies of the P3b to targets. Hypotheses: Similar to the other paradigms, and consistent with behavioural studies of caffeines effects on n-back indexed working memory, the investigators expect caffeine will reduce RTs and will reduce P300 latency in both groups. 