Figure 1: Task and timing. a) Experimental design. i) Overview of session and trial structure.

Monkeys were given either MPH or placebo 15 min prior to the start of simultaneous behavioral and neural recording sessions. Using a passive manipulandum in a 2D plane with position mapped to a cursor on a screen at eye level, monkeys performed a center-out, delayed reaching task with 7 radial targets. To earn a reward, they were required to withhold reaches to cued targets for a randomized delay period (0ms on 10% of trials; drawn from a uniform 30–900ms distribution on the remaining 90%), during which the reach target jittered on screen. Go cue was represented by cessation of target jittering. Experimenters controlled the session duration to hold trial and reward counts roughly steady across treatments (MPH vs. placebo). ii) Example session timing. MPH and placebo sessions were pseudo-randomized to fall on overall similar distributions of weekdays. Sessions the day after MPH sessions were excluded from analysis to minimize potential confounds from stimulant-induced sleep disruption the following day. The experimenter running each session was blinded to the treatment condition. iii) Chronic electrode array placement. Photograph from monkey U’s array implantation surgery showing anatomic location of the three 96-channel Utah arrays in PMd and M1, with surrounding cortical surface landmarks (panel reproduced from ⁴⁴). PMd: dorsal premotor cortex; M1m: primary motor cortex, medial array; M1l: primary motor cortex, lateral array; PCD: precentral dimple; AS: arcuate spur; CS: central sulcus. b) Example reach trajectories by treatment (monkey U, 6 mg/kg dose). Smoothed 2D hand position trajectories (mapped 1:1 to cursor position on screen) from 180 randomly selected trials per treatment condition. Left, placebo sessions; middle, MPH sessions. Colors represent different reach targets. Inset, right: Example overlaid reach trajectories to a single target from MPH sessions (orange) and placebo sessions (blue).