Fig. 1.

Detection task and temporal profile of the DA neurons’ activity. (A) Trials began with a start cue instruction, i.e., when the stimulator probe indented the skin of one fingertip of the restrained hand. The monkey reacted by placing its free hand on an immovable key (key down event). In stimulus-present trials, after a variable prestimulus period (1.5–3.5 s), a vibratory 0.5-s stimulus was presented. Then, after a fixed delay period (3 s), the go cue (stimulator probe tip lifted off the skin) was delivered and the monkey communicated its decision by pressing one of the two buttons (push button event). The reward was delivered immediately after the push button event in correct choice trials. Stimulus-absent trials had the same temporal structure with the only difference that the vibrotactile stimulus was not presented. (B) Mean population firing rate of midbrain DA neurons (black line, ±SEM colored bands) plotted as a function of time for the four trial types. Activity is aligned to the start cue (Left), go cue (Center), and reward delivery (Right). The dashed line indicates the baseline activity (5.1 spikes per second). Before the go cue the activity exhibited a pronounced decay with respect to the baseline in all trial types. (C) Responses of DA neurons at the stimulus onset (SO) in yes-decision trials sorted by stimulus amplitude. Data showed a positive linear increase of the response with the amplitude of the stimulus (R² = 0.98, P < 0.001) (see SI Materials and Methods for more details on data analysis).