Discussion of imalsogreg's comment: Authors used cyclic voltametry to measure dopamine concentrations in realtime (10 readings per second) in freely moving rats in mazes. Dopamine concentration ramps up as rats approach a reward. Instantaneous [DA] correlates better with position-on-track than time-from-start-of-trial. The ramps are more apparent in Ventral-medial striatum (which is considered to be more involved in motivation), than in Dorsal-lateral striatum (which is more strongly linked to motor output). On some trials, [DA] ramped downwards as rats approached food reward. [DA] isn't related to running speed. And the steepness of the ramp is related to the expected size of the upcoming reward (these expectations were constant within training days, not quickly varying - but different parts of the maze won the rats different sizes of reward). Authors suggest that extended ramps of [DA] line the path to distant goals, providing motivation along the way.