Choice between Rewards, Reaction to Delay. Mean (±SEM, n = 6 per sub-population) choice (%) for the large but late (LL) reward in mice, immunized with DAT fragment (right panel) or vehicle alone (left panel), in a delay-of-reward task. Two sub-populations were formed, based on the slope of preference curves (see Methods). Among VEH controls, individuals segregated into either a flexible (with a “steep” profile, black points) or an inflexible (with a “flat” profile, white points) subgroup. Among DAT-i mice, individuals segregated into either an inflexible (with a “stuck” profile, black points) or a flexible (with a “slow” profile, white points) subgroup. * p < .05 between DAT-i and VEH controls; # p < 0.05 when comparing sub-populations (low- versus high-slope) within the same immunization group.