Mice were placed in 40 × 40 cm boxes for 30 min, and their horizontal movement was recorded in six daily sessions. Presented is the day 1 (open bars) vs. day 6 (filled bars) comparison of locomotor results for both the wild-type and DAT-CI mice in both strains (n = 8 C57:129 hybrid; and n = 8 C57-congenic mice). Wild-type mice (n = 8 in each strain) display habituation by the sixth session, whereas DAT-CI mice do not (main effects). Notably, the C57-congenic DAT-CI mice undergo significant “reverse habituation”, causing differences between the strains on day six (double dagger). (*, p < 0.05 within strains; ‡, p < 0.05 between strains). All data are means ± SEM.