Testing was performed using a modification of the methods described in Hirano et al (2005) (link). Briefly, mice (WT = 20; MAO B KO = 13) were individually placed in a dimly-lit (10 lux) Makrolon cages (35 × 28 cm), with 5 cm of fine sawdust, for a 30-min acclimatization period. Subsequently, mice were briefly removed and 20 marbles (1 cm diameter) were placed in each cage, on top of the sawdust. Mice were then returned to the cages, and their behavior was videorecorded for the following 30 min. Measures included the number of buried marbles, and the number and total duration of digging bouts. A marble was considered buried if at least two thirds of its surface area was covered in sawdust. General activity was analyzed by counting the crossings of a grid (5 × 4 squares), as described above.