A two-trial version of the task was used. In the first trial, the mice were forced to choose one of the two arms of a Y-maze by occluding one arm with an opaque partition. Each arm was 15 cm tall and 30 cm long × 5 cm wide clear acrylic, and all angles between the arms were 120°. The mice were then confined to this arm for ≈30 sec and placed into a holding cage while the apparatus was cleaned and both partitions were removed. The mice were then placed back into the stem of the Y-maze and allowed to choose either the arm they had previously entered or the novel one. An entrance was defined as all four paws being within the arm. Because of very high performance in B6D2F2/J and B6D2F3/J mice (the proportion of mice that chose the novel arm was 90%), a different procedure was used for the selected line experiments. On trial 1, no partition was used to obstruct one of the arms, so the mouse was allowed to enter either arm. The mouse was
