2.6. The Novel Object Recognition Test

The novel object recognition test was performed as described previously [20,21]. During the training session, the mice were placed in a test box in which two objects were introduced, and mice were allowed to explore each object for 5 min. The mice were then returned to their cages. After 24 h, one object was replaced by a novel object, and the mice were placed in the test box again. The relative exploration time (time spent exploring the replaced novel object × 100/total exploration time) was measured. One hour before the training session, mice were intraperitoneally administered with a fraction of the nacre extract or PBS. After 30 min, memory impairment was induced by injecting scopolamine (2 mg/kg), and the training session was started 30 min later. The control group was administered with PBS only.