Habituation and Training.

After habituation to the testing arena and the bowls containing the food, the rats were trained on a series of simple discriminations (SD). The aim of the training was for the rats to learn to associate a food reward with an odor or/and a digging medium. The same pair of stimuli was used for all rats during the training, and the positive and negative cues were presented randomly and equally for each rat. The stimuli used in the training were not used later in the actual test. Rats are able to learn to dig reliably in approximately 4–7 days according to the following schedule:

The arena was covered halfway by the lid with mounted camera. The rat was placed and left alone to explore the arena for about 5 min without a bowl in it. After about 5 min a bowl filled with bedding (sawdust) was placed into one corner of the arena, on the side covered with the lid. Roughly five pieces of food was placed in the center of the bowl, on top of the bedding. Only one bowl was used at this point. The goal was only to get the rat to eat the food; it was not timed. The corner, in which the bowl was placed, was randomized so that the rat would not form a side preference. Training stopped after 30 min or after the rat ate 10 pieces of food. These steps were repeated until the rat ate 10 pieces of food during one 30 min session.

After the rat completed the first phase of eating, 10 uncovered pieces of food within one 30 min session, the process of burying the food was initiated little by little. The rat was left in the arena to adjust for about 5 min. One piece of food was placed in the center of the bowl on top of the bedding with the corner randomized. Still only one bowl was used at this stage. When the rat managed to reach about four uncovered pieces, the food was gradually concealed more and more over the next trials until it was deeply buried. The goal was to get the rat to eat five uncovered pieces and five buried pieces.

The rat was left to adjust to the arena for about 5 min before training commenced. At this point, every piece of food was buried, and the corner was randomized according to the learning to dig (LTD) schedule sheet. The goal was to get the rat to eat 10 buried pieces, but still without timing.

The rat was placed in the arena for about 5 min, and after that the holding box was introduced. The rat was placed under the holding box for about 2 min while the baited bowl was placed in the randomized corner according to the LTD schedule sheet. The holding box was relatively large, almost the size of the rats’ home cage (16″ × 20″, 8.25″ H) so the animals could get used to it within the next 1 to 2 days. At this stag,e the rat had only 90 s to find and eat the food during the first four trials and only 60 s during trials five and above. This training phase was continued until the rat retrieved 10 buried pieces within the time limit in a row (consecutive). Once the rat reached the goal of finding 10 buried food pieces within the time limit in consecutive trials, it was considered ready for the exemplar phase, or the ASST pretest. The exemplar, as stated above, consisted of almond versus lemon (odor discrimination) and glass beads versus black gravel (digging medium discrimination). The exemplar phase has to be conducted the day before each ASST to make sure the rat is ready for testing. The complete ASST should not be conducted unless the rat completed the exemplar stage successfully. Exemplars follow the same rules as the ASST; six consecutive correct trials are required to pass.