Like many mammals, rats are attracted to novelty. In order to recognize something in the environment as novel, there needs to be memory for what has been encountered before. The relative attention the animal will devote to the novel compared to the familiar in the environment can therefore be used as a proxy for available memory. The novel object recognition task makes use of this effect and is widely employed in memory research, where it is considered a good model for human declarative memory. With lab- and task-specific optimizations, the task produces robust, reliable results.

This task assesses memory animals have for the identity of a previously encountered object. Identity can be deduced from various sensory inputs received during object exploration. Yet, the vast majority of rat studies introduces novelty by varying visual characteristics of the objects. The stronger the visual differences, the easier it will be to discriminate familiar from novel objects.

In our lab, the task typically consists of three phases – Habituation, Sampling, and Probe.

1. Habituation. Animals are repeatedly exposed to the testing environment. We use an open field, 600 mm X 600 mm X 600 mm. Typically, they explore the empty open field one time per day for four consecutive days.
2. Sampling. Objects are placed in the open field, and rats are returned to explore it. It depends on the experimental question how long and how often this is done. Typically, we place two identical copies of objects in the same positions into the open field during the Sampling trials, usually in opposing corners, with a distance to the wall enough for rats to run around them..
3. Probe. At least one novel object is introduced to the open field, everything else stays the same. Typically, we replace one of the two objects with a new one. If rats explore the novel object longer than the old one, this behaviour is interpreted as novelty recognition and that the animal has memory for the familiar object.

There are many variants to this basic protocol, depending on the experimental question. To name a few, variables that can change are the number of trials in each phase, how long a trial lasts, how long the phases are separated from each other, how many objects are presented, and how many different ones.

All phases are video recorded, and it is good practice that there are no other rats or experimenters in the room in which the open field is placed, as their presence could influence the behaviour of interest. The videos are then later scored by trained observers. In our lab, we do not use software (yet) to automatically score rat exploratory behaviour.

Once the behaviour has been quantified – i.e., the time animals spend exploring objects has been measured – the discrimination index DI (or d) is calculated and statistically analyzed. The index is calculated as follows: DI=[(time exploring novel object)-(time exploring familiar object)]/(time exploring both). DI can take any value between -1 and 1. DI=0 indicates that both objects were explored the same, i.e., the absence of preference or the behaviour that would be expected by chance alone. Di>0 indicates that the novel object was explored more than the familiar one, and DI<0 indicates the opposite. To determine whether DI is significantly different from what would be expected by chance alone, a one-sample t-test comparing DI against 0 can be used.

Our detailed protocols are available at the SOP section.