This month we will be looking at a behavioral measure of rodent memory that is useful for evaluating the role of experimental manipulations on cognition. Novel Object Recognition (NOR) is based on the premise that rodents will explore a novel object more than a familiar one, but only if they remember the familiar one. This tendency is actually shared by humans, as looking time is often used to make inferences about an infant’s memory in the absence of explicit, verbal recognition.
Before training the animals with objects, they are first allowed acclimate to the testing environment, which is nothing more than a large bin equipped with an overhead camera. After a few acclimation sessions, the animals are ready for the training stage. This stage involves the introduction of two identical objects to the environment before allowing the rodent to explore:
Following the training period, the rodent is removed from the environment for a a delay period which can range from 5 minutes to 24 hours, depending on the type of memory being tested. After the delay, the rodent is returned to the bin, where one of the original objects has been replaced by a new one:
The amount of time that the rodents spends exploring each object can be calculated by hand or by using a computer program receiving input from the overhead camera. One company that manufactures such software is Clever Systems, Inc. The literature describes a variety of methods for analyzing results. One technique involves dividing the time spent exploring the novel object by the total time spent exploring either object, yielding % Novel exploration. An alternative technique is used to calculate the discrimination ratio, defined as the difference in exploration time for the objects divided by total exploration time. The method of analysis should be suited to the specific experimental setup.
Though simple by design, NOR is actually quite flexible. For instance, changing the duration of the delay period allows one to selectively test short-term or long-term memory. Alternatively, the NOR protocol can be used to selectively test the effects of an acute drug treatment on a specific stage of memory formation. The experimenter can manipulate memory encoding, consolidation or retrieval by injecting the drug prior to the training, delay or testing period, respectively.
Interested in learning more about NOR?
- I already linked to this paper about alcohol-mediated memory enhancement, but the methods section is worth a second look.
- NOR is also being used to measure cognitive deficits associated with animal models of Alzheimer’s disease.
- Here is an example of a looking time study in human infants, though it is primarily concerned with discrimination between possible and impossible objects rather than memory per se.