The Novel Object Recognition Test in Rodents: Which Are the Essential Methodological Aspects?

The Novel Object Recognition test (NOR) has been introduced by Ennanceur and Delacour in 1988 [1] and can be regarded as a spontaneous Delayed-Non-Matching-to-Sample (DNMS ) [2] test. The test is based on a spontaneous behaviour: the main assumption at the base of this test is that access to novelty (e.g. an object or an environment) can elicit approach behaviours in animals. This apparent ‘unconditioned preference’ for novelty has been used in the NOR test in order to study memory functions, assessing the ability of animals to recognize a novel object in a familiar environment, because they maintain a representation of those is more familiar stored in memory The Novel Object Recognition (NOR) test has been used in different variants, but typically consists in two trials. In the first trial (T1) the animal is exposed to one or two identical objects (sample object). Following the sample object exposure, the animal is returned to his home cage for a retention period. In the second trial (T2), which follows the retention time, the animal is returned to the environment (arena) and presented with a familiar (sample) and a novel object. When the subject ‘remembers’ the previous exposure to the familiar object, it will explore the novel object to a greater degree than that of the familiar one. NOR test doesn’t involve reference memory components (e.g. explicit rule learning), thus it can be considered a “pure” recognition memory test and a valid task to assess working memory. Finally the test doesn’t involve positive or negative reinforces (e.g. food, electric shocks) and this makes NOR comparable to memory tests currently used in humans. All these advantages make NOR test quick and simple to be implemented and, therefore, it has been widely used for assessing mild cognitive impairment in pre-clinical research.