Book Object Recognition. Chapter 3: The Object Recognition Task: A New Proposal for the Memory Performance Study

VALERIA CARLINI

Object Recognition

INTECH

Año: 2011; p. 27 - 41

At present, the studies about learning and memory and the factors that modulate these physiological processes constitute one central issue in neuroscience.  Most relevant experimental works concern with the possible role of neuropeptides on memory performance, focusing in explains into the neurobiological bases of learning and memory. In general, scientists believe that the answer to these questions relies in understanding how the information about a new event is acquired and included in neuronal terms, how this information is modulated and if it is possible to revert age-related or diseases associated cognitive to failures. Numerous tests have been used for studying memory; they differ in several ways other than just the type of information that must be remembered. Other differences include the nature of the motivation or reward, the reinforcement contingencies, and the amount of training required. The behaviors that are measured to assess memory also vary considerably and include conditioned reflexes (e.g., Pavlovian fear conditioning), speed or accuracy of spatial navigation (which can involve either swimming (water maze) or running (radial maze)), and object recognition (e.g., novel object recognition -NOR- or novel object preference -NOP-), being the last one a non spatial memory test. Recently, variants on this novel object recognition task has become widely employed in numerous neuroscience and behavioral pharmacological studies investigating memory processes. In relation to this test, Berlyne (1950) first demonstrated that animals when given equal access to novel or familiar objects spend more time exploring the novel object than the known one, thus displaying a preference for novel stimulation. Afterwards, in 1988, the object recognition test was introduced by Ennaceur and Delacour in order to assess the ability of rats for recognizing a novel object from a known object in a familiar environment. In this test, the interest of an animal in a novel object versus a familiar one, is measured and compared. Non-amnesic animals will spend more time exploring the novel object than the familiar one reflecting the use of learning and memory processes recognition. The absence of any difference in the exploration of the two objects can be interpreted as a memory deficit or, in case of testing an amnesic drug, the effect of this last one. The object recognition task includes two?trials, the first is an acquisition phase or sample phase, also called training phase, and the other one is known as testing phase. Each of them usually has a duration that varies between 2 to 5 minutes. In the training phase, in order to get familiarized with the objects, a rodent is placed in an enclosure and exposed for a set length of time to two identical objects that are located in a specified distance from each other. The animal is then removed from the environment, according to the memory type to assess, and a predetermined amount of time is allowed to pass. The rodent is then retested in the same environment except that one of the two previously used (familiar) objects is replaced with a novel one, that differs from the familiar object in shape, texture and appearance (e.g., a plastic block is replaced with a metal ball). In each phase, the time spent exploring each of the objects is quantified. Traditionally, exploration of an object is defined as time spent with the head oriented towards and within two centimeters of the object. There are four versions of this task, Novel object preference task, Object location task, Temporal order task and Object-in-place task. The first is the traditional task; in the second task, both objects in the test phase are equally familiar, but one is in a new location. The position of the moved object needs to be counterbalanced between animals; the third task comprise two sample phases and one test trial, the positions of the objects in the test and the objects used in sample phase 1 and sample phase 2 are counterbalanced between the animals. The Object-in-place task comprises a sample phase and a test phase separated by a 5 min. In the sample phase, the animals are presented with four different objects and then, in the test phase, two of the objects are repositioned. In recent years, the novelty-preference paradigm has become the most frequently chosen method for assessing object recognition in rats, mainly for practical reasons. The main advantage of this test is that, firstly, an animal can be tested repeatedly with new stimuli in the same session, thus permitting comparisons between subjects in different conditions; and, secondly, the animals do not require extended training. Another advantage is that the familiarization phase is identical for all three versions (with the exception that there are two familiarization phases on the context-memory task). Without a doubt, this test, which is frequently applied, could provide new and original insights about physiological processes of learning and memory.