Extending the spontaneous preference test of recognition: evidence of object-location and object-context recognition

Abstract

The natural preference for novel objects which is displayed by rats has been used as a behavioural index to test object recognition. In this series of experiments the standard spontaneous recognition task was extended to look at other types of recognition memory; memory for place (recognition that an object is in a location where previously there had been no object), memory for object in place (recognition that a specific object has changed position with another object) and memory for context (recognition that a familiar object is in a context different to that in which it was previously encountered). We also included a standard test of object recognition in which successful discrimination relied primarily on visual cues. In addition, we looked at how the differential exploration of objects varied within the 3 min of the test phase. The results showed that rats were sensitive to the changes made in all of the test conditions and that the level of discrimination varied within the 3 min test phase. In the standard condition and the context condition, the first 2 min were found to be the most sensitive period. In the two conditions involving a position change, discrimination was only evident in the first minute.

Introduction

The behavioural assessment of object recognition memory in rats has relied heavily on variants of the delayed matching to sample (DMTS) and delayed non-matching to sample (DNMTS) tasks 1, 13, 19, 21. The main advantage of these classes of task is that an animal can be tested repeatedly with new stimuli in the same session, so permitting within subjects comparisons over different conditions. The main disadvantage of this form of task it that the animals must initially acquire the matching or non-matching rule. This often involves a lengthy training procedure, and there is the possibility that animals may subsequently appear to be impaired because of a difficulty with applying the rule rather that with recognition per se. The lengthy training procedure also creates the possibility that rats may acquire other, unwanted strategies to aid task performance [11].

In response to these problems Ennaceur and Delacour [5]introduced a pair of tasks that take advantage of the spontaneous preference that rats display for novel objects. The standard form of this task [5]involves exposing a rat to two identical copies of an object (sample phase), typically for 3 min. After a delay the rat is then exposed to a novel object and an identical copy of the now familiar object (test phase). Successful recognition is displayed by the rat spending a greater amount of time exploring the novel object during the test period. As this task takes advantage of a spontaneous preference it requires no rule learning and, hence, very little pretraining. The fact that it is not necessary to teach the rats a rule reduces one possible confounding factor, i.e. the ability of the animal to learn or execute the performance rule, compared to other tests of working memory 12, 14such as DMTS or DNMTS tasks. Another advantage is that there is no need for food reinforcement, and this helps to remove other factors that could confound DMTS/DNMTS performance e.g. a change in food palatability. A final possible advantage of this task stems from the proposal that spontaneous tests of recognition provide a closer analogy with tests of recognition given to human subjects 3, 18as such tasks avoid the effects of a lengthy pretraining component.

The primary purpose of the present study was to determine if the standard tasks could be extended to measure other aspects of recognition. Thus, in addition to examining the exploration of a novel object i.e. object recognition [5]and the exploration of an object occupying a novel position i.e. location recognition [7]we tested two further conditions. In the first of these the rats were pre-exposed to two different objects to ensure that they both became familiar. One of these objects was always in a particular distinctive context while the other was in a different, distinctive context. The test session used both objects but only one of the two contexts. The goal was to determine if the rats were sensitive to the change in context that had occurred for one of the familiar objects i.e. did they show preferential exploration of the item in the changed context. In the second new condition rats were exposed to four different objects during the sample phase, each placed in a corner of the test box. In the test phase the same objects were used but two had now switched places. In this way we sought to determine if rats were sensitive to the combination of identity and location i.e. which items were in which locations.

As part of this analysis we also examined the pattern of differential exploration within each test session in order to derive the most reliable measures of object recognition. For this, we compared preference performance during the first, second and third minutes of the test phase of the recognition test. The rationale for this arose from the realisation that a novel object that is extensively explored during the initial part of the recognition test may rapidly become familiar. As a consequence, differential exploration of the two objects is likely to decline or even reverse as the test proceeds. This, in turn, suggests that the standard 3 min test period 5, 6, 7might not correspond to the optimal duration. This possibility was examined across a series of different test conditions. These included two standard tests of object recognition, but in one of them the objects to be discriminated were assembled from exactly the same construction material (Duplo, Lego). This ensured that the test objects would not provide different olfactory or immediate tactual cues as the objects had the same surface texture (although their shapes did differ) i.e. the discrimination relied primarily on visual cues.