Hippocampal damage and exploratory preferences in rats: memory for objects, places, and contexts

Abstract

Rats have a natural tendency to spend more time exploring novel objects than familiar objects, and this preference can be used as an index of object recognition. Rats also show an exploratory preference for objects in locations where they have not previously encountered objects (an index of place memory) and for familiar objects in contexts different from those in which the objects were originally encountered (an index of context memory). In this experiment, rats with cytotoxic lesions of the hippocampal formation were tested on all three versions of the novelty-preference paradigm, with a 5-min retention interval between the familiarization and test phases. Rats with sham lesions displayed a novelty preference on all three trial types, whereas the rats with hippocampal lesions displayed a novelty preference on Object trials but did not discriminate between the objects on Place trials or Context trials. The findings indicate that hippocampal damage impairs memory for contextual or spatial aspects of an experience, whereas memory for objects that were part of the same experience are left relatively intact.

Figure 1

Illustration of the smallest (black) and largest (grey) hippocampal lesions observed at three coronal planes (distance in millimeters from bregma). Cross-hatching represents cortical damage.

Figure 2

Time engaged in object exploration during the familiarization and test phases for each trial type. For Context trials, data are shown separately for the first (1) and second (2) familiarization phases. Error bars represent SEM.

Figure 3

Mean exploration ratios during the retention test for each trial type. The ratio represents the proportion of object-exploration time that was spent exploring the novel object; t_novel/(t_novel + t_sample). The dashed line represents the chance level of performance (i.e., a ratio of 0.50; no discrimination between sample and target objects). Data are shown separately for the first, second, and third minute of the test and for all 3 min of the test combined. Asterisks denote mean ratios that are significantly above chance (one-sample t tests, p < .05). Error bars represent SEM.

Figure 4

Top shows the relative dimensions of the open-field arenas and the positions of the stimulus objects. Objects were positioned at the standard locations (S) on Object trials and Context trials and for the familiarization phase of Place trials. One object was moved to a novel location (n) for the retention test on Place trials. Schematics illustrate the general procedure for Object, Place, and Context trials.