Testing Context Memory using a Variant of the Novelty Preference Test
This version assesses whether a rat remembers that a particular object was encountered in a particular context. Rats are familiarized with two different objects, each for 5 minutes, and each in a unique context (different open fields); exploratory preference is later measured on a test where both objects are presented in one of the two contexts. Normal rats spend more time exploring the object that is now in a context different from that during familiarization. In the Mumby et al., 2002 experiment, the rats with HPC lesions were impaired on the context trials; however, we did not know whether this impairment was due to an inability to remember the contexts, or was it because they did not remember which object appeared in which context. In order to address this question, we performed experiments using a context-shift type of novelty-preference test.
Typically, normal rats show a weakened preference for the novel object when the sample and test phases are conducted in different contexts, indicating that retrieval of contextual cues can facilitate recognition. We hypothesized that if rats with pretraining HPC lesions are unable to remember the context in which they’re exposed to during the learning phase, their performance should not be impaired when the context is different for the test phase. Our results showed that the control rats and HPC rats performed normally when the context remained the same for both the learning and test phase. However, when learning and testing occurred in two different contexts, rats with HPC lesions were impaired while control rats performed normally (O'Brien et al., 2006). The results suggested that rats with HPC lesions must be incorporating the context during learning considering they were impaired when the context was changed for the test phase. We hypothesized that the hippocampus must play an important role in recognizing previously learned objects appearing in subsequent contexts.
We recently conducted experiments similar to this one but using a variant of the open field apparatus, the circular track apparatus. In one experiment, rats were familiarized to objects in one circular track and the test phase was either conducted in the same circular track or a second circular track located in a different room. In this experiment, we found similar results to the O’Brien et al. experiment. In a second experiment, we wanted to determine how novel-object preference is affected by changing the local cues that surround the objects for the test phase. In order to test this, we used one circular track in one room. The compartments of the circular track were contextually distinct (local cues) while the context of the room (global cues) remained the same. On the test phase, half of the objects were moved to a different compartment and the other half remained in their respective compartment. The results showed that when the objects remained in the same compartment for the test phase, rats with HPC lesions and control rats were not impaired on the NOP test. However, when objects were moved to a different compartment for the test phase, rats with HPC lesions were not impaired while control rats were (Piterkin et al., 2008). The phenomena of response habituation and dishabituation may explain why the control rats failed to display a novel-object preference when the sample object was moved to a new place and paired with a novel object. When the sample object was encountered in a different place, the exploratory response that it evoked was dishabituated and the rat therefore explored the sample more than it would have if it hadn’t moved. Thus, the lack of novel-object preference when the object was moved to a different compartment shows that control rats were able to detect that the sample object changed location. According to this interpretation, the rats with HPC lesions showed a novel object preference in the different compartments because they could not remember where the sample object had been previously.
The findings from these experiments are consistent with the view that HPC damage does not cause a general inability to recognize objects, or an inability to encode or store representations of the environment in which these objects are encountered. The results suggest that HPC damage impairs the ability to remember the specific locations of familiar objects within a particular context.