Anterograde Object Recognition after Long Retention Intervals
Fornix lesions have negligible effects on rats' performance on the NOP test, and we have found that cytotoxic (NMDA) lesions of the HPC also fail to disrupt performance with retention delays as long as 24 hours. Others proposed that HPC lesions would impair object recognition after longer delays, but at first, this hypothesis could not be tested, because there had to be a way to demonstrate that normal rats could recognize objects after delays of days or weeks. Neither the DNMS task, nor the standard version of the novelty-preference test can accomplish this. Our solution was to give rats multiple, distributed exposures to the sample object (5 minutes per day on 5 consecutive days) and then test them on their novel object preference. With this modification, normal rats can recognize a sample object on the novelty preference test after delays of up to 7 weeks. We have used this procedure to examine the effects of lesions to the HPC and PeRh on anterograde object recognition after long retention delays, lasting days or weeks.
In one experiment, rats with HPC lesions and control rats were familiarized with a sample object in an open field for 5 minutes a day for 5 consecutive days. Following a 24 h, 1 week or 3 week retention interval, rats were placed back in the apparatus for the retention test. The results showed that the HPC rats’ performance across all three delay conditions was not significantly different from that of control rats (Mumby et al., 2005). These findings suggest that HPC damage spares anterograde object recognition in rats, even after retention intervals lasting weeks.
We also assessed the effects of PeRh lesions on anterograde object recognition using the novel-object preference test, with retention intervals lasting 24 h and 3 weeks. The rats were familiarized with a sample object during the learning phase—5 min per day on 5 consecutive days. Control rats showed a significant novel-object preference after both retention intervals, whereas the rats with PeRh lesions displayed a significant preference only after the 24 hour interval. When the learning phase trial was shortened to a single 5 minute session, the PeRh group was impaired in the 24 h condition (Mumby et al., 2007). The results suggested that the disruptive effects of PeRh damage on anterograde object recognition persist over very long postlearning intervals. Also, the results suggest that object recognition impairments following PeRh damage may not be ubiquitous, and that learning conditions play a significant role in determining the subsequent recognition performance in rats with PeRh damage. In addition to telling us about the contributions of the HPC and PeRh to object recognition, these experiments have yielded information about the temporal limits of object-recognition in normal rats.
We have made significant progress in dissociating the contributions of HPC and PeRh to object- and place-memory under a variety of conditions. In several experiments, we found no evidence that damage to the PeRh impairs learning of a water-maze task, whether working-memory or reference-memory procedures are used (Glenn and Mumby, 1998; Mumby and Glenn, 2000). In contrast, PeRh lesions impair anterograde object-recognition, as assessed by the DNMS task (Mumby and Pinel, 1994), and we have reported that PeRh lesions impair anterograde object recognition on the novelty-preference test when only one familiarization session is provided (Mumby et al., 2002).