Preserved anterograde and retrograde memory of rapidly acquired olfactory discrminations after neurotoxic hippocampal lesions.
ABSTRACT A forced-choice discrimination paradigm was used in two experiments, to evaluate retrograde and anterograde amnesia in rats after hippocampal ablation. In a within-subjects design (Experiment 1), rats were trained on a set of 10 olfactory discriminations 4 weeks before surgery and on a separate set of 10 discriminations 1 week before surgery. In a mixed design (Experiment 2), rats were trained on olfactory discriminations in one of three conditions: condition 1 (10 discriminations at 4 weeks before surgery); condition 2 (10 discriminations at 1 week before surgery); or condition 3 (10 discriminations at 4 weeks before surgery and 10 discriminations at 1 week before surgery). Discriminations in both experiments were rapidly learned, requiring 7-10 trials to reach criterion. After training, half of the rats in each condition received bilateral neurotoxic lesions of the hippocampus, and the other half received sham surgery. One week after surgery, all rats were given a retention test, consisting of a single critical trial for each discrimination. In both experiments, rats with selective hippocampal lesions exhibited preserved retention of these olfactory discriminations with no observable retention gradient. A postoperative acquisition test for two new discriminations indicated that anterograde memory was also preserved, while a postoperative test of spatial learning in the Morris water maze confirmed that the hippocampal lesions impaired spatial learning. Together, these experiments refute the contention that the hippocampus is requisite for (non-spatial) olfactory memory consolidation, storage, or access, despite the condition that the information be rapidly acquired.
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ABSTRACT: Several recent studies have shown a flat retrograde amnesia for spatial information following lesions to the hippocampus in rats and mice. However, the results of the present investigation demonstrate that in rats that presurgically learned a spatial reference memory task based on extramaze cues, a temporally graded retrograde amnesia is evident following lesions to the hippocampus (1, 16, 32 or 64 days after learning) if two conditions are met. First, that a wide range of retention intervals is used, and second, that independent groups of rats are tested, not a single group that learns different spatial discrimination tasks at different times (expt 1). The results of expt 2 show that the hippocampus does not serve as a consolidating mechanism when the spatial task learned presurgically is based on intramaze cues. Taken together, these results indicate that the hippocampus is critical for the storage and/or retrieval of spatial reference information that was learned up to 1 month before hippocampus damage; however, in the absence of the hippocampus, efficient retention can still occur provided that the spatial knowledge was learned in a simple associative manner.European Journal of Neuroscience 11/1998; 10(10):3295-301. · 3.75 Impact Factor
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ABSTRACT: It is widely believed that new memories are stored in the medial temporal lobe structures in the short term, but then are reorganized over time as the neocortex gradually comes to support stable long-term storage. On this view, the medial temporal lobe structures play a time-limited role in information storage. This putative process of reorganization, known as consolidation, is supported by some clinical findings in humans and by some data from nonhuman animals. Here we review prospective studies of retrograde memory in nonhuman animals, with particular emphasis on experimental design. In considering the evidence for a time-limited role for the medial temporal lobe in information storage, we note that there are alternative interpretations for at least some of the findings typically cited in support of the consolidation process. In addition, we suggest that some studies arguing against the consolidation view should probably be given more weight than they have so far received. Finally, we observe that different structures in the medial temporal lobe are unlikely to operate together as a single functional unit mediating a single consolidation process. Although evidence for a time-limited role for medial temporal lobe structures in memory is at present equivocal, future studies that consider some of the alternative accounts we and others have identified will provide a clearer picture of the mechanisms underlying information storage and retrieval in the brain.Hippocampus 02/2001; 11(1):1-7. · 5.49 Impact Factor
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ABSTRACT: A test of socially acquired food preferences was used to study the effects of large lesions to the hippocampal formation (HPC) on anterograde and retrograde memory in rats. In the anterograde test, rats with HPC lesions normally acquired the food preference but showed a faster rate of forgetting than control groups. When the food preference was acquired preoperatively, HPC groups exhibited a temporally graded retrograde amnesia in which memory was impaired when the preference was acquired within 2 days of surgery but not at longer delays. The results support the traditional theory that the HPC contributes to the consolidation of newly acquired information into a durable memory trace that is represented in other brain areas. Consistent with this view, the results indicate that, once a memory trace is consolidated, the HPC does not participate in its storage or retrieval. The possibility is considered that extrahippocampal areas in the medial temporal lobe are needed to maintain a memory trace throughout its existence.Hippocampus 02/2001; 11(1):18-26. · 5.49 Impact Factor