Fimbria-Fornix lesions impair spatial working memory but not cognitive mapping.

Abstract

Three experiments were designed to evaluate the relative merits of two theories of hippocampal function, the cognitive mapping theory and the working memory theory. Rats were tested in a series of maze tasks that varied in memory requirements. In the experiments that required cognitive mapping but not working memory (Experiments 1 and 3), rats with fimbria-fornix lesions reached stable levels of performance that were as accurate as those of control rats, and they also performed accurately during transfer tests, results demonstrating that they used a cognitive mapping strategy to solve the discrimination. In the experiment that required working memory (Experiment 2), rats with fimbria-fornix lesions performed at chance levels during all of postoperative testing, and they distributed their choices randomly between the correct and the incorrect goals. These results are seen as generally refuting the predictions made by the cognitive mapping theory and supporting the predictions made by the working memory theory. Additional analyses (Experiment 3) examined the nature of a transitory impairment exhibited by rats with fimbria-fornix lesions in two tasks that did not require working memory. Postoperatively, the performance of the rats with lesions was initially impaired but recovered to normal levels with a time course similar to that seen during preoperative acquisition. These results are seen as requiring an extension of the working memory theory of hippocampal function, and discussion is focused on the possibility of a temporary retrograde amnesia following fimbria-fornix lesions and the distinctions between different types of memory abilities.

Differential effects of fornix and caudate nucleus lesions on two radial maze tasks: evidence for multiple memory systems

“…1981: Walker andOlton, 1984). The identification of the caudate nucleus as a structure mediating at least some of the learning that is spared following hippocampal damage allows for experiments that can assess these theories by manipulating both substrates.…”

NMDA receptor antagonists decrease GABA outflow from the septum and increase acetylcholine outflow from the hippocampus: a microdialysis study

“…Either the doses of DNQX used lacked of specificity toward non-NMDA receptors or in the awake animal summation of EPSPs and IPSPs (due to recurrent collaterals from the cholinergic neurons) produced no net change in hippocampal ACh outflow. The central cholinergic system plays a fundamental role in memory formation (Bartus et al, 1982) and in particular the septal cholinergic neurons projecting to the hippocampus are involved in the generation of the working memory (Olton et al, 1979;Gray and McNaughton, 1983;Walker and Olton, 1984). In addition, the NMDA receptor system of the hippocampus has been implicated in the induction of long-term potentiation (LTP) (for review, see Collingridge and Bliss, 1987) and NMDA antagonists that block hippocampal LTP disrupt some forms of learning and memory (Morris et al, 1986;Morris, 1989;Davis et al, 1992).…”

Working memory in young rats with lesions to the “general learning system”

“…This was shown not only by the relatively normal performance ofGroups PALL, NIGRA, and COLL on the working-memory test, but by the trivial relationship between working-memory scores and leaming scores on the detour and visual-discrimination tasks (the correlation coefficients ranged from .27 to .32). This outcome is not altogether surprising when considering the fact that the dorsal hippocampus is centrally important in working memory in rats (Olton, 1983;Olton et al, 1979;Walker & Olton, 1984), but may be of little importance in the acquisition of certain sensory-discrimination habits (O'Keefe & Nadel, 1978;Thompson et al, 1986) and problem-box tasks (Thompson et al, 1989b).…”

“…One session of four such trials, in which the position of the correct (an incorrect) arm varied from right to left in a prearranged "random" order, was given each day for 20 days. This task would tap working memory to the extent that the correct arm is changed from one forced TUn to the next, requiring the animal to remember during the choice TUn wh ich arms were associated with footshock and safety during the preceding forced TUn (see Walker & Olton, 1984). A pilot study revealed that normal rats perform on this task at about the 80%-85% level of accuracy.…”

“…For comparative purposes, rats with early lesions to the dorsal hippocampus were also investigated. Although rats with these limbic forebrain lesions have not been found to manifest a generalized leaming impairment (Thompson et al , 1986), they have been reported to be deficient on workingmemory tasks (Olton, 1983;Olton, Becker, & Handelmann, 1979;Walker & Olton, 1984). Thus, the inclusion of this group would provide additional data on the extent to which a working-memory deficit predicts leaming impairments on other kinds of tasks.…”