The hippocampus, space, and human amnesia

Hippocampal dysfunction during aging I: deficits in memory consolidation

Numerous ablation studies indicate a critical role for the hippocampal system in establishing or consolidating certain types of memory. Normal aging manifests by selective neurobiological changes in the hippocampal formation and on performance of tasks that require a functional hippocampus, including retention of contextual fear conditioning. To determine if impairments in the consolidation process contribute to memory dysfunction in aging, middle-aged and aged rats were fear conditioned and subsequently received dorsal hippocampal lesions or sham surgery after a 1, 7, 14, or 28-day interval. During retention tests, middle-aged rats exhibited a temporally graded retrograde amnesia of contextual fear conditioning, whereas aged rats manifested contextual memory impairments at all intervals. We postulate that the lack of consolidation in aged animals relates to previous findings of age-related changes in neuroanatomy and neurophysiological plasticity. The present findings suggest that impaired hippocampal consolidation contributes to age-related learning and memory deficits.

Equivalent impairment of spatial and nonspatial memory following damage to the human hippocampus

The hippocampus has sometimes been proposed to function as a cognitive map, a memory system that stores information about allocentric space. Work with experimental animals and memory-impaired patients has raised difficulties with this view by showing that the hippocampus is not performing an exclusively spatial function. However, the possibility has remained that the hippocampus plays a special role in spatial memory or a disproportionately large role in spatial memory compared to other kinds of memory. This study compared spatial and nonspatial memory in amnesic patients with lesions of the hippocampal formation or diencephalon. Subjects studied an array of 16 toy objects and were subsequently tested for object recall, object recognition, and memory for the location of the objects. Control subjects were tested after long retention intervals in order to equate their object memory performance with that of the patients. The main finding was that, when the performance of amnesic patients on the object memory tests was matched to the object memory performance of control subjects, spatial memory performance of the amnesic patients also matched the spatial memory performance of the control subjects. The results were the same for the two groups of patients. These findings suggest that the hippocampus is not especially involved in spatial memory. Spatial memory is simply one instance of a broader category of memory that requires the hippocampus. While cognitive mapping in its most abstract sense may describe hippocampal function, our results support alternative formulation, suggesting that the hippocampus is necessary for the rapid acquisition of relational, configural, or declarative (as opposed to purely spatial) information.

Memory disturbances following ibotenic acid injections in the nucleus basalis magnocellularis of the rat

The behavioral effects of lesions of the nucleus basalis magnocellularis (NBM) on two spatial discrimination tasks (place navigation and cross maze) were examined in the rat. These tasks were designed to test reference memory. Lesions by bilateral injection of ibotenic acid into the NBM led to a severe and permanent impairment in the learning of the cross maze task. In the learning of the place navigation task, the rats with lesions showed only a transient deficit. Immediately after the removal of the platform, the rats with lesions explored the quadrant (NE) previously containing the platform as long as controls and above chance levels. The rats with lesions did not extinguish exploration like the controls, seen as a reduction both in time spent in the NE quadrant and in swimming activity. Taken together, the results showed that (1) NBM lesions impair reference memory, but (2) spare other aspects of memory. On the basis of the results in the place navigation task, procedural memory was assumed to remain intact after lesion of the NBM. Biochemical assays of choline acetyltransferase (ChAT) in various brain regions in the lesioned animals demonstrated a reduced ChAT activity in the neocortical projections of the NBM but not in the hippocampus. However, it cannot be decided from this work whether behavioral deficits result from the lesion of cholinergic or of non-cholinergic cells in the NBM.