Cerebral ischemia: are the memory deficits associated with hippocampal cell loss?

The long-standing notion that damage restricted to the hippocampal formation is sufficient to produce a significant global memory deficit derives from clinical data. Specifically, it is based on the observation that transient global ischemia, which leads to partial cell loss within the hippocampal formation but not in other brain areas important for memory, can produce global amnesia in humans. This view is, however, challenged by a number of experimental findings. First, in both monkeys and rats, there is evidence that ischemia disrupts delayed object recognition, a memory process found to be largely intact following selective hippocampal lesions. These findings indicate that damage confined to the hippocampal formation cannot account for all aspects of the ischemia-induced memory impairments. Second, although some groups of hippocampal neurons are the most prone to degeneration following ischemia, a wide array of extra-hippocampal damage has been observed in all species, for which the precise extent and distribution may well be underestimated by conventional histological evaluations of ischemic brains. Partial neuronal degeneration reported in regions such as the rhinal areas, medial dorsal thalamic nucleus, or cingulate cortex may contribute to varying degrees to ischemia-induced memory deficits. Third, experimental studies have failed to generate a general consensus on the correlation between extent of hippocampal cell loss and memory performance. In sum, the experimental studies do not, as yet, support the view that hippocampal damage is solely responsible for ischemia-induced memory deficits. Rather, they suggest that both the intra- and extra-hippocampal damage contribute to the pattern of memory impairments observed following ischemia. Consequently, although animals with global and focal ischemia represent valuable models for neuropathological and therapeutic studies, they may not be so useful in assessing the role of the hippocampal formation and its sub-components in memory processes.