Mnemonic and neuropathological effects of occluding the posterior cerebral artery in Macaca mulatta

To investigate experimentally the mnemonic and neuropathological effects of blockage of the posterior cerebral arteries (PCA), a cerebrovascular accident that can lead to global anterograde amnesia in humans, we permanently occluded these arteries bilaterally in six monkeys and then evaluated their performance on a visual recognition task, after which we assessed the extent of their ischemic infarcts. The latter showed substantial individual variation, ranging from almost no damage in one case to massive unilateral injury of both the ventromedial o occipitotemporal cortex and hippocampal formation in another. In the four remaining cases, however, the infarcts fell within a narrow range, being confined almost entirely to the hippocampal formation and parahippocampal gyrus, and then only to restricted portions of these structures, unilaterally in one case, and bilaterally in the three others. Performance on the recognition task was related to the presence and bilaterality of the hippocampal injury. Thus, the case without any hippocampal damage performed at a rate equal to that of normal controls; the case with unilateral hippocampal damage was mildly impaired; and the three cases with bilateral infarctions, involving between 20 and 55% of the hippocampal formation, showed substantial impairment, with scores averaging 20% below those of normal controls. The only subfields of the hippocampus damaged in common in these cases were CA1 and CA2. Paradoxically, the memory loss found in these three animals with only partial bilateral hippocampal damage was significantly greater than that found in animals with total bilateral ablation of the hippocampal formation, whose scores averaged only 10% below those of normal controls. Possible explanations for this extremely puzzling outcome are proposed.