Amnesia and neglect: beyond the Delay–Brion system and the Hebb synapse

Abstract

Hippocampal damage in people causes impairments of episodic memory, but in rats it causes impairments of spatial learning. Experiments in macaque monkeys show that these two kinds of impairment are functionally similar to each other. After interruption of the Delay–Brion system (hippocampus, fornix, mamillary bodies and anterior thalamus) monkeys are impaired in scene–specific memory, where an event takes place against a background that is specific to that event. Scene–specific memory in the monkey corresponds to human episodic memory, which is the memory of a unique event set in a particular scene, as opposed to scene–independent human knowledge, which is abstracted from many different scenes. However, damage to the Delay–Brion system is not sufficient to explain all of the memory impairments that are seen in amnesic patients. To explain amnesia the specialized function of the hippocampus in scene memory needs to be considered alongside the other, qualitatively different functional specializations of other memory systems of the temporal lobe, including the perirhinal cortex and the amygdala. In all these specialized areas, however, including the hippocampus, there is no fundamental distinction between memory systems and perceptual systems. In explaining memory disorders in amnesia it is also important to consider them alongside the memory disorders of neglect patients. Neglect patients fail to represent in memory the side of the world that is contralateral to the current fixation point, in both short– and long–term memory retrieval. Neglect was produced experimentally by unilateral visual disconnection in the monkey, confirming the idea that visual memory retrieval is retinotopically organized; patients with unilateral medial temporal–lobe removals showed lateralized memory impairments for half–scenes in the visual hemifield contralateral to the removal. Thus, in scene–memory retrieval the Delay–Brion system contributes to the retrieval of visual memories into the retinotopically organized visual cortex. This scene memory interpretation of hippocampal function needs to be contrasted with the cognitive–map hypothesis. The cognitive–map model of hippocampal function shares some common assumptions with the Hebb–synapse model of association formation, and the Hebb–synapse model can be rejected on the basis of recent evidence that monkeys can form direct associations in memory between temporally discontiguous events. These advances in the understanding of memory mechanisms in the medial temporal lobe also suggest a possible cause of amnesia in Korsakoff patients who have intact temporal lobes; the memory impairments that are produced by aspiration lesions of the amygdala in the monkey are now known to be largely the consequence of damage to fibres of passage projecting from the hypothalamus to the cortex, and it is possible that damage to axons in the hypothalamus might underlie Korsakoff amnesia. Our general conclusion is that the primate brain encompasses widespread and powerful memory mechanisms which will continue to be poorly understood if theory and experimentation continue to concentrate too much, as they have in the past, on the hippocampus and the Hebb synapse.