As a result of this transformation, the “gist” of the episode is retained, and the factual aspects are extrapolated into the semantic system, with the unique features of the individual events gradually lost over time. The question still remains however: “ how can patients with DA acquire semantic information in the presence of hippocampal damage?” Theoretical accounts of semantic memory formation begin with the memory of a unique experience (the memory of which is dependent on the hippocampus) and these experienced memories subsequently become consolidated and stored in the cortex over time, , see also. The dissociation between episodic and semantic memory described above is now reported in both large group studies of patients with DA, and several single case studies, ,, ,,. In this way, patients with DA appear to use their knowledge of the world to construct a general representation of events in the absence of the ability to reconstruct the specific experiential aspects of the episodes. Of note, Jon and other patients with DA, appear to use their good semantic memory to produce a reasonable response to questions relating to specific episodes, thus giving the impression that they do not have a memory problem at all. This is all the more impressive considering that their dictionary of factual world knowledge is gradually amassed in the presence of early hippocampal damage, long before any overt signs of memory ability has emerged. Consequently, Jon, and other patients with this developmental form of amnesia, are able to acquire a remarkable repository of semantic information about the world such that they are up to date with current affairs, major news items, and new discoveries, etc. As a consequence of his hippocampal damage, Jon has a severe deficit in his episodic memory abilities, but given the apparent integrity of his parahippocampal cortex, the putative neural substrate for processing non-contextual information, his semantic memory system is relatively intact. “I climbed the stairs today because the lift was out of order”. “I always take the lift”) are supported by a different set of brain regions than memory for episodes that are bound in a specific spatial-temporal context (e.g. Theoretical models of human memory posit that the semantic memory system and the episodic memory system are dissociable (see Squire and Zola for an alternative view), such that memory for non-contextual facts (e.g. Why is Jon so confident that he always takes the lift, when he has no episodic memory of doing so? If Jon has no recollection of his life events as they occur, how does he learn what he typically does? Jon was questioned about his memory of this event “How do you know that you took the lift today?”. When he arrived at the laboratory, he had no recollection of having climbed the stairs, and confidently reported that he had taken the lift as normal. On one such visit, the lift at the underground station was out of order, and Jon had to climb the 171 steps to the surface (the equivalent of some 14 floors).
To do so, he travels to an underground train station nearby, then takes the lift to the street level and walks the remainder of the journey. Jon frequently visits our laboratory in London. Throughout his childhood and adult life, he has had difficulty remembering episodes from his past. Jon sustained severe bilateral hippocampal damage as a result of hypoxic-ischaemic events that occurred when he was a neonate. This is illustrated with an anecdote from patient Jon, a well-documented case of developmental amnesia (DA. A conundrum exists in the neurodevelopmental literature where patients with early hippocampal damage exhibit profound amnesia for their life events, but are able to form semantic memories which generalise across those same events.
0 kommentar(er)
