A striking pattern of preservation of recent relative to remote autobiographical memories has been reported for patients with semantic dementia (Graham & Hodges, 1997; Snowden, Griffiths & Neary, 1996). This is the reverse of the classic “Ribot effect ” found in patients with amnesia following damage to the hippocampal complex. Graham and Hodges (1997) suggest that the profile for semantic dementia patients is, in fact, a steplike function rather than a gradient, with preserved memories for a period of about 1.5 years, and essentially equal impairment for all earlier memories. The apparent double dissociation between semantic dementia and amnesic patients is consistent with the “standard ” model of memory, in which the hippocampal complex plays a time-limited role in the acquisition and storage of memories, while long-term storage involves regions of the temporal neocortex (e.g. Squire, 1992). Within this framework, the relatively intact hippocampal structures of semantic dementia patients support good memory for recent events, while the loss of more remote memories can be attributed to atrophy of the temporal neocortex. The semantic dementia results provide a challenge to alternative accounts that do not include a time-limited role for th
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