Patients with probable Alzheimer's disease (AD) not only have great difficulty retrieving memories and information but also suffer from distortions of memory (Dalla Barba et al., 1995, Dalla Barba et al., 1999, Dalla Barba and Wong, 1995, Budson et al., 2000, Budson et al., 2002 and Balota et al., 1999). One of such memory distortions is intrusion. At a general level, intrusion can be defined as the unintentional recall of inappropriate information in a laboratory-learning task such as word-list recall and story recall (Dalla Barba and Wong, 1995). More specifically, following Dalla Barba et al. (2002), intrusion is operationally defined in this study as the production of a word or other story component that deviates from the original, to-be-remembered story.
Intrusion has been reported in a variety of pathological conditions including dementia (Butters, 1987, Dalla Barba et al., 1995, Dalla Barba and Wong, 1995 and Granholm and Butters, 1988), amnesia (Butters, 1987, Dalla Barba, 1993b, Dalla Barba and Wong, 1995 and Kopelman, 1987), depression (Dalla Barba and Wong, 1995 and Loewenstein et al., 1991), and also, on some occasions, in normal subjects (Butters, 1987, Dalla Barba, 1993b, Dalla Barba et al., 2002, Granholm and Butters, 1988 and Kopelman, 1987).
Fuld et al. (1982) have shown that disruption of cholinergic systems in normal subjects and in patients with AD plays an important role in the production of intrusions and concluded that intrusions were sufficiently characteristic of AD to be helpful diagnostically. However, Dalla Barba and Wong, 1995 and Dalla Barba et al., 1995 have shown that intrusions, although frequently observed in AD, are far from being specific to this disease since they can be equally observed in depression and in non-AD amnesias.
Dalla Barba and Wong (1995) found that AD patients made a high proportion of intrusions when attempting to retrieve a list of words not related to one another. When AD patients were asked to retrieve a list of semantically associated words, the production of intrusions was found to be associated with the presence of a semantic memory deficit, as defined by abnormal performance on tests that utilize semantic memory stores (e.g., the ability to generate words in a specified category over 1 min).
These findings were replicated in another study on AD patients and patients with depression (Dalla Barba et al., 1995).
In a more recent study, Dalla Barba et al. (2002) found that normal subjects produced significantly more intrusions in story recall when they were asked to perform a secondary task during the encoding of the to-be-remembered story as compared to when they were asked to perform a secondary task during the retrieval of the story. The authors concluded that encoding processes have a greater role in eliciting intrusions than retrieval processes. This idea has been formalised in Dalla Barba's and co-workers interpretation of confabulation (Dalla Barba et al., 1997, Dalla Barba et al., 1999, Dalla Barba et al., 2002 and Dalla Barba, 2000). They argued that confabulation in episodic memory is the result of a condition in which conscious retrieval processes are no longer able to operate a “fine grain” search in long-term memory. In this condition, events that do not have a strong representation in long-term memory cannot be retrieved. Only more stable representations in long-term memory are used, with the result that habits or semantic information are considered in a personal framework. In fact, there is clinical and experimental evidence that confabulating patients often mistake their habits or personal semantic information for specific unique episodes. For example, when asked what they did the previous day, confabulating patients often report as memories what they usually do in their daily life (Burgess and McNeil, 1999, Dalla Barba, 1993a, Dalla Barba, 2000, Dalla Barba et al., 1997 and Dalla Barba et al., 1999). Although admitted to the hospital, they may say that on the previous day they went to work or they went out shopping: acts that were part of their routine life.
In keeping with Dalla Barba and co-workers' previous studies, we suggest the hypothesis that in patients who produce intrusions, over-learned information interferes with episodic recall, i.e., the retrieval of specific, unique past episodes. We tested this hypothesis in AD patients using an experimental paradigm aimed to elicit intrusions. We asked AD patients and normal controls to recall different types of short stories: one unknown story, (similar to the Logical Memory test in the Wechsler Memory Scale-Revised (Wechsler, 1987), one well-known fairy-tale, (Cinderella), and one “modified” well-known fairy-tale (Little Red Riding Hood is not eaten by the wolf). According to the above hypothesis, patients should produce more intrusions for the modified well-known fairy-tale than for the other types of stories, because original, firmly established information concerning the fairy-tale interferes with the recall of its modified version.
