In Chapter 2 we assessed episodic memory in patients with SA to test whether deficits were parallel across semantic and episodic tasks. SA patients have difficulty retrieving links between concepts (e.g. CAMEL – CACTUS) in the presence of alternative associations that are not relevant for the task at hand (e.g. SUNFLOWER, TREE, ROSE). In such circumstances, control demands are high because the required link between concepts is not dominant in the representational system and irrelevant associations can cause interference. In this chapter, we extended this work to verbal paired associate tasks tapping episodic memory. After learning lists of item-pairs (e.g. SUN – PHOTO), participants were asked to retrieve the second member of the pair when probed with the first (e.g. ‘What was SUN paired with?’). This task requires a novel link between items to be recovered in the face of potential interference from both existing knowledge (i.e. SUN – CLOUD,eliciting false memories), and irrelevant episodic associations trained shortly before the critical judgments (giving rise to proactive interference). Previous work in the semantic domain has shown that when control demands are high, cues can increase performance in SA patients to near-normal levels, presumably because these cues reduce the need for internal constraint over retrieval (Jefferies and Lambon Ralph, 2006; Jefferies et al., 2008b; Soni et al., 2009; Noonan et al., 2010). Here we tested if cues (probing the initial phonemes of the target, e.g. S…)
were also helpful in episodic memory. In addition, we examined whether interference was promoted not only by semantic links in episodic tasks, but also by episodic memories (i.e. newly learnt associations) in semantic tasks. If LIFG is responsible for resolving competition between memories both within and between these representational systems, difficulties should be found in both instances. Finally, since SA patients’ semantic difficulties are multimodal (i.e. extend across both verbal and pictorial stimuli; Corbett, Jefferies and Lambon Ralph, 2009), we expected to find episodic impairment using both words and pictures.
We first tested sensitivity to cues using the verbal paired associate recall task from the Wechsler Memory Scale. Accuracy was impaired relative to controls’ and patients’ erroneous responses were often words semantically related to the probe (e.g. they responded HEAVEN -instead of LADDER,when probed with STAR), but when phonological cues were provided patients’ performance reached controls’,
showing initial evidence of episodic deficits when control demands are high. Despite this impairment, face recognition from the Wechsler Memory Scale was intact, showing that the SA cases were not amnesic. Since not all the patients could attempt cued recall, due to poor language production, we further assessed interference using a recognition version of the verbal paired-associates task, in which both semantic and episodic associations were available. In Experiments 1 and 3, patients were trained on unrelated and related (e.g. SUN – PHOTO or HOTEL – GUEST) word and picture pairs respectively. The
patients’ tendency to select semantic distractors (e.g. CLOUD or BEDROOM) during retrieval was measured. In the verbal version (Experiment 1), patients found it more difficult to learn new associations
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unsupported by existing knowledge (i.e. they had significantly lower performance for unrelated pairs), showed intrusion of semantic knowledge (i.e. they chose semantic distractors – e.g. CLOUD –
significantly more often than controls) and when their response was related to the probe they had little insight about their accuracy. In verbal paired associates when existing knowledge was not supportive at encoding (i.e., all word pairs were unrelated; Experiment 2), performance was still impaired, suggesting that individuals with SA have difficulty forming and retrieving novel links between semantically-unrelated items, even when interference from long-term semantic knowledge was minimised (i.e. semantic distractors were not available). Together these results show that episodic memory benefits from coherence with existing knowledge in SA but retrieval is also prone to false memories when irrelevant yet dominant semantic distractors are presented alongside the target.
The design of Experiments 1, 2 and 3 also tested interference from episodic memories. These experiments followed an AB/AC structure (e.g. 1st list:
SUN – PHOTO, 2nd list:
SUN – STEEPLE), designed to elicit proactive interference. Patients showed an abnormal rate of interference from previously learnt episodic associations (in Experiment 1 and 2 but not in Experiment 3). Finally, to examine interference from episodic memories during semantic decisions, a novel task was designed (Experiment 4). Participants were trained on associations between unrelated words (such as TEA-MONEY) and, after showing evidence of consolidated learning, performed a semantic association task where they had to retrieve links between concepts (such as TEA - LEAVES). Critically, in half of the trials, one of the options was the episodic trained distractor (i.e. MONEY) – and patients but not controls showed impaired performance when episodic associations where available. The number of semantic errors made in Experiment 1 (i.e. in an episodic task) and episodic errors in Experiment 4 (i.e. in a semantic task) positively strongly correlated, as did accuracy for semantic and episodic control tasks. Finally, patients’ accuracy was equally impaired using words and pictures (Experiment 1 and 3), suggesting that control processes may interact with heteromodal memory representations also in the episodic domain.
To summarize, SA patients presented a) deficits beyond semantic cognition when retrieval demands were high (e.g. without cues), b) difficulty suppressing irrelevant semantic knowledge but also interference from recently learnt material during both semantic and episodic tasks and c) multimodal episodic and semantic retrieval deficits. Collectively, these results show evidence of parallel deficits across memory domains. The evidence is in line with an equivalent role of LIFG in resolving competition between memory representations that are not only verbal in nature. However, one potential difference between modalities was observed: in the non-verbal task, there was no evidence of an increased rate of semantic false memories or proactive interference, although overall accuracy was still lower for patients than for controls. Picture stimuli are less abstract and more distinctive and rich than words; in addition, the hippocampus – which is intact in the patients – has strong coupling with visual- spatial regions. This might have helped patients discard novel semantically-related stimuli (i.e. semantic false memories) and items presented in previous trials (i.e. proactive interference). In Chapter 3, we further examined episodic memory using mostly non-verbal tasks. We used source memory tasks
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requiring decisions between two alternative options that remained constant across trials (preventing participants to take advantage from perceptual novelty to discard the competing option). This also allowed to identify circumstances when patients failed to resolve competition using non-verbal material.