What influences the accessibility of conceptual knowledge? Evidence from Experimental Psychology, Neuropsychology and Brain Stimulation

Previous studies have shown that accessibility of conceptual information declines when sets of semantically-related items are presented repeatedly, although the underlying basis of this effect is debated – it is unclear if comprehension can decline without massed repetition of individual items, or if this effect is restricted to lexical retrieval in picture naming. Furthermore, declining comprehension has been characterised as arising from both ‘too much activation’ (i.e., on-going strong activation of competitors) and ‘too much inhibition’ (i.e., a failure to overcome inhibition which may facilitate the earlier retrieval of semantically-related targets). The thesis explored the impact of experimental manipulations (speed of presentation; strength of association between category and target item; modality of presentation; type of semantic decision required), on the magnitude of declining comprehension in healthy young adults. Comprehension declined even without individual item repetition, especially for strongly-associated targets (which may have accrued more competition or inhibition). The effect was found irrespective of presentation modality and more strongly at fast presentation speeds (when there was less time to overcome competition/inhibition). Next, the thesis examined the impact of ageing and semantic aphasia on changes in comprehension within the continuous categorisation paradigm. In these populations, controlled retrieval of conceptual information is thought to be weakened (relative to younger adults and healthy controls without aphasia). This should exaggerate declines in comprehension that reflect difficulty overcoming competition, but reduce the effect if it arises from the inhibition of competitors on earlier trials. The results were in line with the second hypothesis, since older adults and patients with semantic aphasia maintained their performance throughout the categories, unlike younger adults. Lastly, the thesis examined how this effect is modulated by transcranial electrical stimulation delivered to a key brain region implicated in semantic control – left inferior frontal gyrus (LIFG). Stimulation of LIFG attenuated the effect of declining comprehension, perhaps because initial retrieval was facilitated (potentially reducing the inhibition of related information), and/or because subsequent target selection was strengthened. Together, these results provide a more comprehensive account of what drives declining performance in continuous categorisation in healthy young adults who have the capacity to strongly engage semantic control

Intrinsic connectivity of left ventrolateral prefrontal cortex predicts individual differences in controlled semantic retrieval

Control processes allow us to constrain the retrieval of semantic information from long-term memory so that it is appropriate for the task or context. Control demands are influenced by the strength of the target information itself and by the circumstances in which it is retrieved, with more control needed when relatively weak aspects of knowledge are required and after the sustained retrieval of related concepts. To investigate the neurocognitive basis of individual differences in these aspects of semantic control, we used resting-state fMRI to characterise the intrinsic connectivity of left ventrolateral prefrontal cortex (VLPFC), implicated in controlled retrieval, and examined associations on a paced serial semantic task, in which participants were asked to detect category members among distractors. This task manipulated both the strength of target associations and the requirement to sustain retrieval within a narrow semantic category over time. We found that individuals with stronger connectivity between VLPFC and medial prefrontal cortex within the default mode network (DMN) showed better retrieval of strong associations (which are thought to be recalled more automatically). Stronger connectivity between the same VLPFC seed and another DMN region in medial parietal cortex was associated with larger declines in retrieval over the course of the category. In contrast, participants with stronger connectivity between VLPFC and cognitive control regions within the ventral attention network (VAN) had better controlled retrieval of weak associations and were better able to sustain their comprehension throughout the category. These effects overlapped in left insular cortex within the VAN, indicating that a common pattern of connectivity is associated with different aspects of controlled semantic retrieval induced by both the structure of long-term knowledge and the sustained retrieval of related information

When comprehension elicits incomprehension: Deterioration of semantic categorisation in the absence of stimulus repetition

Repetition improves retrieval from memory; however, under some circumstances, it can also impair performance. Separate literatures have investigated this phenomenon, including studies showing subjective loss of meaning following “semantic satiation”, slowed naming and categorisation when semantically-related items are repeated, and semantic “access deficits” in aphasia. Such effects have been variously explained in terms of habituation of repeatedly-accessed representations, increased interference from strongly activated competitors, and longer-term weight changes reflecting the suppression of non-targets on earlier trials (i.e., retrieval-induced forgetting). While studies of semantic satiation involve massed repetition of individual items, competition and weight changes at the conceptual level should elicit declining comprehension for non-repeated items: this pattern has been demonstrated for picture naming but effects in categorisation are less clear. We developed a paced serial semantic task (PSST), in which participants identified category members amongst distracters. Performance in healthy young adults deteriorated with ongoing retrieval for non-repeated words belonging to functional categories (e.g., picnic), taxonomic categories (e.g., animal) and feature-based categories (e.g., colour red – “tomato”, “post box”). This decline was greatest at fast presentation speeds (when there was less time to overcome competition/inhibition), and for strongly-associated targets (which may have accrued more inhibition to facilitate earlier target categorisation). Deteriorating performance was also seen across words and pictures, consistent with a conceptual locus. We observed a release from deteriorating categorisation following a switch to a new category, demonstrating that this was not a general effect of time on task. Patients with semantic aphasia, who have deficient semantic control, maintained their performance throughout the categories, unlike younger adults: this finding is hard to reconcile with accounts of declining performance that propose a build-up of competition, since the patients should have had greater difficulty resolving such competition. These results instead suggest that declining performance on our goal-driven categorisation task was linked to the use of a controlled retrieval strategy by healthy young adults. Patients may not have inhibited related non-target knowledge to facilitate initial categorisation like younger volunteers, and consequently they were less vulnerable to declining comprehension in this paradigm. Together, these results demonstrate circumstances which produce declines in continuous categorisation in healthy adults

Shared processes resolve competition within and between episodic and semantic memory: Evidence from patients with LIFG lesions

Semantic cognition is supported by two interactive components: semantic representations and mechanisms that regulate retrieval (cf. ‘semantic control’). Neuropsychological studies have revealed a clear dissociation between semantic and episodic memory. This study explores if the same dissociation holds for control processes that act on episodic and semantic memory, or whether both types of long-term memory are supported by the same executive mechanisms. We addressed this question in a case-series of semantic aphasic patients who had difficulty retrieving both verbal and non-verbal conceptual information in an appropriate fashion following infarcts to left inferior frontal gyrus (LIFG). We observed parallel deficits in semantic and episodic memory: (i) the patients’ difficulties extended beyond verbal materials to include picture tasks in both domains; (ii) both types of retrieval benefitted from cues designed to reduce the need for internal constraint; (iii) there was little impairment of both semantic and episodic tasks when control demands were minimised; (iv) there were similar effects of distractors across tasks. Episodic retrieval was highly susceptible to false memories elicited by semantically-related distractors, and confidence was inappropriately high in these circumstances. Semantic judgements were also prone to contamination from recent events. These findings demonstrate that patients with deregulated semantic cognition have comparable deficits in episodic retrieval. The results are consistent with a role for LIFG in resolving competition within both episodic and semantic memory, and also in biasing cognition towards task-relevant memory stores when episodic and semantic representations do not promote the same response

Training flexible conceptual retrieval in post-stroke aphasia.

Semantic therapy in post-stroke aphasia typically focusses on strengthening links between conceptual representations and their lexical-articulatory forms to aid word retrieval. However, research has shown that semantic deficits in this group can affect both verbal and non-verbal tasks, particularly in patients with deregulated retrieval as opposed to degraded knowledge. This study, therefore, aimed to facilitate semantic cognition in a sample of such patients with post-stroke semantic aphasia (SA) by training the identification of both strong and weak semantic associations and providing explicit pictorial feedback that demonstrated both common and more unusual ways of linking concepts together. We assessed the effects of this training on (i) trained and untrained items; and (ii) trained and untrained tasks in eleven individuals with SA. In the training task, the SA group showed improvement with practice, particularly for trained items. A similar untrained task using pictorial stimuli (Camel and Cactus Test) also improved. Together, these results suggest that semantic training can be beneficial in patients with SA and may show some degree of generalization to untrained situations. Future research should seek to understand which patients are most likely to benefit from this type of training

Training flexible conceptual retrieval in post-stroke aphasia.

Semantic therapy in post-stroke aphasia typically focusses on strengthening links between conceptual representations and their lexical-articulatory forms to aid word retrieval. However, research has shown that semantic deficits in this group can affect both verbal and non-verbal tasks, particularly in patients with deregulated retrieval as opposed to degraded knowledge. This study, therefore, aimed to facilitate semantic cognition in a sample of such patients with post-stroke semantic aphasia (SA) by training the identification of both strong and weak semantic associations and providing explicit pictorial feedback that demonstrated both common and more unusual ways of linking concepts together. We assessed the effects of this training on (i) trained and untrained items; and (ii) trained and untrained tasks in eleven individuals with SA. In the training task, the SA group showed improvement with practice, particularly for trained items. A similar untrained task using pictorial stimuli (Camel and Cactus Test) also improved. Together, these results suggest that semantic training can be beneficial in patients with SA and may show some degree of generalization to untrained situations. Future research should seek to understand which patients are most likely to benefit from this type of training