Semantic control deficits impair understanding of thematic relationships more than object identity

Abstract

Recent work has suggested a potential link between the neurocognitive mechanisms supporting the retrieval of events and thematic associations (i.e., knowledge about how concepts relate in a meaningful context) and semantic control processes that support the capacity to shape retrieval to suit the circumstances. Thematic associations and events are inherently flexible: the meaning of an item changes depending on the context (for example, lamp goes with reading, bicycle and police). Control processes might stabilise weak yet currently-relevant interpretations during event understanding. In contrast, semantic retrieval for objects (to understand what items are, and the categories they belong to) is potentially constrained by sensory-motor features (e.g., bright light) that change less across contexts. Semantic control and event understanding produce overlapping patterns of activation in healthy participants in left prefrontal and temporoparietal regions, but the potential causal link between these aspects of semantic cognition has not been examined. We predict that event understanding relies on semantic control, due to associations being necessarily context-dependent and variable. We tested this hypothesis in two ways: (i) by examining thematic associations and object identity in patients with semantic aphasia, who have well-documented deficits of semantic control following left frontoparietal stroke and (ii) using the same tasks in healthy controls under dual-task conditions that depleted the capacity for cognitive control. The patients were impaired on both identity and thematic matching tasks, and they showed particular difficulty on non-dominant thematic associations which required greater control over semantic retrieval. Healthy participants showed the same pattern under conditions of divided attention. These findings support the view that semantic control is necessary for organising and constraining the retrieval of thematic associations.

The neurocognitive basis of knowledge about object identity and events: dissociations reflect opposing effects of semantic coherence and control

“…We found that these contrasts produced overlapping clusters in IFG, pMTG and pre-supplementary motor area-in other words, across much of the semantic control network. In a second study [75], we found that patients with SA (who have semantic control deficits) and healthy participants under dual-task conditions (eliciting depleted control resources) showed greater difficulty on the retrieval of weak thematic relations, which require semantic control to identify a linking context, relative to strong thematic relations and identity matching for general and specific concepts. These findings support the view that the semantic control network might make an important contribution to event understanding by allowing the flexible retrieval of knowledge in a way that reflects the requirements of the context.…”

“…The review of studies above provides evidence that left pMTG supports semantic control (along with other regions such as IFG), but there is also compelling research showing that pMTG supports action understanding [46,47] and tool recognition [48] and shows a stronger response to verbs than nouns [49][50][51], consistent with the response profile expected for a thematic hub. In recent studies, we have considered whether these ideas are related [5,75]. One possibility is that judgements involving verbs (relative to nouns) and actions (relative to objects) often have higher control demands, and/ or a stronger need to shape conceptual retrieval to suit the context.…”

Controlled semantic cognition relies upon dynamic and flexible interactions between the executive ‘semantic control’ and hub-and-spoke ‘semantic representation’ systems

“…This network constantly interacts with the hub-and-spokes structure to promote efficacious selection of task-relevant knowledge from a multitude of information in the representational database. These two neural systems can be selectively lesioned, causing double dissociation : Whereas ATL atrophy leads to semantic dementia (erosion of semantic memory with preserved capability of semantic control), frontoparietal stroke leads to semantic aphasia (inability to select appropriate pieces of semantic information, exacerbated by infrequent or unfamiliar meaning, while semantic memory per se is intact; for example, see Thompson et al., 2018 , Thompson et al., 2017 ). It is noteworthy that we chose to focus on the IFG in the present study owing to its established role in selecting context-relevant semantic information, particularly under unfamiliar or ambiguous situations.…”

Neurocognitive patterns dissociating semantic processing from executive control are linked to more detailed off-task mental time travel

“…The verbal fluency score is the contrast of the category condition and letter condition (category -letter). The previous research 31 shows that category fluency is more dependent on semantic memory, while letter fluency is more executively demanding. Picture naming tasks, the four mountains tasks, RAMP were summarised with accuracy scores.…”

“…Behaviourally, mode 1 (r = 0.57, p = 0.036) is reflected by positive loadings on task switching (both flexibility and inhibition) and picture naming, with relatively weak negative loadings on the specific feature matching and strong semantic association task. These two tasks both required the ability to access semantic knowledge but not flexibility in semantic processing 31 . In neural terms, functional connectivity was dominated at the positive end of the dimension by lateral visual, the left temporoparietal junction and the left ventral lateral prefrontal cortex.…”