Neural network dynamics of speech comprehension – the role of the angular gyrus
Speech comprehension is often challenged by acoustically adverse listening conditions (e.g. background noise or compromised signal quality). When perceiving speech in noise, successful speech comprehension relies on additional factors, such as semantic context. Previous work has shown that comprehension of sentences is facilitated by rich semantic context (e.g. “The ship sails the sea” vs. “Paul discussed the sea”). This contextual comprehension benefit is found to be largest at intermediate levels of intelligibility and is accompanied by enhanced engagement of left angular gyrus (AG). Furthermore, there is converging evidence for the recruitment of domain-general adaptive control networks when listening conditions become challenging. For instance, it has been shown that the extent to which cingulo-opercular regions are recruited has substantial predictive value for the behavioural outcome. However, it remains unclear how domain-specific speech networks and domain-general control networks interact and influence each other during successful speech comprehension.
To address these questions, we conducted an event-related fMRI experiment. Stimuli were derived from the German version of speech in noise (SPIN) sentences (Kalikow et al.), with the sentence-final word being embedded in either high or low semantic context. Sentences were presented auditorily at six levels of intelligibility tailored to each subject individually using an adaptive staircase procedure. During continuous scanning, participants performed an overt sentence repetition task.
At the behavioural level, we found significantly higher accuracy scores for sentences with high opposed to low semantic context. Univariate fMRI group-analyses revealed broad bilateral activation in superior temporal, as well as pre- and postcentral regions, together with left inferior frontal gyrus for increasing intelligibility. In contrast, regions that showed greater activation for decreasing intelligibility encompass bilateral AG, precuneus, superior and inferior frontal regions. Furthermore, we found significant context x intelligibility interactions in fronto-temporo-parietal regions: left AG, left supramarginal gyrus and posterior portions of the middle and inferior temporal lobes showed a stronger increase in activation for high predictable sentences, left inferior frontal regions as well as bilateral insulae exhibited a stronger increase for low predictable sentences. These preliminary results provide further evidence for a strong context-dependent recruitment of left AG in degraded speech comprehension.
As a next step, task-specific changes in effective connectivity between left AG and other nodes within the speech-specific network as well as across domain-specific and domain-general networks will be investigated to provide deeper insight into the functional dynamics of speech comprehension under adverse listening conditions at a network level.