Understanding spoken language often occurs under suboptimal listening conditions, such as processing degraded speech input or conversing in the presence of a competing talker. Current neuroimaging studies suggest a role of frontal regions in compensating for degraded speech and that of cingulo-opercular attentional network in allocating attentional resources between different tasks under distraction. However, no studies so far have explored the combined effects of acoustic degradation and distraction on the speech processing network.

Using functional magnetic resonance imaging (fMRI), we investigated the neural basis of processing degraded speech under divided attention. We examined brain responses of listeners performing a sentence recognition task concurrently with a visuospatial orientation task. Twenty British-English speakers in a factorial design completed a primary sentence recognition task for speech degraded to 4 and 8 bands of noise vocoding together with the visuospatial dual task at two difficulty levels. Participants performed worse in the hard than in the easy condition for both tasks. Performing the harder speech task, compared to the easier speech task, resulted in increased activity in bilateral anterior cingulate gyri and bilateral anterior insula, reflecting an upregulation of the executive network during effortful listening. In particular, participants’ self-rated attention to the speech task was negatively correlated with the overall activity in insula across conditions, suggesting a modulating role of insula on the attention to degraded speech processing. Contrary to our expectation, no significant differences in neural activity were found when contrasting the hard and easy visual conditions (behavioural accuracy: 0.65 and 0.8). As such, our study showed the engagement of cingulo-opercular attentional network and insula in the processing of degraded speech under divided attention.