One Salience Network, Two Functions?

What’s the science?

The anterior insula and dorsal anterior cingulate cortex (dACC) are two brain regions that are often active together as part of the brain’s ‘salience network’. A ‘salient’ stimulus is one that is able to capture our attention easily. The anterior insula is active during many cognitive and emotional processes - this has been assumed to be due to its role in orienting attention towards salient stimuli. However, the role of the anterior insula and dACC in specific aspects of the attention-capturing process in the anterior insula and dACC have not actually been delineated. This week in Cerebral Cortex, Han and colleagues performed two experiments using both emotional and emotionally neutral events to assess the functional roles of these two brain regions in salience and emotion.

How did they do it?

Experiment 1: Fifteen healthy young adults participated. While performing functional magnetic resonance imaging (fMRI), the authors had participants look for ‘target’ images of a dining or living room which were presented in rapid succession (sequentially) among other (distraction) pictures on a screen (144 images total per trial, 135 trials). Participants were asked to press one of two buttons when they saw a living room or dining room. In a few trials, a 10-second movie was presented instead of distraction pictures. This movie was either emotional (e.g. of a person in pain or a spider on an arm), or emotionally neutral (e.g. waves or swirls). This task can help to measure the brain’s involvement in processing salient and behaviourally relevant stimuli.

Experiment 2: Fourteen healthy young adults participated. While performing fMRI, a stream of digits was presented in rapid succession, interspersed with letters indicating to the participant had to perform one of two tasks: either to judge either whether the number was odd or even or to judge whether the digit was smaller or bigger than 4 (indicated with a button press). Other letters were also interspersed between digits to indicate whether to stay on the same task (‘hold’ cue) or switch to the other task (‘switch’ cue). This task can help to measure the brain’s involvement in attention switching.

In both experiments, the authors compared brain activation between conditions/tasks.

What did they find?

Experiment 1: As expected, trials in which video clips were presented lowered performance (reaction time), likely because they captured attention and distracted from the task (finding the target - living room or dining room scene). When the fMRI response was examined, the authors found that both the anterior insula and dACC were active at the onset and offset of the emotionally neutral and emotional stimuli (video clips). However, the anterior insula only was also active throughout the presentation of the emotional clips. This indicates that the anterior insula is not simply active during relevant changes in the environment (at the beginning and end of the clip). A smaller sustained response was also found in two other brain regions; the thalamus and putamen. Overall, the authors suggest that the anterior insula and dACC are active when attention is captured by behaviourally significant events.

Experiment 2: The authors found that the dACC was more active during switch cues than hold cues, while there was no such difference for the anterior insula. Therefore, the dACC may be involved in attention set switching/goal directed behaviour (i.e. updating), while the anterior insula is involved in detection of behaviourally relevant events. In a whole brain analysis, another brain region activated by ‘switch’ cues was the medial superior parietal lobe.

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What’s the impact?

This study demonstrates, via fMRI, the different functional roles of the anterior insula and dACC in fine grain detail. The anterior insula is active during behaviourally relevant events - and the dACC is active during attention switching. We now know more about how these regions function within the brain’s salience network.

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S.W. Han et al., Functional Fractionation of the Cingulo-opercular Network: Alerting Insula and Updating Cingulate. Cerebral Cortex (2018). Access the original scientific publication here.