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The insular cortex, the fifth cortical lobe, hidden deep within the Sylvian fissure in the primate brain, is active during many different tasks ranging from the detection of primary interoceptive events to the integration of hedonic, motivational, and social signals. With its position as a rich club hub within the salience network and its vast ranging network of connections with cortical and subcortical brain structures, the most anterior agranular part of the insula is ideally positioned to evaluate salient events and allocate resources to accordingly optimize homeostasis.
Whilst functional imaging has implicated the activity of the anterior insula in many different interoceptive, emotional, and cognitive functions, local electrical microstimulation (EMS) has revealed its influence in autonomic regulation. Such influence has been proposed to be asymmetrical, with parasympathetic and sympathetic processes being predominantly represented in the left and right anterior insula, respectively. In monkeys, one architectonic area within the ventral anterior insular cortex (VEN area) hosts two specialized neuronal morphotypes, the fork neuron and the von Economo neuron (VEN). The VEN, with its interconnections with both, prefrontal cortex and brainstem autonomic centres, has been attributed a unique role in the maintenance of homeostasis and autonomic responses as well as neurobiological mechanisms contributing to subjective awareness.
To further our understanding of the insula’s role, especially of the VEN area, in autonomic processes and to uncover if any observed asymmetry in autonomic regulation would also be mirrored by an asymmetric whole-brain activity regulation, we conducted EMS in the left and right VEN areas in three anaesthetized macaque monkeys during high-resolution functional magnetic resonance imaging (fMRI). In addition, to examine basic whole-brain activity changes, we investigated the role of the left and right VEN area in the salience network and in the dynamic control of the default mode network and the central executive network.
Using a general linear model analysis, we found the VEN area to be functionally connected to monosynaptic and polysynaptic cortical and subcortical brain regions. However, the alternate stimulation of the left and right VEN area did not reveal any asymmetry in global brain activity response patterns. Employing a seed-based correlation analysis, we found that EMS reduced the insula’s functional correlations with mainly contralateral cortical areas, while correlations to ipsilateral and midline structures remained constant. Further, we showed that EMS led to a shutdown of the insula’s functional correlation with the contralateral salience and default mode networks. However, most remarkably, right-sided – but not left-sided - EMS shut down the central executive network bilaterally and conspicuously increased the ipsilateral salience network.
The present results strengthen the insula’s role as a major hub in the salience network, and its ability to not only switch global brain activity between networks but also to regulate these networks. While this regulation appears to be asymmetric and might influence emotional, cognitive, and behavioural processes, global brain activity, evoked by EMS, did not display asymmetries. It did, however, indicate functional connections to monosynaptic and polysynaptic cortical and subcortical brain areas, underlining the insula’s ability to integrate polymodal stimuli in an interoceptive self-referential domain. This could contribute to the detection and evaluation of salient events, regulation of autonomic processes, and perhaps engendering self-awareness in humans. To further investigate the role of the VEN area in the regulation of network activities, the present experiment should now be replicated in alert monkeys involved in passive or active behavioural paradigms. With the introduction of salient physiological and/or environmental challenges, which are most likely to engage the insula, the crucial role of the VEN area in brain and body regulation could be revealed. |
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