The Salience Network

Think of a debate between two of your main brain networks representing your internal and external mind. The two featured speakers would be standing on stage preparing to share their points of view. The debate moderator then decides which speaker has the floor and determines how much the audience hears from each side.

In the same way a moderator only lets one person speak at a time, the salience network moderates the activity of the DMN and the CEN.

The salience network also plays a critical role in processing pain, emotion, reward, and motivation³ in connection with the limbic system. In these cases, the moderator decides how much the human body “hears” signals involving emotional response.

As with any successful moderator, the role is to support and guide the speakers. The moderator handles switching between speakers and conversation topics subtly and quickly. The salience network is no different. The SN regulates and transitions between networks swiftly and imperceptibly to address the changing demands of the moment.⁴

While serving such an important function, the salience network is a relatively recent discovery which marks an incredible stride in our understanding of the human brain. Before pivotal studies in 2007, the areas of the brain now considered part of the salience network used to be dissociable⁵ from the task processing⁶ of the default mode network or the central executive network. Now, we understand that functional areas in the salience network play less of a role in conducting tasks than they do for moderating control of internal and external processing.

The main functional areas, or nodes, of the salience network are located in the anterior cingulate, the anterior insula,^4,8and the presupplementary motor areas.⁹ The SN also includes nodes in the amygdala, hypothalamus, ventral striatum, thalamus, and specific brainstem nuclei.⁴ anterior cingulate cortex (ACC), medial temporal network, parahippocampal gyrus, olfactory lobe, and the ventral tegmental area (VTA).

Why we think this network is worth considering in decision-making:

The salience network plays a key role in a number of important cognitive, emotional, and motivational function.

Evidence that this network is responsible for useful function in humans:

SN’s main components are insula and anterior cingulate cortex with three key subcortical structures, the amygdala, the ventral striatum, and the substantia nigra/ventral tegmental area.¹ The anatomical connections between the components of SN in cortical level are well established in humans using diffusion tensor imaging.^2,3 In addition, the neural architecture of the two cortical components of the SN shares distinct characteristics (e.g. von Economo neurons).⁴ Also, developmental differences have been shown both in the structural and functional connectivity level in the cortical components of SN, which was suggested to account for the cognitive maturation with age.⁵ SN mainly supports detecting salient stimulus to recruit functional networks while between activation and deactivation of DMN and CEN.⁶

Consequences of damage to this network:

Akinetic Mutism: Using lesion network mapping, Darby et (2019) have shown that lesions in the SN network cause disruptions in volition and cause akinetic mutism or abulia.⁷ This finding was also consistent with prior studies of disorders of free will perception, such as PNES or catatonia that identified structural and functional abnormalities in the same network. Disruptions to this network for depression, obsessive-compulsive disorder, or chronic pain introduced impairments in volition as well.^8,9,10

Dementia: It has been shown that SN connectivity pattern can successfully predict different dementia types such that decreased connectivity in the SN correlated with the behavioral variant frontotemporal dementia whereas increased connectivity with the Alzheimer’s Disease. Moreover, in the behavioral variant frontotemporal dementia group, clinical severity positively correlated with the loss of SN connectivity.¹¹

Schizophrenia: SN abnormalities are found in schizophrenia¹² and correlated with the severity of reality distortion.¹³ Findings also highlights the importance of the SN in regulating network switching and the role of these processes in higher cognition.¹⁴ In addition to functional connectivity impairments, studies found volumetric grey matter changes in SN regions in patients with schizophrenia.¹⁵

ADHD: Additional studies identified lower connectivity in the anatomical correlates of SN in ADHD.¹⁶ Using machine-learning algorithms, Sato et al could establish an abnormality matrix derived from the connectivity patterns of SD regions, and this matrix could successfully classify adults with ADHD.¹⁷

Other Mental Disorders: In obsessive-compulsive disorder, abnormalities in salience network connectivity measures related to symptom severity as well as decreased sustained attention.¹⁸ Impairments in connectivity in SN associated with higher trait anxiety,¹⁹ ruminative brooding²⁰ and depression.²¹ Volumetric grey matter changes in SD also identified in depression patients.¹⁴