Neuronal Networks: The Salience Network

What is salience?  Fundamentally it is a value judgment that determines where your brain will place its limited resources.  There are a lot of things that could draw our attention.  The world is full of sights, sounds, smells, pressures, temperatures, stretches.  Our mind is full of thoughts.  Without a salience network, we wouldn’t know what matters and what doesn’t.  We would just randomly scan our thoughts and the environment and hope what we are noticing at any point in time is what will help keep us alive.  That is a losing evolutionary strategy.

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Transcript

Welcome to PsyDactic - Residency Edition… 

What is salience?  Fundamentally it is a value judgment that determines where your brain will place its limited resources.  There are a lot of things that could draw our attention.  The world is full of sights, sounds, smells, pressures, temperatures, stretches.  Our mind is full of thoughts.  Without a salience network, we wouldn’t know what matters and what doesn’t.  We would just randomly scan our thoughts and the environment and hope what we are noticing at any point in time is what will help keep us alive.  That is a losing evolutionary strategy.

When we see something moving in space, we need to be able to decide if it is worth watching.  Is it alive?  Does it have volition? When we smell something we need to know if it might be toxic, if it is familiar, if it could be food, if we could be it’s food.  Failure to implement a more cautious movement strategy when approaching the edge of a cliff could result in dire consequences. While our attention networks help us identify details in our environment (like a cliff), our salience network tells us what details are important (a cliff might result in a fall).  If five people are looking at you, four of them are smiling and one of them is scowling, then you need to be able to identify the scowling person as something needing attention.  Being able to identify that something looks different from other things requires your dorsal and ventral attention networks and, of course, your visual cortex.  To be able to care that something needs your attention requires your salience network.  If you hear footsteps behind you in an abandoned house that you went into alone, then you better prioritize finding out what that is.

Your salience network is used to prioritize stimuli, whether it is internal or external.  It is made up of your dorsal anterior cingulate cortex and the anterior insula which communicates extensively with subcortical regions in your ventral striatum, ventral tegmentum, and amygdala.  If you can remember to the last episode when I asked to to lift up the operculum of the frontal lobe and stair beneath, then you may remember that the insula was tucked comfortably underneath.  The cingulate gyrus is that part of the cortex of the brain that is hugging your corpus callosum.

Your ventral tegmentum and ventral striatum are midbrain structures that are part of your mesolimbic system.  What this means is that part of your salience network, the cortical parts of the insula and the cigulate gyrus are predominately glutamatergic and the other part (the ventral tegmentum and ventral striatum) are predominately dopaminergic.  I will shameless refer you back to Episode 7 - OCD Brain Space where I discussed the cortico-striatal-thalamo-cortical circuit and how glutamate responsive neurons predominate in cortical regions and dopaminergic and gabaergic neuron predominate in striatal regions.

Your ventral tegmentum and ventral striatum are important in reward and motivation (hence the dopamine), and also in aversion and avoidance (with their connections with the amygdala).  Your salience network, then, is integral in identifying stimuli and deciding not only if something is of interest, but what kind of interest is it.  Should I explore more? Should I move toward and embrace it? Should I move away? Should I avoid it?  Have I seen this before and how did it affect me?

The ventral striatum contains the nucleus accumbens which you may recognize as being one of the key players in the mesolimbic dopamine system (which includes our reward system) and consequently in addiction.  Clinicians often advise those with substance use disorders to avoid temptation because for someone who struggles with substance use, their salience network might assign a distracting amount of value to thoughts or perceptions about the substance, or to planning for acquiring the substance.  A paper from 2015 in the Neurobiology of Disease by Liang et al found that the default mode network and salience network have reduced connections in individuals with cocaine dependence versus healthy control. (Interactions between the Salience and Default-Mode Networks Are Disrupted in Cocaine Addiction Xia Liang,1,2 Yong He,2 Betty Jo Salmeron)  In more general language, this means the part of our brain that is more self referential and that adds social and past context to our thoughts may not be communicating as actively with the part of our brain that is assigning value to stimuli in our environment.  It seems to me that this might make it biologically harder for these individuals to think about the consequences of their decisions.  Liang et al could not comment over whether this reduced connectivity was a result of cocaine addiction or existed prior to cocaine addiction.

Now let’s imagine someone with schizophrenia who is trying to pick out salient aspects of their environment or their own thoughts.

If you think about the diagnostic criteria for schizophrenia, we usually break them up into two kinds of symptoms, positive and negative.  Positive symptoms include hallucinations, delusions, and disorganized speech or behaviors.  The negative symptoms are avolitional: such as a lack of motivation, a lack of affect, or a paucity of speech.  These seem like very different kinds of things.  Delusions are when we seem to assign too much salience to our thoughts or things we assume about our perceptions of the environment around us resulting in phenomena like thought broadcasting, paranoia, or culturally derived conspiratorial assumptions (like I’m being watched by the government, or I have been chosen by god for a special mission).  Hallucinations are unsubstantiated yet real seeming internal perceptions.  Disorganized speech and behaviors seem like desperate and ineffectual attempts to affect the world around us.  But avolition is a lack of interaction with the world.  It is hard to know what’s going on inside of a brain that is not doing things or saying things.

Schizophrenia itself refers to a splitting of the mind or more broadly a splitting of the self.  A splitting of the self seems to involve the default mode network which is the network that tends to reference ourselves.  It was the first network I talked about in this series.  Disorganized behavior seems to involve our executive functions.  I haven’t yet discussed our executive networks.  That is for the next episode.

But how is the salience network involved in something like this?  It seems like the positive symptoms (especially delusions) are more likely to directly involve the salience network than negative symptoms or disorganized symptoms.  However, avolition might result from an inability to assign value to otherwise salient social cues.  It's obviously not a simple problem.

Would we expect the part of our brain that gives salience to our thoughts or the environment to be extra-connected or under-connected to other regions of the brain?  It’s not obvious to me.  It could be that the Salience Network is over-active, assigning too much significance to transient details, resulting in paranoia.  Or it could be that it is underactive, and not properly assigning salience or double checking whether something should be considered significant, so the rest of our mind is allowed to run wild with actually unimportant details.

Kowalski and colleagues in the J. Clin. Med. 2021 described psychosis like this: “On a phenomenological level, aberrant salience is described as interpreting irrelevant as relevant, highly familiar as novel, having sharpened senses, keenness for things normally deemed negligible or a sense of newly found insight or understanding.”  I love that quote so I will say it again…

Since the salience network has broad functions that work together to assign value to internal and external stimuli and is intimately involved with our reward system, we might suspect that some parts of this system might be overactive while others are underactive.  For example, the ventral striatum in some studies has been shown to be overactive, giving a sense of reward where none is warranted.  The insula and dorsal anterior cingulate in some studies is underactive, which may explain why the rest of the brain gives too much attention to details that truly have no real significance.  They have lost connection with what is truly salient.  In short, in psychosis, our brains reward us for perceptions, actions, and thoughts that have no real value.

That is a very basic oversimplification of how the salience network might malfunction in psychosis.

But none of our brain networks work alone.  Among neuroscientists, there is a rising paradigm called the Triple Network Model.  It posits that there are three main networks in our brains that are dysfunctional in most primary psychiatric disorders.  These networks include the Default Mode Network, the Salience Network, and the Central Executive Network.

In the next episode, I will discuss the Central Executive Network.  It is a network that sends tentacles out into other networks in the brain in order to communicate and receive feedback necessary to accomplish tasks that require strategy, that require planning and prioritization.  After that, I will discuss how, in a similar way that a family or a business or an administration cannot function well without a balance of feedback from its members, none of these networks can function effectively without the others.

I am Dr. O and this has been an episode of PsyDactic Residency Edition. (1–8)
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