Saturday, August 30, 2008

Emotions and the insula

I've been meaning for some time to write more about a brain region known as the insula. Looks like now is as good a time as any.

This is a big topic. We had one previous, tentative discussion here. It dealt with involvement of the insula in the "feeling" of fairness and how moral/ethical decisions are made.

Let's start with a review of what the insula actually is, anatomically. It is a part of the brain's cerebral cortex, the outermost layer of the brain, which in humans ranges from 2 to 4 mm in thickness. Although part of the cortex, the insula is located fairly deep within the brain, in an especially deep fold (or "sulcus") of the cortex, between the temporal lobe and the inferior parietal cortex.

Since the insula is adjacent to the temporal lobe on each side of the brain, it is made up of two separate parts, on the right and left sides of the brain. It is found that one side or the other, often the part on the right side, may be more important than the other in particular circumstances. Sometimes the critical area is even smaller, such as the anterior or posterior region of the insula on one side.

A good reason for considering the insula is that it seems to be deeply involved in mediating various emotions in humans. However, the evidence is somewhat indirect.

One important clue is that when a part of the brain including the insula is damaged due to disease or trauma, significant changes in emotions of the affected individual are observed. For example, smokers who experience damage to the insula suddenly cease to feel pleasure from smoking, and they are able to quickly give up the habit. (See here.)

Another clue is that fMRI brain scans show significantly increased activity in the insula when experimental subjects feel strong emotions. There are many example of this. In fact, the relationship seems so predictable that activity in the insula as revealed in fMRI scans is taken to be a marker for emotional experience.

However, there's a lot more we would like to know about exactly how the insula is involved with emotion. For instance, is there anything special about neurons found in the insula, some genes expressed in those neurons more than in neurons elsewhere? Or is the insula's role simply a matter of how it is connected to other parts of the brain? Are there particular hormones or neurotransmitters that are important for processing of emotions in the insula?

In order to even think about such questions, we need to look more closely at what is known about emotions themselves, from classical psychological studies.

To begin with, psychologists distinguish between different "levels" of emotion, not in the sense of intensity, but in the degree to which a particular emotion is more or less "fundamental" and related to basic instinctual needs or aversions. Fundametal emotions are such things as hunger, lust, craving, and fear. It is easily imagined that most animals with a developed central nervous system have such emotions.

Other emotions are a little more abstract, such as happiness, sadness, anger, disgust, anxiety, joy, surprise, delight, pleasure, and amusement. One supposes that substantially fewer animal species experience such "higher" emotions.

And then there are emotions which are pretty much limited to complex social animals that have a rich social life – for example, group loyalty, a sense of fairness or unfairness, altruism, trust, pride, shame, envy, jealousy, greed, grief, etc.

A whole lot more could be (and has been) said about such a categorization of emotions or alternative categorizations, but this will serve to provide an idea of what is meant by "emotion".

There are very good reasons for studying emotions per se. Here are some of them:

  • Emotions play a large role in making moral and ethical judgments. The sense of "fairness" and empathy, as well as various other emotions (especially social ones), come in here. This was discussed here.
  • Another special case of emotional involvement with decision making is found in politics. For instance, our attitudes towards particular candidates have a lot to do with our emotional reactions to the candidates as people – e. g., whether or not they give us a feeling of trust or security or empathy. (Some discussion here.)
  • Emotions play a significant role in economics. For instance, choices made about investments depend a lot on whether one has emotions of fear (in troubled times) or greed (when the economy seems to be doing well).
  • Emotions play a large role in decision making in general. Everyone understands the importance of "gut feelings" here. The emotions a person typically has in certain situations have a large effect on the decisions the person makes in such situations.
  • Critiques of artificial intelligence and certain forms of cognitive psychology assert that intelligent behavior is not simply a matter of algorithmically applying rules and heuristics to known facts. The idea is that computers in their present form are not capable of fully human intelligent behavior because they are not embodied in the way humans are, and in particular they lack emotions. Such critiques have been posed by many philosophers and cognitive scientists, such as Hubert Dreyfus, and George Lakoff. The latter is a representative of the viewpoint known as "embodied philosophy".

We're not going to address all those issues right away. They just serve to remind us of things where more knowledge of the insula and emotions might help our understanding.

As an example of very recently reported research on the insula and emotions, we have this:

Why An Exciting Book Is Just As Thrilling As A Hair-raising Movie (8/12/08)
We all know, however, that reading a book describing the same scene can be similarly gripping. This week, in a paper published in the online, open-access journal PLoS ONE, Mbemba Jabbi, Jojanneke Bastiaansen and Christian Keysers show us why.

At the NeuroImaging Center of the University Medical Center Groningen of the University of Groningen (the Netherlands), Jabbi and colleagues compared what happens in our brains when we view the facial expressions of other people with the brain activity as we read about emotional experiences. ...

"Our striking result," said Keysers, "is that in all three cases, the same location of the anterior insula lit up. The anterior insula is the part of the brain that is the heart of our feeling of disgust. Patients who have damage to the insula, because of a brain infection for instance, lose this capacity to feel disgusted. If you give them sour milk, they would drink it happily and say it tastes like soda."

Prof. Keysers continued, "What this means is that whether we see a movie or read a story, the same thing happens: we activate our bodily representations of what it feels like to be disgusted– and that is why reading a book and viewing a movie can both make us feel as if we literally feel what the protagonist is going through."

To summarize: the question was about similarities (and differences) in brain response to the emotion of disgust, which may be stimulated in three different ways: actual experience (tasting something very unpleasant), observation of facial expressions of others experiencing disgust, and subjective imagination of disgust.

It was already known that fMRI scans showed activity in the insula and an adjacent region (the frontal operculum) – collectively termed the IFO – as a result of experiencing or observing the emotion of disgust. The research showed that very similar activity occurred as a result of imagining disgust.

Further reading:

Why real and imagined disgust have the same effect (8/13/08) – New Scientist article about the research

Emotional Thrills From A Movie (or a Book) – blog post on this research

A Common Anterior Insula Representation of Disgust Observation, Experience and Imagination Shows Divergent Functional Connectivity Pathways – August 2008 research article in PLoS ONE

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