By Emma Young
Love it or loathe it, Forrest Gump has now gone way beyond introducing “Life is like a box of chocolates” and “Run, Forrest! Run!” into our vernacular. It’s been used to do something truly remarkable: to reveal the location of a map of emotions in the human brain.
This new work, published in Nature Communications, shows that a spherical bit of cortex, about three centimetres in diameter, represents not only the kind of emotion we’re feeling in any given moment, but how strongly we’re feeling it. In revealing objective brain-based correlates of our feelings, the work potentially has all kinds of implications for psychiatry.
Giada Lettieri at the IMT School for Advanced Studies in Lucca, Italy, and colleagues recruited 12 participants to watch an edited version of the movie. Throughout, they each reported on their feelings of happiness, sadness, anger, fear, surprise and disgust, providing intensity ratings on a scale from 1 to 100.
The team averaged out these ratings across participants during different scenes, to get an overall picture of how emotions changed throughout the movie. But as well as conceptualising emotions as discrete feelings like happiness or anger, the team also examined three measures that together could also account for how participants were feeling. The first was a measure of polarity — the positivity, or negativity, of the viewers’ emotional state at any given point in the film. The second was a measure of complexity — of feeling mixed, vs single, emotions. The final measure was of emotional intensity.
The team then compared this data with brain scans made using fMRI of 15 other people while they had watched the same film. This fMRI work had been done earlier by another team in Germany, who had made their data openly available for use by other researchers.
The results revealed that the emotional ratings across the course of the movie were related to specific patterns of activity in a small region of the right temporo-parietal cortex. It wasn’t the case, though, that “happiness” was represented in one discrete part, and “anger” in another, say.
Instead, different “gradients” of neuronal activity in this region represented emotional polarity, complexity, and intensity. The three overlapping gradients in different orientations “allows a gamut of emotional experiences to be represented in a single patch of cortex”, the researchers note.
They call this process “emotionotopy” and add that its organisation is similar to the way certain properties of visual information (such as the distance of a stimulus image from the fovea and the stimulus’s location in the visual field) are mapped in the primary visual cortex.
Overall, the work certainly does suggest that this bit of cortex is responsible for mapping what we’re feeling in a given moment, and how strongly we’re feeling it. And this, at least, seems to be common to all people. “In the current study, ratings of the emotional experience elicited by an American movie in Italian participants explained brain activity in German subjects,” note Lettieri and her colleagues. “This suggests that the topographic representation of emotion dimensions exists regardless of linguistic or micro-cultural differences.”
More movies should now be used to test the generalizability of the results, they add. If the findings do hold up, an understanding of how emotions and their intensity are represented in the brain might in theory help with research into disorders of emotion, such as depression and phobias. “These studies are getting psychiatry closer to other fields of medicine in finding objective biological correlates of feelings, which are subjective states,” comments study co-author Pietro Pietrini.