Emotion increases the similarity between neural representations of pictures, but not their perceived similarity

Abstract

Emotional similarity refers to the tendency to group stimuli together because they evoke the same feelings in us, even when they look different and have different semantic meanings. It is still unclear which features define the similarity space of emotional categories. Additionally, whether emotional stimuli are perceived as more similar than neutral ones, and whether this difference is paralleled by differences in their neural representations, has never been investigated. We conducted a series of experiments to quantify behavioural similarity, and one that used fMRI and Representation similarity analysis to compute neural similarity. We hypothesised that the similarity between emotional stimuli will be greater than between non-emotional stimuli, paralleled by higher neural similarity among emotional stimuli. We tested these hypotheses with two measures of similarity and two different databases of complex negative and neutral pictures, the second of which allowed us to control semantic similarity. For the first time, we found a decoupling between subjective and objective measures of emotional similarity. Pictures taken from two distinct emotional and neutral categories were perceived as equally similar; however, their neural similarity was higher. This effect was detected in brain clusters localised in a constrained search volume. We conclude that features representing participants similarity space may have different weights in these regions than they do in explicit ratings.