An fMRI study on the role of serotonin in reactive aggression

Abstract

Reactive aggression after interpersonal provocation is a common behavior in humans. Little is known, however, about brain regions and neurotransmitters critical for the decision-making and affective processes involved in aggressive interactions. With the present fMRI study, we wanted to examine the role of serotonin in reactive aggression by means of an acute tryptophan depletion (ATD). Participants performed in a competitive reaction time task (Taylor Aggression Paradigm, TAP) which entitled the winner to punish the loser. The TAP seeks to elicit aggression by provocation. The study followed a double-blind between-subject design including only male participants. Behavioral data showed an aggression diminishing effect of ATD in low trait-aggressive participants, whereas no ATD effect was detected in high trait-aggressive participants. ATD also led to reduced insula activity during the decision phase, independently of the level of provocation. Whereas previous reports have suggested an inverse relationship between serotonin level and aggressive behavior with low levels of serotonin leading to higher aggression and vice versa, such a simple relationship is inconsistent with the current data.

Figure 1. Trial timing and behavioural results.

A Time course of a single trial under high provocation. The trial began with a 12-s preparation phase. The participant saw the opponent for the upcoming trial and had to select the punishment. After the reaction-time task proper, the participant was informed about the selection of the opponent. Finally feedback was given about the outcome and the participant had to either press a button for the punishment or the temperature of the thermode was increased. B Average punishment selections under low (grey) and high (black) provocation separately for low (filled bars) and high (stripes) trait aggressive participants (median split; n = 28). C Average punishment selection in low (grey) and high (black) provocation trials, separately for the tryptophan depleted (TRP-) and balanced (BAL) group across the four runs.

Figure 2. Imaging results for the decision phase.

A shows the main effect of provocation with an increased BOLD response in the dorsal ACC and below the corresponding beta values separately for the two groups (TRP- left and BAL right). In B depicted the main effect of the group factor with a higher BOLD response in the insula for the BAL group. Below the corresponding beta values separately for the two groups (TRP- left and BAL right).

Figure 3. Imaging results for the outcome phase (first run).

A depicts the results of the correlational analysis for the outcome phase during the first run. Participants with a higher behavioral provocation effect showed an increased neural provocation effect in the caudate nucleus, dorsal ACC, insula and right inferior frontal gyrus. B For visualization purpose only, the correlation of the average difference in beta values in the caudate nucleus (left) and ACC (right) with the behavioral provocation effect (effect size d, selection high – low provocation, relative to the pooled standard deviance) is shown with the best linear fit. Participants of the TRP- group are indicated with a cross; BAL participants are shown with a circle.