From an evolutionary perspective, passive fear is older than active fear. Terburg: ‘Passive fear, or freezing, is a primal instinct. Reptiles still do it: they often remain completely motionless when faced with danger.’ Mammals that appeared later in the evolutionary process have the basolateral amygdala which can suppress this reptilian response.

Unpleasant shock

For their study, the UU researchers used test subjects who had suffered damage to their basolateral amygdala. A healthy control group also took part. ‘Both groups sat in front of a computer. Pictures then appeared on the screen, and the participants were instructed to push a button within a certain amount of time: a sort of “flight” response, essentially. If they didn’t do this in time, they were given a small, unpleasant shock.’ In contrast to the healthy control group, the test subjects with brain damage exhibited more passive fear responses. This was observed both in their behaviour and in the activation of their brain stem. ‘Because the functioning of their basolateral amygdala is impaired, it is also harder for them to suppress their passive fear.’