abstract Jonathan Roiser

Jonathan Roiser (Institute of Cognitive Neuroscience, University College London, U.K.)

Bonsai trees in your head: the powerful influence of reflexive processes on goal-directed decision-making

Decision-making in the real world is tricky, because the decisions we make now affect future choices, and future choices and outcomes should guide current decisions. The exponentially increasing number of combinations of future choices and actions means that brute-force approaches to sequential decision-making only work for trivially small problems. Using a computational modelling approach to analyse responses on a deterministic sequential decision-making task, we demonstrate a novel and powerful influence on goal-directed decision-making in humans, "pruning", a simple reflexive process that cuts down (or prunes) an expanding decision tree to a computationally manageable size. Pruning involves automatically discounting sequences of decisions that feature large negative outcomes, no matter what the overall outcome; it is different to loss aversion. Our participants used this pruning strategy even when it was disadvantageous, and the tendency to prune was related to mild depressive symptoms. In a follow-up neuroimaging study we replicated this behavioural pattern, and found that pruning was associated with responses in the subgenual anterior cingulate cortex and periaqueductal grey, which are implicated in mood and anxiety disorders. Finally, initial psychopharmacological evidence supports the hypothesis that pruning is influenced by serotonin signalling. We interpret these findings within a theoretical framework that relates Pavlovian behavioural inhibition to serotonin and mood disorders.