Intraparietal stimulation disrupts negative distractor effects in human multi-alternative decision-making

There has been debate about whether addition of an irrelevant distractor option to an otherwise binary decision influences which of the two choices is taken. We show that disparate views on this question are reconciled if distractors exert two opposing but not mutually exclusive effects. Each effect predominates in a different part of decision space: (1) a positive distractor effect predicts high-value distractors improve decision-making; (2) a negative distractor effect, of the type associated with divisive normalisation models, entails decreased accuracy with increased distractor values. Here, we demonstrate both distractor effects coexist in human decision making but in different parts of a decision space defined by the choice values. We show disruption of the medial intraparietal area (MIP) by transcranial magnetic stimulation (TMS) increases positive distractor effects at the expense of negative distractor effects. Furthermore, individuals with larger MIP volumes are also less susceptible to the disruption induced by TMS. These findings also demonstrate a causal link between MIP and the impact of distractors on decision-making via divisive normalisation.

Automated summary

There is a debate on whether the addition of an irrelevant distractor option affects the decision-making between two choices. This study shows that there are two opposing effects of distractors, with each effect dominant in a different part of the decision space. By disrupting the medial intraparietal area (MIP) through transcranial magnetic stimulation (TMS), the positive distractor effect increases while the negative distractor effect decreases. Furthermore, individuals with larger MIP volumes are less susceptible to the effects of TMS. These findings establish a causal link between MIP and the impact of distractors on decision-making.