Investigating the Mechanisms of Decision-Making Using Non-Invasive Brain Stimulation

This study aims to investigate which of two brain regions is important for which decision-making strategy. The investigators suppose that one brain region - the \*striatum\* - is important for repeatedly choosing objects that have previously been paired with rewards (strategy 1). They suppose that a different region - the \*hippocampus\* - is important for extending such reasoning to RELATED objects (strategy 2).

The investigators will repeatedly ask young and healthy subjects to choose between two objects. The two objects are repeatedly drawn from a pool of seven. Relations between the seven objects have been learned on the first day of the experiment. The investigators will record subjects' choices and will use an automated way to assess the extent to which any given subject uses either decision-making strategy.

* On one day, subjects will perform the task while their right hippocampus is being stimulated with non-invasive electrical stimulation ("condition A"). * On another day, subjects will perform the same task but with their striatum being stimulated ("condition B"). * On yet another day, a control stimulation ("condition C") will be applied that looks and feels like the real stimulation but does not influence brain activity.

Which stimulation (A, B, C) happens on which day is assigned by computer code and differs between subjects. Subjects do not know, and investigators mostly do not know, whether real or control stimulation is applied on each day.

The investigators will check whether subjects use strategy 1 more when their striatum is being stimulated and strategy 2 more when their hippocampus is being stimulated, as compared to when the control stimulation is applied.