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

Weight between map-based generalization and stimulus-outcome association-based decision-making

"Weight" references a parameter that is extracted from recorded choice behavior by means of computational modeling. Three basic models (one map-based, two stimulus-outcome association-based) are fit to each subject's choice data from each session. Two hybrid models - combining the map-based with either of the stimulus-outcome association-based components - are also fit. The weight parameter will be extracted from the hybrid model that fits most subjects' behavior best under the control condition (high-frequency stimulation). It describes the extent to which the subject relies on the map-based relative to the association-based component/strategy for making their choices. Given the presumed roles of hippocampus in map-based and striatum in association-based decision-making, the investigators expect the weight parameter to increase under hippocampal tTIS and to decrease under striatal tTIS relative to high-frequency control stimulation.

Choice accuracy

Whether, out of two simultaneously presented stimuli, the one associated with the higher reward has been chosen on a given trial. Early after a shift in reward contingencies, using a map-based strategy should increase the probability of choosing correctly. This is because the new reward contingencies may not have been experienced yet (disabling association-based strategies) and so can only be inferred (requiring map-based strategies). Both strategies should be equally successful later after a shift once all new contingencies have been experienced. Averaged across all trials, the investigators therefore expect to observe higher accuracy under hippocampal and lower accuracy under striatal tTIS as compared to control stimulation. The investigators also expect subjects' individual choice accuracies to positively correlate with their map-based weight parameter estimates.

Choice response time

Time between presentation of the two stimuli for the present trial and the subject's choice of one of the two stimuli. The investigators will analyze the effects of hippocampal and striatal tTIS as compared to control stimulation on response time in an exploratory manner.

Proportion of subjects best fit by different computational models

Different candidate models are fit to the recorded choice data, two of which are on the "stimulus-outcome association-based" and one on the "map-based generalization" side. Hybrid models (= weighted sum of one stimulus-outcome association-based element and the map-based generalization element) are also fit. WAIC (Watanabe, 2010) will be used to determine which model fits which subject best under any given stimulation condition. The investigators expect the proportion of subjects best described by the map-based (association-based) model(s) to increase (decrease) under hippocampal tTIS as compared to control stimulation, with the opposite pattern showing under striatal tTIS. Note: Should ≥ 50 % of subjects perform at chance level in the proximity judgments in the first session (last eight blocks), the investigators will use subjects' \*subjective\* representations (as collected at the end of the last choice task session) instead of the objective ones as a basis for the map-based model.

Influence of true and past rewards on choice

The investigators will use (a) the difference in the most recently observed values of the stimuli presented to the subject's left vs. right and (b) the difference between the actual, current values of these stimuli (which may not have been observed yet) to predict whether the subject will have chosen the left vs. right stimulus. Subjects pursuing a stimulus-outcome association-based strategy will exclusively rely on directly experienced reward contingencies (strong regression weight of (a) but not (b)). Subjects generalizing across related stimuli will be able to infer a stimulus' current value even if that one has not been observed recently (strong regression weight of (b) but not (a)). The investigators expect the regression weights of (a) to increase and (b) to decrease under striatal tTIS (opposite under hippocampal tTIS) as compared to control stimulation.

Gender

Not an outcome variable but a variable that will be collected to characterize the study sample.

Age

Not an outcome variable but a variable that will be collected to characterize the study sample.

Center for Epidemiological Studies Depression Scale

German version (Hautzinger \& Bailer, 2003). This is not an outcome measure but a variable that will be collected once at the beginning to characterize the study sample. It will not be analyzed as a function of stimulation condition. Sum scores range from 0 to 60, where higher scores indicate higher depressive symptom load.

Edinburgh Handedness Inventory

Oldfield (1971). German translation. This is not an outcome measure but a variable that will be collected once at the beginning to characterize the study sample. It will not be analyzed as a function of stimulation condition but will be explored as a potential moderator of stimulation effects. Per side (left and right), between 0 and 20 "+"s can be marked, where a higher number of "+"s indicates stronger use of the respective hand.