Effect size at the peak midbrain voxel in each learning rate.

<p>The effect sizes at the peak midbrain voxel across the eight learning rates are shown for each model (filled squares and thick lines for the WITH model, open circles and dotted lines for the WITHOUT model). Vertical axis represents the effect size (parameter estimates for the regressor of the reward prediction error <i>δ</i>(<i>t</i>)) at the peak voxel averaged for 23 subjects based on the data up to session 3. The effect size was significantly greater for the WITH model than for the WITHOUT model in most learning rates (two-tailed paired <i>t</i>-test: *, <i>p</i><.05; **, <i>p</i><.01). Error bars represent ±1 s.e.m.