Figure 3. Correlations between midbrain and left caudate activity: voxels demonstrating a significant inverted-U shaped effect.

 Top. Mean Z scores of correlations of midbrain-caudate connectivity divided into span groups and drug conditions. Error bars indicate standard error. Bottom. Mean Z scores of correlations of midbrain-caudate connectivity with performance on the listening span test during placebo or drug condition. Working memory span was measured as the number of words recalled on the listening span task (see Methods).