Higher GABA concentration in the region around human SMA predicts smaller negative compatibility effect (NCE) across individuals (a). This result was replicated in a second cohort. b, There was no correlation between the NCE and GABA concentration in dorsolateral prefrontal cortex (DLPFC), parietal cortex, anterior cingulated cortex (ACC) or inferior frontal gyrus (IFG). Positioning of these MRS voxels is shown for one participant (yellow rectangles on inset brains). Note that although the word cortex is included in some labels (to follow standard abbreviations for these regions) all voxels necessarily included both grey and white matter. Filled symbols and bold R-values reflect measurements from the second cohort. GABA concentration measurements are stated in institutional units (i.u.). All p-values are given 2-tailed but uncorrected for multiple comparisons; the main relationship of interest between the NCE and the SMA was specified a priori [5], but even if it had not been (and the first p-value is corrected), the replication demonstrates that the relationship is robust.