Fig. 4. GABA in SMA region does not correlate with other potential mediating factors.

(a: age, b: prime identification, c: mean reaction time, d: error rate, e: error compatibility effect, and f: fraction of grey matter in SMA MRS voxel) R-values are the correlation coefficient obtained putting both cohorts together; R1- and R2-values are the coefficient obtained for cohort 1 and 2 separately. There was also no significant correlation of any of these factors with the NCE (all |R|, |R₁| or |R₂| < 0.44, p> .16). Most importantly, when these factors were controlled for, the (partial) correlation (R_p) between the NCE and GABA in the SMA region remained. When controlling for the amount of grey matter, R_1p = 0.8, p< .003, R_2p = .53, p< .04, one-tailed. Similarly when controlling for age (R_1p = 0.77, p< .005, R_2p = .62, p< .035), average speed (R_1p = 0.84, p< .001, R_2p = .61, p< .03), prime visibility (R_1p = 0.8, p< .003, R_2p = .55, p< .03, one-tailed) and error rate (R_1p = 0.75, p< .008, R_2p = .51, p< .045, one-tailed). Note that as a neurotransmitter, the concentration of GABA is expected to higher in grey matter (GM) than in white matter, so one might predict a correlation between GM volume and GABA. However, the GM proportion in the voxel was very similar across participants (i.e. it was well controlled for), so there was little opportunity for a correlation to be revealed. GM proportion ranged from 49% to 54% in cohort 1 and from 46% to 55% in cohort 2. The essential point is not whether GM correlates with GABA, but that this relationship does not account for the correlation of GABA with the NCE.