In 229 healthy volunteers aged 18–55 (mean 30 + 9.2), we found that measures of GABA in the dACC decrease with age. In a subset of 171 subjects, age was associated with reduced performance on the WCST. The WCST is a problem-solving task, thought to index executive function, in which the subject must adapt dynamically to ongoing feedback in order to correctly assign cards to categories (color, shape, or number) that shift unpredictably during task performance. This task has been found to be sensitive to damage to the frontal lobe and has been associated with brain activation in multiple cortical areas, including the ACC [37]. GABA/Cre in the dACC was also associated with WCST performance and statistically mediated the effects of age on WCST performance.

Association of cognitive variables with age

The existence of age effects on WCST performance is well established [38]. Other cognitive variables, including processing speed and verbal memory, were not significantly associated with age in our study cohort. Several factors in addition to stringent multiple comparisons corrections likely account for this. First, the sample had a restricted age range (18–54 years, with two-thirds of the sample between 20 and 39), and it may be that the WCST is more sensitive than other cognitive measures to age-related performance changes within this age range [39]. Also, we excluded individuals with IQ < 70, evidence of learning disabilities, and/or age-related brain abnormalities, such as WM hyperintensities, all of which might account for some age-related cognitive decline in an unselected sample.

Effects of age on metabolites

Both our measures of GABA+ decreased with advancing age. This is consistent with findings by Gao et al. [18] for GABA+/Cre, and those of Rowland et al. [20] for GABA/water, but not with those of others [22, 24, 25] for GABA/water. The most significant difference between our study and the ones mentioned above may be in the age-based correction of the T2 of water that we applied. We show here (see Supplementary Material) that the T2 of water declines significantly with age in GM; this observation has important implications for all MRS signals reported as ratios to water (sometimes referred to as “absolute” values) since this approach would result in inflated estimates of metabolite concentrations at older ages. Had we not performed the correction for age-related decline in T2 of water, GABA+/water would not have been associated with age in a statistically significant manner (results not shown). It is important to note that the T2 of water has been reported to increase with age using imaging methods such as turbo spin echo or gradient recalled sequences [40, 41]. We used the same spectroscopic PRESS sequence for the water T2 calculation and for the metabolite determination. Spectroscopic sequences may be weighted toward T2*, which has been shown to decrease with age [40, 42], and may therefore be a more suitable method for correction of T2 of water in MRS studies.

Associations of GABA with cognition

Here we observed a statistically significant association of GABA+/Cre with WCST performance, accounting for about 6.2% of the variance, which survived covariation for sex and age. This is consistent with the relationship between GABA+ and a more general cognitive task (the Montreal Cognitive Asessment) found by Porges et al. [19] in a group of older patients. The fact that two well-powered studies, ours and that of Porges et al. [19], converge in finding a relationship between anterior cingulate GABA and a complex cognitive ability declining with age adds import to this result, especially given the frequent lack of replicability of imaging findings. Moreover, the overlap between the findings of these two studies bolsters the argument that GABAergic transmission is involved in cognitive decline well before the appearance of frank cognitive deficits. It is therefore possible that alterations of GABA transmission might be very early occurrences in the chain of events leading to cognitive disturbances of aging.

No other ratio to Cre reached statistical significance, although 95% CIs for betas overlapped across ratios. The latter finding may be due to (1) the rather shallow slope of the association of GABA+/Cre with WCST performance; (2) to the fact that all ratios have the same denominator, which reduces their mathematical independence; and (3) to the fact that there is a biological correlation among some of the metabolites (e.g., NAA has been associated with glutamate levels [43] and GABA and Glu are in the same metabolic pathway).

The WCST is a widely used index of executive function and dynamic, feedback-guided problem solving. The association of GABA+/Cre with WCST performance in the dACC was not found in the WM, likely implying a selective cortical mechanism consistent with the higher concentrations of GABA and Cre in the cortex as compared to WM [31]. The association of GABA+/Cre with WCST performance was likely due to both the numerator and the denominator of the ratio, since both GABA+/water and Cre/water were significant and fully independent of each other in the results shown in Table S2 (they explained about 2.5% and 4% of the variance in WCST performance, respectively, while age explained about 5%).

Proportion of CSF and GM in the voxel were associated with age, as expected, but not with WCST performance when age was accounted for. Thus, we lacked statistical evidence to support adding CSF or GM proportion in our models. However, we cannot rule out that with improved measurement precision, effects of CSF and GM might have been detectable. These variables did not account for a significant portion of variance in WCST performance when introduced in the same model as GABA+/Cre, therefore their potential role as independent mediators of the relationship of age to WCST performance is likely to be weaker than, and independent of, GABA+/Cre.

Mediation analysis

GABA+/Cre in the dACC was a statistically significant mediator of the association between age and WCST performance (Table 4), suggesting that reduced GABA and increased Cre in the dACC are factors contributing to poorer problem solving and cognitive flexibility with age. Few other investigations have explored mediation effects of metabolites measured with MRS on cognitive function. Gomar et al. [44] examined the precuneus in a sample older than 50 years of age and found that associations of Cho/Cre with a composite measure of broad cognitive ability varied as a function of APOE genotype (including APOE-4), but these investigators did not examine GABA. CSF and measures of Alzheimer pathology (e.g., estimates of amyloid concentration) were not included in this analysis. We found only weak evidence for a decline in IQ with age (p < 0.01 uncorrected) within the relatively younger age range considered here. Rowland et al. [20] demonstrated a modest role for GABA in the dACC in predicting verbal working memory performance, but these authors did not test for age effects. Porges et al. [19] did not report beta values making it difficult to determine if their results were likely to support a mediation effect similar to the one found here.

Our findings are in line with literature from animal models showing a contribution of GABA-related deficits of inhibition to cognitive aging [45]. Since spectroscopy detects total metabolite levels rather than synaptically active GABA, our data cannot be interpreted to reflect inhibitory function per se. Despite this consideration, however, experiments in animal models and cell culture where intracellular concentration of GABA was increased by inhibition of GABA transaminase resulted in increased cytosolic and vescicular GABA [46], as well as depolarization- and electrical stimulation-evoked release of endogenous GABA [47, 48], indicating some degree of equilibrium between intra- and extracellular compartments. The MRS technique used here does not provide information on GABA-a or GABA-b receptors, which are important in modulating cognition, at least in animal models [49]. Our findings implicating GABAergic transmission might also be compatible with the observation that β-amyloid accumulation can cause selective dysfunction of parvalbumin positive inhibitory interneurons [2], and that inducing overexpression of sodium channels (Nav1.1, produced by SCN1A) in these cells can ameliorate memory dysfunction in animal models of Alzheimer disease [50, 51]. A contribution of energy-related mechanisms, possibly indirectly indexed by the Cre peak, is also suspected to play a part in cognitive aging [52, 53], and a hypothesis has been advanced that inhibitory interneurons with high energy demands are particularly important for cognitive decline [54].

Some limitations of this study are discussed in the Supplementary Material.