Inhibitory and excitatory mechanisms in the human cingulate-cortex support reinforcement learning
The dorsal anterior cingulate cortex (dACC) is crucial for motivation, reward- and error-guided decision-making, yet its excitatory and inhibitory mechanisms remain poorly explored in humans. In particular, the balance between excitation and inhibition (E/I), demonstrated to play a role in animal studies, is difficult to measure in behaving humans. Here, we used magnetic-resonance-spectroscopy (1H-MRS) to examine these mechanisms during reinforcement learning with three different conditions: high cognitive load (uncertainty); probabilistic discrimination learning; and a control null-condition. Subjects learned to prefer the gain option in the discrimination phase and had no preference in the other conditions. We found increased GABA levels during the uncertainty condition, suggesting recruitment of inhibitory systems during high cognitive load when trying to learn. Further, higher GABA levels during the null (baseline) condition correlated with improved discrimination learning. Finally, excitatory and inhibitory levels were correlated during high cognitive load. The result suggests that availability of dACC inhibitory resources enables successful learning. Our approach establishes a novel way to examine the contribution of the balance between excitation and inhibition to learning and motivation in behaving humans.