Over recent years there has been an upsurge in interest in Magnetic Resonance Spectroscopy (MRS) as a non-invasive method to quantify neurotransmitter concentrations in discrete regions of the human brain. In a typical experiment, a voxel is placed within a region of interest, such as the primary motor cortex (M1) and the concentration of neurochemicals within that region can be assessed with a temporal resolution sufficient to allow detection of phasic changes.
There has been particular interest in using this technique to quantify changes in GABA, the major inhibitory neurotransmitter. Initial studies demonstrated the sensitivity of MRS to detect decreases in GABA within M1 after interventions such as learning,4
ischaemic nerve block5
and transcranial stimulation techniques such as transcranial direct current stimulation (tDCS).6,7
These interventions are all known to increase cortical excitability, and are thought to induce Long-Term Potentiation (LTP)-like plasticity.
In addition to investigating group mean changes in GABA following plasticity-induction paradigms, MRS has more recently been applied to probe the neurochemical factors underlying inter-individual differences in behavioral performance. In a number of distinct regions of the brain a subject's resting GABA concentration is closely correlated with that person's ability to perform a task dependent on that region.1–3
This relationship between GABA concentration and behavior is not observed for control brain regions not thought to be critical for task performance.
As well as shedding light on associations between GABA and steady state behaviors, MRS can also be used to investigate inter-individual differences in subjects' ability to perform dynamic tasks such as learning a novel sequence of motor movements. In our recent study3
we demonstrated a close relationship between the degree to which tDCS decreases GABA in an individual on one day and that subjects' ability to learn a novel task, such that subjects with a more responsive GABA system (i.e., those who showed the greatest decrease in GABA with tDCS) were also subjects who showed the greatest ability to change their behavior through learning. This finding suggests that the ability to decrease GABA within the cortex is important for the early-stages of motor learning to occur in humans.
However, despite these promising results, it is not yet clear how directly MRS measures of GABA relate to synaptic activity; nor how accurately we can assess the relative contributions of GABA and glutamate using MRS.