A considerable amount of evidence suggests the nucleus accumbens, located close to the bottom of the brain, as a key player in many functions such as aversion, reward, reinforcement and, in the case of this study also, in motivated performance. The researchers involved understood, as many do, that individuals vary a great deal in their ability and capacity to undertake motivated behaviour and that motivational problems such as apathy are typical in neurodegenerative and psychiatric disorders. However not much is known about the neurochemical underpinnings of individual differences in motivation, and so, given previous knowledge, these researchers decided to target the nucleus accumbens.
In order to assess and quantify motivation the team devised what is known as a “monetary incentive force (MIF) task”, where the aim was a ‘do more and get paid more’ type of method. Individuals participating would carry out a task that would increase in effort (which was measurable) and would be paid amounts of money corresponding to their effort.
Individuals were also scanned with 1H-MRS. 1H-MRS (1H magnetic resonance spectroscopy, also called “proton magnetic resonance spectroscopy”) is an advanced brain-imaging technique that can selectively measure the concentration of neurochemicals, like metabolites and neurotransmitters, in the brain; it is also used in clinical settings to ascertain neurological disorders. 1H-MRS was applied at ultra-high-field in the nucleus accumbens of 43 men to measure metabolites – specifically to inquire whether levels of glutamate (Glu), glutamine (Gln), GABA or their ratios predict individual differences in the effort-based motivated task performance.
Participants were instructed to squeeze a hand-held dynamometer, which measures grip and hand strength, to a required threshold for them to get either 0.2, 0.5, or 1 Swiss franc. This was repeated for 120 consecutive trials, which made carrying out the task a bit challenging. The point of the test was that the varying amounts of money would drive the individuals to determine whether they would invest energy and carry out the task as asked at each trial. The team also carried out the test under isolation and in group conditions to examine differences in performance under self-motivated or competition settings.
Since performance is the result of numerous cognitive, motor and physiological processes, the team applied computational modelling to estimate the best-fitting parameters that should be measured regarding utility, effort and performance functions. This meant they were able to interrogate if specific neurotransmitter amounts predicted certain motivational functions. The model-based analysis showed that the driving force of performance – and ultimately motivation – is found in the ratio of two neurotransmitters in the nucleus accumbens: glutamine and glutamate.
The analysis showed that accumbal Gln-to-Glu ratio specifically relates to stamina, i.e. the capacity to maintain performance over long periods. There were positive correlations between Gln and Gln-to-Glu ratios, respectively, and success rate, while negative correlations between Gln and Gln-to-Glu, respectively, and effort perception. So, glutamine and Gln-to-Glu ratio related to better performance and reduced effort perception. The analysis also discovered that competition seemed to boost performance from the onset of the task, especially for participants with low Gln-to-Glu in their nucleus accumbens.
The team’s findings give new insights within the field of motivation neuroscience, by implicating accumbal Gln and Glu balance on the prediction of specific computational components of motivated performance. This approach and results can help developing therapeutic strategies, which includes nutritional interventions, based on targeting metabolism to combat deficits in effort engagement.
Original source: A Strasser, G Luksys, L Xin, M Pessiglione, R Gruetter, and C Sandi. “Glutamine-to-glutamate ratio in the nucleus accumbens predicts effort-based motivated performance in humans”. Neuropsychopharmacology. (July 2020).