Munich study, 31 volunteers: prefrontal tDCS cut reaction time to 0.98 s but increased fatigue

Researchers from the University of Munich (Germany) stimulated the dorsolateral prefrontal cortex, a brain area associated with mental effort, with direct current in 31 healthy volunteers. As reported by Nauka w Polsce, citing the journal PNAS, stimulation improved performance on a difficult memory task but also led to greater mental fatigue afterward naukawpolsce.plNauka w Polscenaukawpolsce.plOpen source ↗.

Transcranial direct current stimulation (tDCS) is a noninvasive method of brain neuromodulation. Electrodes are placed on the scalp and a very weak direct current, usually 0.5–2 mA, is passed for a period of time. The method can raise or lower the excitability of specific brain regions.

Experiment design

tDCS is studied and used as a supportive therapy in depression, ADHD, concentration problems, stroke rehabilitation, epilepsy, neurodegenerative diseases and chronic pain. The procedure is painless; a person may feel slight itching, tingling or warmth under the electrodes.

In the Munich work, electrodes were placed on the heads of 31 volunteers with no health problems. During a half-hour session in an MRI scanner, participants spent 20 minutes on a difficult "3-back" memory test, which requires remembering and identifying an item that appeared three positions earlier in a sequence. Afterwards they were offered 1 euro for an easy task or 2 euros for a harder one. Randomly chosen volunteers were stimulated during the test and did not know their brains were being stimulated; the comparison group received sham (placebo) stimulation naukawpolsce.plNauka w Polscenaukawpolsce.plOpen source ↗.

Results and glutamate

Stimulated participants did better on the hard tasks, responding faster and more accurately than those who were not stimulated. Their mean reaction time was 0.98 s, against 1.12 s in the sham group. However, the stimulated participants later also reported greater mental fatigue.

After the tests, magnetic resonance spectroscopy, a noninvasive technique, showed a higher level of the neurotransmitter glutamate in the dorsolateral prefrontal cortex of people who received tDCS. The higher the glutamate level, the more likely the participants were to choose the 1-euro task over the 2-euro one.

The authors of the study (DOI: 10.1073/pnas.2534136123) say this may indicate that mental fatigue results from an accumulation of glutamate, which in high concentrations can be harmful to cells. They conclude that when mental fatigue is felt, taking a break is worthwhile naukawpolsce.plNauka w Polscenaukawpolsce.plOpen source ↗.