Zapping the ear’s vagus nerve might boost motivation efficiency in severe depression

Non-invasive stimulation of the vagus nerve may help people with severe depressive symptoms allocate their effort more efficiently, according to a study published in the Journal of Affective Disorders.

Major depressive disorder can involve more than persistent sadness. Many people experience reduced pleasure and motivation, making it difficult to decide whether an activity is worth the energy it requires.

Inefficient effort is not limited to doing too little. Previous research suggests that some people with depression may also expend substantial effort when it provides no additional benefit.

Vagus nerve stimulation may provide one way of influencing these processes. The vagus nerve carries signals between the brain and major organs and is connected with brain systems involved in mood, reward, and motivation. Although surgically implanted stimulators are used for some cases of treatment-resistant depression, transcutaneous, meaning through the skin, vagus nerve stimulation, or tVNS, delivers mild electrical pulses through the ear without surgery.

Led by Paul A.G. Forbes of Goethe University Frankfurt, the researchers studied 98 adults with an average age of 35.2 years, including 63 females. Fifty-three participants had major depressive disorder, while 45 were non-depressed controls.

Free daily newsletter

Each participant attended two sessions in a randomly assigned order. During one, stimulation was delivered to an area of the right ear connected to the vagus nerve. During the other, a sham, or placebo, stimulation was applied to the earlobe as a baseline for comparison. Participants were not told which condition they were receiving, and tests indicated that the blinding process was successful.

Participants then completed 54 trials involving a hand-grip device. On each trial, they selected an easy squeeze requiring 50 percent of their maximum grip strength or a hard squeeze requiring 90 percent. Sometimes working harder produced a larger payment; at other times, both choices produced the same payment.

An efficient participant would therefore choose the harder squeeze when it earned more money but conserve energy when both options paid equally. This allowed the researchers to distinguish reluctance to make a worthwhile effort from unnecessary effort that provided no additional reward.

The primary analysis found no significant evidence that tVNS affected participants with depression differently from controls.

However, secondary analyses suggested that symptom severity mattered. Among the 19 participants with severe symptoms, the estimated proportion of efficient choices increased from 89.4 percent during sham stimulation to 94.6 percent during tVNS. No corresponding improvement occurred among participants with mild to moderate symptoms, and the stimulation actually increased unnecessary effort among those with no or minimal symptoms.

The improvement in the severely depressed group was not caused by a greater willingness to work for larger rewards. Instead, these severely affected participants became less likely to select the harder squeeze when it paid exactly the same as the easy choice. Their estimated rate of unnecessary high-effort choices declined from 10 percent under sham stimulation to 1.8 percent under tVNS.

As the researchers summarized, short-term tVNS improved reward-effort efficiency in patients with severe depressive symptoms. They proposed that stimulation might help people adjust their effort more flexibly as potential rewards change.

Caution is required when interpreting these results. The findings were based on secondary analyses of a small subgroup of only 19 severely depressed participants, which the researchers noted increases the likelihood of a false-positive result. Furthermore, participants completed an artificial task during a single session, so the study cannot show that stimulation improves everyday motivation, reduces depression, or produces lasting benefits.

The study, “Transcutaneous vagus nerve stimulation enhances reward-effort efficiency in severe major depressive disorder,” was authored by Paul A.G. Forbes, Emily Brandt, Mareike Aichholzer, Carmen Uckermark, Aicha Bouzouina, Leona Jacobsen, Jonathan Repple, Jonathan Kingslake, Christine Reif-Leonhard, Andreas Reif, Carmen Schiweck, and Sharmili Edwin Thanarajah.