Research Frontiers Auricular Vagus Nerve Stimulation and Anhedonia

Introduction: The Research Question

Anhedonia refers to a marked reduction or loss of the ability to experience pleasure, interest, or enjoyment. It often appears as diminished interest in ordinarily rewarding activities such as food, social interaction, entertainment, sex, or achievement. In depressive states, people may also find it harder to invest effort for a potential reward, learn from feedback, or make positive choices when the payoff is uncertain. A 2026 study published in the Journal of Psychiatric Research examined whether transcutaneous auricular vagus nerve stimulation (taVNS) affects these distinct reward-related processes, while also measuring heart rate variability (HRV) and self-reported anxiety.

This was a single-session, randomized crossover study. taVNS had a clearer effect on willingness to exert effort for rewards among participants with higher depressive symptoms. It did not improve reward learning, and autonomic responses varied according to baseline HRV and depressive symptoms. The findings suggest that taVNS effects may depend on physiological and psychological state.

Background: Distinct Components of Anhedonia

Depression-related anhedonia includes several processes, including reward responsiveness, reward learning, reward valuation, and effort-based decision-making. Treating these processes as one general measure of pleasure makes it difficult to determine which component an intervention affects.

The researchers therefore used two behavioral tasks. The Probabilistic Reward Task (PRT) assesses whether participants gradually favor a stimulus that is rewarded more often. The Effort Expenditure for Rewards Task (EEfRT) asks participants to choose between a low-effort option with a smaller reward and a high-effort option with a larger reward. It captures the practical question of whether a reward seems worth the effort.

Vagal pathways are involved in motivational regulation and reward processing, but previous taVNS studies have produced mixed findings. Differences may reflect stimulation parameters, study design, and participants’ baseline autonomic state. Heart rate variability, particularly the root mean square of successive differences (RMSSD) between consecutive heartbeats, is commonly used as a time-domain marker of parasympathetic activity. This study treated baseline HRV as a potential moderator.

Methods: A Randomized Crossover Design

The study included 68 participants, 50 of them women, with a mean age of 23.3 ± 4.35 years. Based on the Beck Depression Inventory-II (BDI-II), 34 participants were assigned to the higher-symptom group (BDI-II ≥ 14) and 34 to the lower-symptom group (BDI-II < 14). These groupings do not constitute clinical diagnoses. The study excluded people with several medical, neurological, or psychiatric conditions and those taking relevant medications.

Each participant received active taVNS on one day and sham stimulation on another, in randomized order, with at least 3 days between sessions. Active stimulation was delivered to the left cymba conchae, while sham stimulation was delivered to the left earlobe. Stimulation used 30 Hz, 0.1 mA, and a 250 µs pulse width, with a 30 s on/30 s off duty cycle for approximately 1 hour. The pre-task stimulation period lasted 15 minutes. Participants could not identify the stimulation condition from their experience.

Electrocardiography was recorded continuously. RMSSD was calculated during a 5-minute baseline period and the pre-task stimulation period. The behavioral battery included the PRT and EEfRT; the latter focused on high-effort choices at 12%, 50%, and 88% reward probabilities. Subjective state was assessed with a Visual Analogue Scale (VAS).

Figure 1. A, overall experimental procedure. VAS, Visual Analogue Scale; ECG, electrocardiogram; HRV, heart rate variability; PRT, Probabilistic Reward Task; EEfRT, Effort Expenditure for Rewards Task. B, example trial in the PRT. C, example trial in the EEfRT.

Results: Changes in Effort Allocation

taVNS did not increase HRV uniformly across participants. HRV analyses included 62 participants, divided by the median baseline RMSSD of 40.4 ms into low-HRV (n = 30) and high-HRV (n = 32) groups. The overall active-versus-sham difference was not significant (F(1,54) = 1.26, P = 0.267). However, the interaction among stimulation condition, baseline HRV stratum, and depressive-symptom group was significant (F(1,54) = 4.73, P = .034, η² = .014). Estimated marginal means suggested that the direction of the response varied across combinations of baseline HRV and depressive symptoms. Because the interaction was small and subgroup sizes were limited, this finding should be viewed as a moderating signal rather than a validated individual classification rule.

In the EEfRT, participants generally selected the high-effort option more often when reward probability and reward magnitude increased. The effect of taVNS appeared in a higher-order interaction involving stimulation condition, depressive-symptom group, reward magnitude, and reward probability (β = 0.93, SE = 0.40, z = 2.32, P = .02). Relative to sham, active taVNS mainly increased high-effort choices among participants with higher depressive symptoms when the probability of winning was low and the reward was large. EEfRT analyses included 64 participants; four were excluded because they did not make the required choices.

Reward learning, as measured by the PRT, did not change significantly. The main effect of stimulation condition was not significant (F(1,61) = 0.51, P = .476, η² = .003), and neither was the condition-by-group interaction (F(1,61) = 0.16, P = .691, η² = .001). Participants reported less anxiety after active stimulation than after sham stimulation (F(1,66) = 7.63, P = .007, η² = .050), with this effect observed in both groups. No significant differences were found for motivation, fatigue, boredom, or optimism.

Figure 2. A,the main effect of stimulation condition on self-reported anxiety measured with the VAS. B, three-way interaction among stimulation condition, depressive-symptom group, and HRV stratum. lnHRV, natural-log-transformed HRV.

Figure 3. Effects of taVNS on the probability of choosing the high-effort option as a function of winning probability and reward magnitude. A, 12% winning probability. B, 50% winning probability. C, 88% winning probability.

Mechanistic Interpretation: State-Dependent Effects

The study places the behavioral effect of taVNS more precisely. Stimulation may influence how people weigh reward magnitude, winning probability, and effort cost, particularly when a reward is uncertain but potentially worthwhile. The pattern does not support a broad improvement in reward learning or subjective pleasure.

The authors propose that effort allocation under uncertain reward may be more modifiable in people with higher depressive symptoms. EEfRT choices, however, cannot demonstrate that taVNS repaired a specific neural pathway or improved clinical symptoms. The HRV findings also support a state-dependent account in which baseline autonomic state may alter the direction of the response. The higher-symptom group unexpectedly had higher baseline HRV than the lower-symptom group, unlike the trend reported in much of the depression literature. The high proportion of women in the sample may have contributed to this pattern.

Future Directions: Toward Replication

Future studies should use larger clinical depression samples, repeated stimulation, and longer follow-up to test whether changes in task-based effort allocation translate into everyday motivation and functioning. They should also preregister stratification factors such as baseline HRV, sex, stimulation intensity, and ear location, while measuring physiological responses during tasks and adding inflammatory or neuroimaging markers. Replication across laboratories and populations will be needed before the potential clinical value of taVNS for anhedonia can be assessed with confidence.

Reference

Grimaldi R, Uzun E, Schettino M, Pizzagalli DA, Treadway MT, Ottaviani C. Targeting Distinct Facets of Anhedonia via Transcutaneous Auricular Vagus Nerve Stimulation: Effects on Heart Rate Variability and Reward-Based Behavior. Journal of Psychiatric Research. 2026. doi:10.1016/j.jpsychires.2026.09.004