This clinical trial aims to evaluate whether transcutaneous auricular vagus nerve stimulation (taVNS), a non-invasive brain stimulation method, can improve attention and memory in veterans with traumatic brain injury (TBI) and depression and/or posttraumatic stress disorder (PTSD). The study seeks to answer two main questions:

1. Can active taVNS improve attention and memory compared to sham (placebo) stimulation?
2. Does taVNS affect heart rate variability (HRV)?

taVNS delivers a gentle electrical current to the vagus nerve through electrodes placed on the ear, targeting brain areas involved in attention and memory without requiring surgery.

This study uses a crossover design, meaning all participants will experience two sessions: one with active taVNS and one with sham stimulation. The sham session feels similar but does not deliver actual stimulation, allowing researchers to compare the two and understand taVNS's effects on the brain.

In a single visit, participants will:

• Complete eligibility screening (questionnaires and vital signs).
• Undergo two sessions (one active and one sham), randomly assigned.
• Perform attention tasks before and after each session.
• Have their heart rate monitored during the sessions.

The findings will help determine whether taVNS could be an effective treatment for improving attention and memory in veterans with TBI.

Attention and memory problems are common in veterans with traumatic brain injuries (TBI), particularly when accompanied by depression and/or posttraumatic stress disorder (PTSD). Current treatments for these issues often have limited effectiveness or unwanted side effects. Transcutaneous auricular vagus nerve stimulation (taVNS) is a non-invasive technique that uses gentle electrical impulses delivered through electrodes placed on the ear. This stimulation targets the vagus nerve, which plays a key role in regulating brain areas involved in attention and memory. By enhancing the activity of these brain regions, taVNS shows potential as a novel treatment option for improving cognitive functions like attention and memory.

In this study, researchers aim to determine whether taVNS can enhance attention and memory in veterans with TBI. Participants will complete a single study visit lasting 2.5 to 3 hours. The visit includes two taVNS sessions, one active and one sham (placebo), to compare their effects. Sham stimulation mimics the sensation of taVNS but does not deliver electrical current to the vagus nerve. This design allows researchers to isolate the specific effects of taVNS.

Procedures:

1. Screening (40 minutes):

   Participants will begin by completing questionnaires about their medical history, memory concerns, alcohol and substance use, mood, and demographics. Vital signs (heart rate, blood pressure, and temperature) will also be measured to confirm eligibility.

2. Session 1 (approximately 40 minutes):

   Participants will complete a computer-based attention task that involves responding to specific visual cues on a screen. This task takes about 10 minutes. taVNS will then be delivered using electrodes placed on the left ear. For this session, participants will either receive active stimulation or sham stimulation, randomly assigned. The stimulation lasts 20 minutes. After the stimulation, participants will repeat the attention task to assess changes in performance.

3. Break:

   Participants will have a short break between sessions, during which they can rest or engage in light activities.

4. Session 2 (approximately 40 minutes):

The same sequence of procedures will be repeated, but participants will receive the alternate type of stimulation (active or sham) in this session. The order of active and sham stimulation is randomly assigned, and participants will not know which session is delivered first.

Throughout both sessions, participants' heart rate variability (HRV) will be monitored using a lightweight chest device. HRV provides insights into how the body responds to stimulation and helps evaluate taVNS's physiological effects.

Inclusion Criteria:

• Ages 25-64
• Right-handedness
• Veterans with a history of deployment to Operation Iraqi Freedom (OIF), Operation Enduring Freedom (OEF), Operation New Dawn (OND) or other post 9/11 war on terrorism
• History of PTSD and/or depression
• Military related mild traumatic brain injury
• If taking psychotropic medication, demonstrate stability for 3 months
• If taking stimulants, washout period of 12 hours

Exclusion Criteria:

• History of neurological, cardiovascular, or pulmonary disease
• Cardiac arrhythmia (all types)
• Active suicidal ideation
• Visible wounds on skin of the left ear
• Medical implants such as cardiac defibrillators, pacemakers, or deep brain stimulators
• Pregnancy
• Completed taVNS in the past 4 weeks
• Current substance use disorder (exception: mild cannabis use disorder allowed)
• Current moderate or severe alcohol use disorder
• Major cognitive disorder

Individual participant data (IPD) will not be shared because this is an exploratory, feasibility trial designed to assess the potential effects of transcutaneous auricular vagus nerve stimulation (taVNS). As a pilot study, the primary goal is to evaluate the intervention's feasibility and gather preliminary data, rather than generate findings intended for broader generalization. To protect participant privacy and confidentiality, all data will be reported in aggregate form without identifying individual participants, in accordance with ethical guidelines and institutional policies.