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.