Inclusion Criteria:
1. are aged 18-79 years older;
2. have histologically confirmed Stage 0, I, II, or III breast cancer;
3. had completed their primary cancer treatment (surgery, radiotherapy, chemotherapy) and are currently on a stable survivorship care plan (e.g., endocrine therapy, supportive care), with no major treatment changes expected during the study;
4. have experienced pain with a severity of 4 or greater out of 10 for at least ten days in the last month;
5. are committed to maintaining the current treatment plan (e.g., endocrine therapy, supportive care) during the study;
6. have reliable internet access;
7. are willing to provide stool samples and undergo fNIRS brain imaging procedures;
8. are able to read and understand English and provide written informed consent.
Exclusion Criteria:
1. have metastatic breast cancer (Stage IV);
2. have a current diagnosis of another active cancer;
3. have a history of significant cardiac conditions, such as bradycardia, arrhythmia, recent myocardial infarction, or heart failure;
4. have been diagnosed with a severe psychiatric illness (e.g., schizophrenia, bipolar I disorder with active psychosis) that could interfere with adherence to study procedures;
5. have active inflammatory or malabsorptive gastrointestinal disorders (e.g., Crohn's disease, ulcerative colitis, celiac disease) that could confound gut microbiota results;
6. have taken antibiotics, probiotics, or gastrointestinal motility agents (e.g., laxatives, prokinetics) within the past 3 months, due to potential disruption of gut microbiota;
7. have a progressive neurological condition (e.g., Parkinson's disease, epilepsy, multiple sclerosis) that may impact fNIRS data quality or study participation;
8. have a history of surgical or pharmacological vagotomy or are currently receiving implanted vagus nerve stimulation therapy, because of potential interference with autonomic regulation and taVNS mechanisms;
9. have an active electronic or metallic implant (e.g., cochlear implant, pacemaker, neurostimulator) or other electronic/metallic device in the head or neck area, which may be contraindicated for taVNS;
10. are pregnant, breastfeeding, or planning to become pregnant during the study period;
11. have had a recent initiation or dose change of pain medications (e.g., opioids, neuropathic agents) within the past 4 weeks.
