A 42-year-old woman from Mumbai presents with chronic lower back pain radiating to both legs, which worsens with activity and improves with rest. She reports that her pain is significantly relieved within 30 minutes of taking a combination of amitriptyline (a tricyclic antidepressant) and gabapentin, even though the pain typically returns after 4–6 hours. Interestingly, she notes that distraction (watching television or engaging in conversation) also provides temporary relief. Which of the following best explains the mechanism by which both pharmacological and psychological interventions provide pain relief in this patient?