We know little on why some TBIs quickly resolve, while others do not. One hint could be a common symptom shared among various chronic pain disorders and TBI: photosensitivity. Patients suffering from chronic pain syndromes often have increased sensitivity to light, as do TBI subjects. Recent research has demonstrated that nociceptive neurons in the brainstem can be activated by light, suggesting extreme photosensitivity and chronic pain are both symptoms of a dysfunctional central pain circuit. If true, photosensitivity thresholds could be used as markers of chronic pain due to altered central pain processing. This could be used not only in subjects with TBI, but other populations susceptible to centralized chronic pain, for example, individuals with Post Traumatic Stress Disorder (PTSD).