A 21-year-old woman incurs a blow to her head from a fall while mountain biking. She then has loss of consciousness for 5 minutes. On examination her deep tendon reflexes are diminished. A head CT scan 6 hours later shows no abnormalities. She recovers over the next week, with no neurologic deficits, but cannot remember this event. During the next year she has irritability, headache, difficulty sleeping, trouble concentrating, and fatigue. Which of the following is the most likely consequence from her injury?