Traumatic brain injury (TBI) is the most common cause of death and disability in young adults. Patients can experience significant problems with concentration, attention, and memory (so called 'cognitive impairments') following TBI. These cognitive impairments can drastically impact on a patient's well-being, and can lead to significant economic and social consequences. Roughly a quarter of TBI patients improve but an equal number deteriorate over time. The investigators know little about why patients vary so much in how they recover. Crucially, the investigators have no treatments to improve brain functioning or recovery after TBI.