to examine the relationship between repeated concussions and late decline of brain function. In addition, all participants agreeing to participate in the study will be asked to will their brains to The Krembil Neuroscience Centre Concussion Project at the Toronto Western Hospital with the consent and full knowledge of their families and doctors. However, it is possible to participate in the research without agreeing to a brain donation.

The Project Team is specifically attempting a clinical-MRI-brain tissue research analysis to determine the exact mechanism of the damage to brain tissue following repeated concussions. This condition is known as chronic traumatic encephalopathy (CTE), and shows an abnormal protein in the brain called tau-protein.

We hypothesize that repeated concussions in retired professional athletes will be associated with abnormalities in neurological, neuropsychological, biofluids and neuroimaging assessments. The primary objectives of the study are:

1. To determine the effects of repeated concussions on neurological, neuropsychological and psychosocial functioning.
2. To determine the relationship between repeated concussions and neuroanatomical abnormalities in brain gray and white matter subserving cognitive and motor functions using structural MRI assessment.
3. To determine the relationship between repeated concussions and changes in the cerebrospinal fluid composition.
4. To (i) establish the infrastructure to conduct ongoing pathological examination of donated brains.
5. In order to investigate whether the effects of repeated concussions on brain function and brain structure are progressive and related to the condition known as Chronic Traumatic Encephalopathy, prospective, longitudinal, follow-up data will be collected

Inclusion Criteria:

fluent in English, and a history of multiple concussions , low exposures , or no concussions (Control group ). Determination of concussion exposure will be determined as follows: Previous concussion will be based on the player's recall of injury and defined as "an injury resulting from a blow to the head that caused an alteration in mental status and one or more of the following symptoms: headache, nausea, vomiting, dizziness/balance problems, fatigue, trouble sleeping, drowsiness, sensitivity to light or noise, blurred vision, difficulty remembering, and difficulty concentrating.

Exclusion Criteria:

Neurological disorders prior to concussions (e.g.: seizure disorder); systemic illnesses known to affect the brain (e.g., diabetes and lupus); a history of psychotic disorder; known developmental disorders (e.g., attention deficit disorder, dyslexia); history of migraines; and, active engagement in litigation.

Retired professional athletes with history or multiple concussions and healthy control group without any history of concussion