Research continues at Purdue 

More recently, a remarkable series of eight studies [31-38] by Purdue scientists as part of an ongoing study of brain changes in high school football players, made a host of significant findings:

• the number of head impacts was related to substantive neurophysiological changes during the course of a football season, with all players sustaining more than 500 cumulative head impacts "flagged" for scoring more poorly on at least one component of the ImPACT neurocognitive test compared to their baseline, and/or displaying a statistically significant difference between pre- and post-season fMRI scans on 11 or more of 116 "regions of interest" in the brain;[31]
• high magnitude hits (over 60 g's of linear force) players accrued over the course of a season were more likely to prompt abnormal biochemical/metabolic responses in regions of the brain responsible for executive and motor function, with abnormal increases in metabolites sometimes followed by metabolic decreases, all dependent on the timing, number, magnitude, and location of blows to the helmet. The findings led the researchers to conclude that, with such "diverse metabolic consequences to accumulating sub-concussive blows, such competing mechanisms could (1) lead to no noticeable differences in overall metabolic levels and (2) ultimately mask symptoms in injured athletes," and provided "further evidence for a cumulative effect of head blows on neural health."^[32]
• players who sustained more than 900 hits over the course of a season were much more likely than players hit less than 600 times in a season to be flagged by ImPACT, fMRI, or both.[33]
• players who averaged more than 50 head impacts per week (coincidentally, the typical number of plays a high school football offense or defense ran in a game) were flagged at a rate of 83% while those who received less than 50 hits per week were only flagged 43% of the time, a threshold the researchers considered significant.[33]
• when tested between 2 and 5 months after the football season ended, 6 out of 10 players had results which were flagged as abnormal, 11 by ImPACT, 12 by fMRI, with 3 flagged by both.[33]  The findings led the researchers to conclude that using a neurocognitive test such as ImPACT, more commonly used by clinicians in measuring the effects of concussion and assisting in making return to play decisions, in combination with fMRI, which is sensitive to more subtle changes in brain physiology that may not be exhibited in cognitive performance, may provide a better assessments of a player's brain health than either measure by itself. 
• where on the helmet a player was hit most rather than the number of hits was the best predictor of changes in the brain, suggesting that a player's style of play may be particularly important in determining brain changes resulting from subconcussive impacts.[34]
• abnormal brain activation patterns while players performed tasks involving visual working memory appeared to be related to exposure to contact: after several months of play, the players exhibited a high rate of deviation from their respective pre-season measures of brain activation, with the amount of abnormal activity increasing during the primary months of contact (August-October), only beginning to drop more than two months after the season ended (October/November), and not returning to baseline again until February-April. [35]    they said, as it suggested that, "even at sub-concussive levels of head impacts, there is neural reorganization and no true return to 'normal,' which, in turn, suggests that neural plasticity could be acting as a compensatory mechanism to keep football players asymptomatic."   As Thomas Talavage, a professor of electrical and computer engineering and biomedical engineering and co-director of the Purdue MRI facility, told Purdue News, [11] "The brain is pretty amazing at covering up a lot of changes. Some of these kids have no outward symptoms, but we can see their brains have rewired themselves to skp around the parts that are affected."
• athletes exposed to RHI exhibited significant abnormalities in the white matter of the brain during the season which increased as the season wore on, and persisted after the season. [38] Interestingly, the data suggested that the greater number of lesser intensity collisions experienced by the members of one football team resulted in injury at the cellular level (inflammation of the axons, which are like cables woven throughout brain tissue), while a lesser number of high intensity collisions experienced by the second team may have been more injurious to the fiber structure of the brain.  While the researchers said it was an "open question" whether one or the other may be of more clinical significance, the bottom line was the same: that the injury to the white matter of the brain was "slowly accumulating, with magnitude and number of events affecting the nature of the observed changes."