It found that players who experienced one or more concussions in their career performed worse in tests that measure cognitive flexibility, complex attention, executive function and processing speed. To put it in layman's terms, that is the ability to understand and process information quickly, to make rapid decisions, to switch attention between tasks and to track and respond to information over long periods of time.
"The problem is we still don't know who's going to get [the cognitive difficulties]," King said.
Work with biomarkers might assist in that process but King says researchers are probably five years away from any definitive answers.
The paper is just the first of several expected from the rugby health study.
New Zealand Rugby has paired up with Statistics NZ to try to determine whether the rates of dementia are higher among elite rugby players between 1950 and 1970 than non-players; and Otago NPC players will wear electronic devices, about the size of a $2 coin, above their ear during their next five home games to measure the impact on players' heads during games.
"That could mean they [rugby administrators] get a surprise just how many head impacts there are," King said. The emergency nurse said he had been "ridiculed" in various quarters for his research that demonstrated that there were many concussions during a game that had no visible effects.
In the first year of his study he saw five "visible concussions" but his testing picked up a further 17 that were unknown at the time. In the second year that figure was two visible concussions and 20 other failed concussion tests.
NZ Rugby's chief medical director Ian Murphy said the research out yesterday has underscored the importance of preventative training and playing techniques.
Murphy said it strengthened the argument for further research.
"While the majority of the tests indicated the average values for all three former player groups were within normal ranges ... the new information provides us with more reliable data to work through as we seek to better understand concussion," Murphy said.
"The research supports the importance of education and promotion of correct training and playing techniques as a preventative measure, which we've been leading for some time."
The publication of the findings came on the same day Cillian Willis became the first professional rugby player to sue for clinical negligence. The halfback is taking action against Sale Sharks over two head injuries he suffered in a game against Swansea in March last year.
Willis, a cousin of legendary centre Brian O'Driscoll, received treatment for a head injury before returning to the game and suffering a second blow. He was replaced and has never played professionally again, retiring at 28. Willis' lawyer described it as a landmark case for the sport.
The story so far
• In March, the Herald reveals five players from the 1964 Taranaki Ranfurly Shield team, including All Blacks Neil Wolfe and Ross Brown, had either died with or were suffering from dementia.
• Over the next week, a number of All Blacks and their families come forward to share their dementia/ CTE stories, including Greg Rowlands, Waka Nathan, Tony Steel, Sandy McNicol and Geoff Old.
• NZ Rugby chief executive Steve Tew responds: "It is a complicated issue and even the highly skilled and trained medical professionals cannot give you a definitive answer on a whole load of really important questions."
• In June, the Herald reveals that the AUT-led, World Rugby-funded research into the health of retired rugby players had created tension, with lead researcher Patria Hume saying: "World Rugby and NZ Rugby did not give approval for the full report and all the results to be released". NZR and WR reject the contention that findings from the study were sanitised.
• NZR partners with Statistics NZ in research to try to determine whether elite rugby players from 1950-70 were more likely to suffer from dementia-related illnesses than non-rugby playing people.
• August 25: A paper in Sports Medicine reveals rugby players with a history of concussion are more susceptible to "cognitive difficulties" when they retire.
