A study involving Gareth Southgate which is examining whether heading and concussion increase the risk of dementia among professional players is around 100 participants short of its ideal target, one of its leaders has said.

Participants, who include England manager Southgate, are asked to perform a number of tests of cognitive function, including recalling elements from a short story and matching faces to names and jobs.

They will also be asked to report on any incidents of concussion during their careers and provide information which will help researchers gauge how frequently they headed the ball in matches and training.

Any member of the Professional Footballers' Association aged 50 or over is eligible to take part, with all the assessments conducted via the telephone or online, but so far the study leaders have only got around 200 of the 300 participants they would ideally like.

He said the study team will be looking for "subtle differences" in how individuals perform.

"It's generally accepted that if you are going to have problems like Alzheimer's later in life, it will begin to show up at an earlier age, that you might score a little bit less on some of these cognitive function tests," he said.

"It doesn't mean you're destined to get Alzheimer's, it just means you're slightly higher risk. The idea is that we can see if there's a problem in the 50-60-70 age group that might predict serious problems later in life."

Professor Pearce said individuals could come forward if they believed they were eligible but had not yet been contacted – the researchers would just need to check that the ex-player was on the PFA's database.

The study will be the first that asks participants to provide information about heading. Performance on the cognitive function tests can be compared to age-matched members of the general population.

"We're not just asking people how many times they head the ball," Professor Pearce said.

"We might ask that, but we ask for a lot of other information about their playing career, what leagues they were in, what position they played, about heading in training and so on. And then we put the data together.

"So it's not like someone says, 'I headed the ball so many times' and that's the value we use – we would use the value for everyone else who has the same sort of characteristics and the same sort of playing career. We believe that we can get reasonable estimates, at least (grouping people) into high, medium and low frequency of how much they head the ball."