However, the 77-year-old betrayed no weakness of mind as he drew upon his experience of soil science to tell assembled politicians, doctors and academics that they needed to look far and wide for the causes of dementia.
''I believe we must ask the right questions to get the right answers ... sciences are all interrelated, and the challenge has to be for a major shift in scientific thinking,'' Mr Thompson said.
Mr Thompson's wife's family has a history of Alzheimer's disease, which made the decision to take part in the trial an easy one, he said.
''I hope to be able to benefit my fellow man,'' he said.
Mr Thompson's experiences in the programme so far have included four hour-long question-and-answer sessions and a range of scans.
The doctor putting him through the programme, clinic director Nick Cutfield, said being able to predict the onset of dementia meant therapies to treat those conditions could be used earlier, enhancing a patient's quality of life.
Study participants will trial a range of therapies and lifestyle changes to gauge their effect on dementia.
''Some of the things that can delay the onset of dementia include leading a healthy lifestyle, including physical activity and mental activity, ensuring that if you have reduced hearing it is treated effectively, and social engagement,'' Dr Cutfield said.
Dementia affects more than 55,000 New Zealanders, and it is estimated 40 people per day are diagnosed.
Each year about 26,000 people develop memory impairment, and dementia is expected to affect up to 150,000 New Zealanders by 2050.
