Michael Valenzuela of the school of psychiatry at the University of NSW has revealed that the finding results from an analysis of the brain scans used during the study.
He says that the finding sheds more light on the link between lifestyle and dementia.
The results of the study also add strength to the evidence that mental exercises, like puzzles and new languages, stave off ageing diseases.
"We've got strong evidence here that people who use their brains more have less brain shrinkage," the Australian quoted Valenzuela as saying.
"I hope people take this as a further call to arms to get out there and use their brains, get engaged in anything from tai chi to world travel, in the knowledge that it may help delay or prevent the onset of dementia," he added.
Valenzuela and his colleagues wanted to determine how mental activity delayed the onset of the degenerative brain diseases, such as Huntington's, Alzheimer's and Parkinson's.
For the purpose, they studied the brains of 60-year-olds over three years, and tested their lifetime mental agility with questionnaires.
The researchers discovered that, of the 50 subjects, people who had been more mentally active over their lives had a larger hippocampus, an important memory centre in the brain.
Among such subjects, the area shrank at half the rate of those who had lower mental activity over the period of three years.
"This is a significant finding because a small hippocampus is a specific risk factor for developing Alzheimer's disease," said Dr. Valenzuela.
Based on his observations, he came to the conclusion that people could prevent themselves from the shrinkage of the hippocampus.
"Our prior research shows the risk for dementia is quite malleable, even into late life," he said.
An article on the study has been published in the journal PLoS ONE.