Scientists have suggested that losing interest in hobbies and pastimes in old age could be a sign of dementia.
Researchers discovered the number of certain neurons in the brain would shrink in apathetic people who weren't depressed.
These neurons, known as grey and white matter, are present in the memory and communication regions of the brain respectively.
Dr Lenore J Launer, lead author of the study, said: "Just as signs of memory loss may signal brain changes related to brain disease apathy may indicate underlying changes.
"Apathy symptoms are common in older people without dementia. And the fact participants in our study had apathy without depression should turn our attention to how apathy alone could indicate brain disease."
Neuron loss is common as people get older, but mass decrease can signal brain disease, so this new research can make it easier for medical professionals to detect dementia at an earlier stage.
More than 4,300 individuals with a combined age of 76 were analysed for the study, all of which were given an MRI scan.
That was followed by a questionnaire designed to measure apathy symptoms, including lack of interest, emotion, energy and generally dropping activities and other interests.
The report, published in the journal Neurology, concluded those who displayed at least two of the symptoms had an average of 1.4 per cent less grey matter in their brain, as well as 1.6 per cent less white matter.
Dr Launer said: "If these findings are confirmed, identifying people with apathy earlier may be one way to target an at-risk group."
Approximately 820,000 people in the UK have been diagnosed with dementia, while 25 million people in the country know a close family member or friend with the disease.
It is estimated that it costs the economy around £23 billion, more than heart disease and cancer combined.
Find out about dementia care and support services at Barchester care homes.