The study also suggests changes in blood vessels in the eye could be linked to the development of dementia.
Scientists say scanning for the spots and examining the eye's blood vessels could be a "valuable tool in Alzheimer's disease monitoring".
David Allamby, laser eye surgeon and medical director of London's Focus Clinic said: "The eyes have long been thought of as a window to the body, where the health of the eye is a reflection of the general health of the person.
"You can often detect the early signs of heart disease, diabetes and even brain tumour through a simple eye test.
"And the eye's retina is part of the central nervous system, and shares many structural and functional features with the brain.
"So examining the eye could therefore be a monitoring tool for the changes in the brain also."
The study was led by Queen's University's Dr Imre Lengyel, whose team scanned the eyes of 117 patients aged between 60 and 92.
A quarter of people with Alzheimer's have the yellow spots
He discovered 25 per cent of patients with Alzheimer's have the yellow spots behind their eyes. While hard drusen are considered to be harmless, soft drusen are known to be a risk factor for developing age-related macular degeneration, which is vision damaging.
Researchers also found people with Alzheimer's have thicker blood vessels in their eyes, slowing blood flow.
They say this could be a warning sign for vascular dementia, in which the brain declines because of reduced blood flow.
Eye scanning cheaper than brain scans
Dr Lengyel said: "A brain disease is difficult to diagnose through the eye.
"But we can use ophthalmologic imaging to chart progression of the disease and monitor the effectiveness of medications used to combat it.
"It's also about finding cheaper methodologies.
"Brain scanning costs thousands of pounds per scan. And you can't perform scans often because they use radioactive tracers which can be harmful to the body.
"An eye imaging session would comprise 20 seconds of time and is very non-invasive.
"It's exciting. We really hope that eye imagining will be a powerful tool in monitoring Alzheimer's progression."
There are 850,000 people with dementia in the UK, and numbers are predicted to rise to more than a million by 2025, according to the Alzheimer's Society.
Alzheimer's disease is the most common type of dementia, affecting 62 per cent of those diagnosed.
Other types of dementia include vascular dementia, which affects 17 per cent of patients, and mixed dementia which affects 10 per cent of those diagnosed.
How the research was carried out
Dr Lengyel and his team used state-of-the-art ultra-widefield (UWF) retinal imaging scanning equipment on a group of 117 patients from west London.
They ranged in age from 60 to 92, and 56 of them had Alzheimer's while 48 acted as "controls" for the experiment.
He discovered that one in four of the Alzheimer's patients had hard drusen, but only one in 25 of those without the disease had them.
Dr Lengyel said: "At the two-year follow up examination conducted on those who were still able to comply, we found that there were more areas associated with drusen deposition in Alzheimer's disease.
"But we also observed a consistently higher increase in drusen numbers in all patients when compared to controls."
Thicker blood vessels could point to bad blood flow to the brain
In his study Dr Lengyel also found people with Alzheimer's have thicker blood vessels in their eyes.
He revealed: "The team were able to use imaging to measure the thickness of the vessels in the eye.
"What they found was that in Alzheimer's disease patients there are measurably thicker vessels.
"And if the vessels widen, then the blood flow slows down.
"Our gut feeling is that the eye is mirroring what's occurring in the brain. Although it's not exactly the same, it's the same type of change."
Writing in the journal Ophthalmic Research he added: "If the observations made in this initial study remain robust, then routine UWF imaging might achieve an earlier and thereby clinically opportune and cost-effective method for monitoring the progression of Alzheimer's disease and identifying individuals at a high risk of developing Alzheimer's disease."
Symptoms of dementia include memory loss, confusion and problems with speech and understanding. Dementia is a terminal condition.
Roughly one in six people over the age of 80 have dementia - but there are also more than 40,000 people under 65 with dementia in the UK.
There is currently no cure for Alzheimer's disease, but there is medication available that can temporarily reduce the symptoms.
Early intervention is critical
David Allamby added: 'Previous studies looking at the relationship between the eyes and Alzheimer's had looked at a protein known as beta-amyloid plaque.
"Research showed that elevated levels of this biomarker was a precursor for dementia, with Alzheimer's sufferers having more than twice as much of this protein in their retinas as non-sufferers.
"This new study instead looks at hard drusen as a biomarker, found in a different area of the eye.
"It could prove valuable indeed when it comes to predicting Alzheimer's, particularly as early medical intervention is so critical when it comes to treating this terrible condition."
