Some 25 participants took part in the trial, which saw them have to read a written word related to a phrase they heard moments before.
Brain scans showed that slower responses are 'aberrant' in individuals who will go onto develop Alzheimer's disease, the study published in Neuroimage Clinical found.
Dr Katrien Segaert, lead author, said: "Our findings were unexpected as language is usually affected by Alzheimer's in much later stages of the onset of the disease.
"It is possible this breakdown of the brain network associated with language... could be a crucial biomarker used to identify patients likely to develop Alzheimer's."
How was the study carried out?
Participants were healthy elderly people, along with mild cognitive impairment (MCI) patients. They were fitted with EEG (electroencephalogram) skull caps.
Detecting electrical activity in the brain allows researchers to establish how quickly they processed words shown to them on a computer screen.
Previous research has found that when a person is shown a written word, it takes 250 milliseconds for the brain to process it - activity which can be picked up on an EEG.
What do the experts say?
Dr Hilda Hayo, chief executive of Dementia UK, said the charity "welcomes anything that can speed up the process of getting a diagnosis of dementia".
Clare Walton, research manager at Alzheimer's Society, said 'research like this give us hope' of finding a way to diagnose the disease early on.
However, she warned that it was too early to draw any firm conclusions and that larger studies are obviously needed to confirm the theory.
MCI is estimated to be suffered by up to 20 per cent of people aged over 65. It is not a type of dementia, but a person with MCI is more likely to go on to develop it.
Dementia: The facts
Officials estimate there to be around 47 million dementia sufferers across the world, with nearly 10 million new cases reported each year.
In the US, the devastating disease, which is currently incurable, affects more than five million, while 850,000 suffer in the UK.
Diagnosing the disease earlier would offer hope of prescribing drugs when they are more likely to work, and encouraging lifestyle changes that can stave off dementia.
One reason medications have so often failed in trials is they are given to patients when the devastating neurological illness has already taken hold.
TAKE THE TEST TO DETERMINE YOUR RISK OF ALZHEIMER'S...
The research, published in the journal Neuroimage Clinical, revolved around reading a phrase aloud to the 25 participants.
Phrases used in the study included 'a type of wood' and 'a breakfast food' - all others have been devised by MailOnline.
A second later, the volunteers were shown a word that was either congruent (oak, pancake), or just an every day noun.
Those who took longer to recognise if the word was relevant to the phrase, measured by EEG tests, displayed early signs of cognitive impairment.
Say this phrase out loud: A type of breakfast food
Read this word: Pancake
Say this phrase out loud: A type of wood
Read this word: Oak
Say this phrase out loud: A type of metal
Read this word: Copper
Say this phrase out loud: A type of tea
Read this word: Earl Grey
Say this phrase out loud: A type of car
Read this word: Mercedes
Say this phrase out loud: A type of shape
Read this word: Square
Say this phrase out loud: A type of vegetable
Read this word: Broccoli
Say this phrase out loud: A type of fruit
Read this word: Apple
Say this phrase out loud: A type of clothing
Read this word: Jeans
Say this phrase out loud: A type of alcohol
Read this word: Beer
It should be noted that the phrases created by MailOnline may not pose the same scientific accuracy as the ones devised by researchers. The first part of the test is also meant to be conducted aloud.
