An early symptom of B12 deficiency is anaemia, but taking large amounts of folic acid treats the anaemia without treating the B12 deficiency.

BBC: Vitamin B 'puts off Alzheimer's'

But Dr Robert Green, who led the study, suggested depression could adversely affect the brain or could be a very early symptom of the disease.

BBC: Clue to old age memory loss

Failure to develop this trick is an early symptom of autism, a syndrome whose main underlying feature is an inability to understand that other people have minds, too.

ECONOMIST: Now, it seems, even the bird-brained have theories of mind

Short-term memory loss is the hallmark early symptom of Alzheimer's.

CNN: Aging workforce means dementia on the job could rise

Cognitive decline can be an early symptom of dementia.

BBC: Hearing loss in 'brain decline link'