Individuals with Alzheimer's have amyloid beta plaques built up in their brains in addition to the atrophy of the brain. The plaques have been a primary focus of much of the research into the disease. However, one of the great mysteries of Alzheimer's is how individuals who have lots of amyloid beta plaques in their brain can have normal cognitive functions and little if any atrophy. When this was discovered, it provided another path of study to examine.
Brain samples needed to be collected from individuals with Alzheimer's disease and those without symptoms of Alzheimer's prior to their death. The samples from the apparently healthy individuals had to be examined to find those with high concentrations of amyloid beta. Once those were identified, scientists could then look to see what else was different between the two groups of samples.
Recent findings indicate that there is a difference in the concentration of oligomers in the two groups. An oligomer is a large molecule consisting of a very few bonded molecules of the same type. There are many different types of oligomers such as those found in oils, proteins or nucleic acids. In this case, the oligomers are actually amyloid beta molecules which have bonded together in small groups but have not bonded enough to form plaques. The individuals who had been diagnosed with Alzheimer's had amyloid beta oligomers in their brain in addition to the plaques.
Just one more piece of the puzzle.