A new study on mice may reveal that memories still exist for Alzheimer’s patients but they just cannot recall them for some reason, according to the Washington Post in "Scientists reawaken memory in mice that had a condition resembling Alzheimer's."
The scientists genetically engineered mice, so they would have a condition similar to Alzheimer's. They then introduced a lemon to the mice and gave the mice a small electric shock, so the mice would associate the scent of lemons with being shocked.
When the scientists later gave the mice lemons, the mice did not appear to remember the shock, until the scientists used a laser to stimulate the neuron in the brains of the mice that were associated with the original memory.
When they gave the mice lemons again, the mice did seem to remember the shock and act in anticipation of it.
This suggests that the memories of Alzheimer's patients are not erased, but the patients just are not able to access the memories.
Reference: Washington Post (July 29, 2017) "Scientists reawaken memory in mice that had a condition resembling Alzheimer's."