Any area of the brain cortex can be the target of Alzheimer disease. The problem is caracterized by two types of depositions in the brain.
The first one are called "amyloid plaques", also known as "senile plaques". These depositions are the result of the aggregation of a protein called beta-amyloid between the neurons.
The other one is termed "neurofibrillary tangles". These are also formed by a deposit, which is however created inside the neurons, and linked to a protein named "tau protein".
What is behind the apparition of these aggregates is still unclear. A history of head trauma or mild traumatic brain injuries can result in that kind of deposition and are linked to an early onset of dementia. That said, the problem could have multiple other origins.