What is happening when someone has dementia? It's important to understand the job of the brain because it is the guiding system, the maintenance system, and the managing system of the body. Learn more about the role of the limbic system, the prefrontal cortex, the sensory motor strip, and the importance of the occipital lobe for seeing and doing. It is estimated that with most dementias the brain shrinks one third of it's original size causing many changes in structural and chemical ability. Dementia is not a memory problem, it means brain failure and causes many changes in structural and chemical function.