What happens when the brain changes drastically?
Do you know anyone who has Alzheimer's disease? If you haven't heard of this disease, look at this page by the Alzheimer's Association for more detailed information. In summary, people with Alzheimer's and other similar dementias tend to show the following symptoms:
In other words, the disease affects behavior in significant ways. Would you guess that there are drastic changes in the structure or function of the brain?