To test the protection of bilingualism against Alzheimer's disease (AD), Bialystok et al. (2007) examined the hospital records of monolingual and bilingual patients who had been diagnosed with various types of dementia. After controlling for various cognitive and other factors, the researchers found that bilinguals experienced the onset of symptoms and were diagnosed approximately 3–4 years later on average than monolinguals. This was replicated with patients all diagnosed with AD. It is important to stress, however, that the studies did not show that bilingualism directly prevents one from having AD, but rather enables functional cognition for a longer period of time; it delays the onset of symptoms for those with the disease. This was confirmed by the finding that when monolingual and bilingual Alzheimer's patients' brains were scanned, bilinguals actually had more pathology (signs of disease) and damage than the monolingual patients. This suggests that active use of the two languages protects against the symptoms of the disease; areas of the brain that enable cognitive control may have benefited from the bilingual experience and so improve cognitive function in older age. 