Dementia is a chronic or persistent disorder of the mental ability process caused by brain diseases or injuries marked due to personality changes, memory disorders, and impaired reasoning. Dementia is not a specific disease. It's a group of symptoms related to a decline in memory or other thinking ability skills enough to reduce a person's ability activities. Various kinds of dementia are associated with particular types of brain cell damage in particular regions of the brain.

Memory
Communication and language
Ability to focus and pay attention
Reasoning and judgment
Visual perception