Comprehension is a complex process, which involves recognition, language and judgment, amongst other functions. Each of these is themselves a complex process requiring multiple brain chemicals Brain chemicals
Chemicals released by the neurons in the brain in order to communicate. that become disruptive in dementia.

People with dementia, often have difficulty with planning. The person you care for might know what they want to do and how to do it, but not be able to put the actions together to perform the task. Sometimes, the person with dementia does not understand something that they see or hear. This makes it difficult for them to follow what is happening on a television program or to tell someone about something they are reading in the newspaper.