By the time the person reaches the later stages of dementia, they are likely to have significant memory loss and cognitive difficulties. Recent memories may be lost completely (for example, what they had for breakfast or when they last saw a friend) and they may only remember parts of past memories.
The person may believe they are living in an earlier time period from their life (for example, when they were at school). This can mean they say things and behave in ways that don't make sense to those around them. The person may also confuse those around them for someone else (for example, thinking their partner is their sister). The person may respond and experience emotions related to how they felt in the past. The person's emotions are often related to how they're currently seeing their situation - for instance, they might become distressed because they believe they need to go and collect their children from school but they are being prevented from doing this.
The person may no longer be able to recognise themselves or other people such as their partner, friends and family. This may also be due to them believing they are in a different time period, and this can be very difficult for the person and those around them. The person may become upset when looking at themselves in the mirror or think there are strangers in the house, for example. It can be extremely difficult when someone with dementia is not able to remember their own family or close friends. Don't take this personally. This memory loss is caused by the progression of the dementia.
Even if the person with dementia is not able to place someone they are still likely to experience feelings they associate with that person. For example, they may still be prompted when they see someone familiar to feel safe and happy around them. Keeping in touch with the people they know where possible will help them to continue to have these positive feelings and enjoy their company.