Also, many drugs can disturb the eyes' ability to focus well. Poor vision can increase the risk of falls and injury. It can also make communication harder. Vision plays a big role in communication. We watch the facial expressions of other people as they speak and as they listen to what we have to say.
Eye contact seems to mean a lot.
Also, we often use gestures, like pointing or a shrug of the shoulders, instead of words. Dementing illnesses like Alzheimer's disease produce problems with memory, paying attention, concentration, perception and judgment. The impaired person will forget names and will have trouble coming up with words. The person may have a hard time understanding and remembering what you say. The person may also seem to see things or persons that are not really there.