Seems to stare right through you
When it happens
Why it happens
Seeming to "look without seeing" or gazing downward without any eye contact is the end result of the brain being unable to process most forms of communication.
What you can do
Don't try to coax the person into an interaction; he or she literally can't manage it.
Try not to be too upset by this phenomenon; it's disconcerting but not intentional.
Do continue to talk normally; he or she can probably still take some comfort from the sound of your voice.
Do use physical contact to connect: stroking a hand or knee when you talk, touching the back lightly when you approach.
Use body language to support your speech, such as smiling when you talk, which may or may not be "read" by the person with dementia.