How can i help my husband with "brain blindness? He looks at at me, but he doesn't see me. It's take the time, then i can tell he sees.

My husband has Dementia, the big problem it's his eyes, it is not a vision - doctor
said, it his brain. Doctor couldn't check his eyes because he cant follow. Sometimes my husband said "Oh, hi!', like he sees me first time for whole day. He walks very slow and stops many times, looking at the floor or somewhere.